Parse::Eyapp - Extensions for Parse::Yapp
use Parse::Eyapp; use Parse::Eyapp::Treeregexp; sub TERMINAL::info { $_[0]{attr} } my $grammar = q{ %right '=' # Lowest precedence %left '-' '+' # + and - have more precedence than = Disambiguate a-b-c as (a-b)-c %left '*' '/' # * and / have more precedence than + Disambiguate a/b/c as (a/b)/c %left NEG # Disambiguate -a-b as (-a)-b and not as -(a-b) %tree # Let us build an abstract syntax tree ... %% line: exp <%name EXPRESSION_LIST + ';'> { $_[1] } /* list of expressions separated by ';' */ ; /* The %name directive defines the name of the class to which the node being built belongs */ exp: %name NUM NUM | %name VAR VAR | %name ASSIGN VAR '=' exp | %name PLUS exp '+' exp | %name MINUS exp '-' exp | %name TIMES exp '*' exp | %name DIV exp '/' exp | %name UMINUS '-' exp %prec NEG | '(' exp ')' { $_[2] } /* Let us simplify a bit the tree */ ; %% sub _Error { die "Syntax error near ".($_[0]->YYCurval?$_[0]->YYCurval:"end of file")."\n" } sub _Lexer { my($parser)=shift; # The parser object for ($parser->YYData->{INPUT}) { # Topicalize m{\G\s+}gc; $_ eq '' and return('',undef); m{\G([0-9]+(?:\.[0-9]+)?)}gc and return('NUM',$1); m{\G([A-Za-z][A-Za-z0-9_]*)}gc and return('VAR',$1); m{\G(.)}gcs and return($1,$1); } return('',undef); } sub Run { my($self)=shift; $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, ); } }; # end grammar our (@all, $uminus); Parse::Eyapp->new_grammar( # Create the parser package/class input=>$grammar, classname=>'Calc', # The name of the package containing the parser firstline=>7 # String $grammar starts at line 7 (for error diagnostics) ); my $parser = Calc->new(); # Create a parser $parser->YYData->{INPUT} = "2*-3+b*0;--2\n"; # Set the input my $t = $parser->Run; # Parse it! local $Parse::Eyapp::Node::INDENT=2; print "Syntax Tree:",$t->str; # Let us transform the tree. Define the tree-regular expressions .. my $p = Parse::Eyapp::Treeregexp->new( STRING => q{ { # Example of support code my %Op = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/'); } constantfold: /TIMES|PLUS|DIV|MINUS/:bin(NUM($x), NUM($y)) => { my $op = $Op{ref($bin)}; $x->{attr} = eval "$x->{attr} $op $y->{attr}"; $_[0] = $NUM[0]; } uminus: UMINUS(NUM($x)) => { $x->{attr} = -$x->{attr}; $_[0] = $NUM } zero_times_whatever: TIMES(NUM($x), .) and { $x->{attr} == 0 } => { $_[0] = $NUM } whatever_times_zero: TIMES(., NUM($x)) and { $x->{attr} == 0 } => { $_[0] = $NUM } }, OUTPUTFILE=> 'main.pm' ); $p->generate(); # Create the tranformations $t->s($uminus); # Transform UMINUS nodes $t->s(@all); # constant folding and mult. by zero local $Parse::Eyapp::Node::INDENT=0; print "\nSyntax Tree after transformations:\n",$t->str,"\n";
Parse::Eyapp (Extended yapp) is a collection of modules that extends Francois Desarmenien Parse::Yapp 1.05. Eyapp extends yacc/yapp syntax with functionalities like named attributes, EBNF-like expressions, modifiable default action (like Parse::RecDescent autoaction), grammar reuse, automatic abstract syntax tree building, syntax directed data generation, translation schemes, tree regular expressions, tree transformations, scope analysis support, directed acyclic graphs and a few more.
Parse-Eyapp LALR parsing engine provides mechanisms for the dynamic resolution (i.e. at parsing time and not at grammar compilation time) of shift-reduce and reduce-reduce conflicts that can not be satisfactorily solved using static precedences. Parse-Eyapp also provide means to solve the problem of languages where the token's type depends upon contextual information like in the well known PL/I statement:
if then=if then if=then
The documentation is distributed among several files:
To get familiar with eyapp read Parse::Eyapp::eyappintro and Parse::Eyapp::debuggingtut. It assumes a reader familiar with parsing techniques. Familiarity with yacc or yapp, RecDescent, ANTLR or similar tools will help but it is not indispensable.
eyapp
yacc
yapp
RecDescent
ANTLR
The document Parse::Eyapp::defaultactionsintro describes the use of default actions and how to deal with grammar reuse and factorization.
The document Parse::Eyapp::eyapplanguageref describes the Eyapp language.
Parse::Eyapp can be used to generate data that conforms with a given grammar. The tutorial Parse::Eyapp::datagenerationtut shows how.
The document Parse::Eyapp::translationschemestut describes the use of Translation Schemes inside the Eyapp language.
The Treeregexp language is described in Parse::Eyapp::Treeregexp. Treeregexp is a language to describe transformations of abstract syntax trees.
Read Parse::Eyapp::Scope to know about the functionalities provided for Scope Analysis.
A set of basic miscellaneous support functions are described in Parse::Eyapp::Base. Several of these functions are related to the dynamic use of methods and subroutines.
Don't forget to read the section "BUGS AND LIMITATIONS"
The examples used in this document can be found in the directory examples/Eyapp accompanying this distribution. As a general rule, each pod/tutorial has an associated subdirectory of examples/Eyapp. Thus, the examples used in the documentation of Parse::Eyapp::eyappintro can be found in examples/Eyapp/eyappintro; The examples mentioned in Parse::Eyapp::Node can be found in examples/Eyapp/Node. There are a few exceptions however. For those exceptions the relative location where the file can be found will be mentioned.
examples/Eyapp
examples/Eyapp/eyappintro
examples/Eyapp/Node
This section describes the syntax of the Eyapp language using its own notation. The grammar extends yacc and yapp grammars. Semicolons have been omitted to save space. Between C-like comments you can find an (informal) explanation of the language associated with each token.
Parse-Eyapp/lib/Parse/Eyapp$ eyapp -c Parse.yp | cat -n 1 %token ASSOC /* is %(left|right|nonassoc) */ 2 %token BEGINCODE /* is %begin { Perl code ... } */ 3 %token CODE /* is { Perl code ... } */ 4 %token DEFAULTACTION /* is %defaultaction */ 5 %token EXPECT /* is %expect */ 6 %token HEADCODE /* is %{ Perl code ... %} */ 7 %token IDENT /* is [A-Za-z_][A-Za-z0-9_]* */ 8 %token LITERAL /* is a string literal like 'hello' */ 9 %token METATREE /* is %metatree */ 10 %token NAME /* is %name */ 11 %token NAMINGSCHEME /* is %namingscheme */ 12 %token NOCOMPACT /* is %nocompact */ 13 %token NUMBER /* is \d+ */ 14 %token OPTION /* is (%name\s*([A-Za-z_]\w*)\s*)?\? */ 15 %token PLUS /* is (%name\s*([A-Za-z_]\w*)\s*)?\+ */ 16 %token PREC /* is %prec */ 17 %token PREFIX /* is %prefix\s+([A-Za-z_][A-Za-z0-9_:]*::) */ 18 %token SEMANTIC /* is %semantic\s+token */ 19 %token STAR /* is (%name\s*([A-Za-z_]\w*)\s*)?\* */ 20 %token START /* is %start */ 21 %token STRICT /* is %strict */ 22 %token SYNTACTIC /* is %syntactic\s+token */ 23 %token TAILCODE /* is { Perl code ... } */ 24 %token TOKEN /* is %token */ 25 %token TREE /* is %tree */ 26 %token TYPE /* is %type */ 27 %token UNION /* is %union */ 28 %start eyapp 29 30 %% 31 32 # Main rule 33 eyapp: 34 head body tail 35 ; 36 #Common rules: 37 symbol: 38 LITERAL 39 | ident #default action 40 ; 41 ident: 42 IDENT 43 ; 44 # Head section: 45 head: 46 headsec '%%' 47 ; 48 headsec: 49 #empty #default action 50 | decls #default action 51 ; 52 decls: 53 decls decl #default action 54 | decl #default action 55 ; 56 decl: 57 '\n' #default action 58 | SEMANTIC typedecl symlist '\n' 59 | SYNTACTIC typedecl symlist '\n' 60 | TOKEN typedecl symlist '\n' 61 | ASSOC typedecl symlist '\n' 62 | START ident '\n' 63 | PREFIX '\n' 64 | NAMINGSCHEME CODE '\n' 65 | HEADCODE '\n' 66 | UNION CODE '\n' #ignore 67 | DEFAULTACTION CODE '\n' 68 | TREE '\n' 69 | METATREE '\n' 70 | STRICT '\n' 71 | NOCOMPACT '\n' 72 | TYPE typedecl identlist '\n' 73 | EXPECT NUMBER '\n' 74 | EXPECT NUMBER NUMBER '\n' 75 | error '\n' 76 ; 77 typedecl: 78 #empty 79 | '<' IDENT '>' 80 ; 81 symlist: 82 symlist symbol 83 | symbol 84 ; 85 identlist: 86 identlist ident 87 | ident 88 ; 89 # Rule section 90 body: 91 rulesec '%%' 92 | '%%' 93 ; 94 rulesec: 95 rulesec rules #default action 96 | startrules #default action 97 ; 98 startrules: 99 IDENT ':' rhss ';' 100 | error ';' 101 ; 102 rules: 103 IDENT ':' rhss ';' 104 | error ';' 105 ; 106 rhss: 107 rhss '|' rule 108 | rule 109 ; 110 rule: 111 optname rhs prec epscode 112 | optname rhs 113 ; 114 rhs: 115 #empty #default action (will return undef) 116 | rhselts #default action 117 ; 118 rhselts: 119 rhselts rhseltwithid 120 | rhseltwithid 121 ; 122 rhseltwithid: 123 rhselt '.' IDENT 124 | '$' rhselt 125 | '$' error 126 | rhselt 127 ; 128 rhselt: 129 symbol 130 | code 131 | '(' optname rhs ')' 132 | rhselt STAR 133 | rhselt '<' STAR symbol '>' 134 | rhselt OPTION 135 | rhselt '<' PLUS symbol '>' 136 | rhselt PLUS 137 ; 138 optname: 139 /* empty */ 140 | NAME IDENT 141 ; 142 prec: 143 PREC symbol 144 ; 145 epscode: 146 | code 147 ; 148 code: 149 CODE 150 | BEGINCODE 151 ; 152 # Tail section: 153 tail: 154 /*empty*/ 155 | TAILCODE 156 ; 157 158 %%
The semantic of Eyapp agrees with the semantic of yacc and yapp for all the common constructions.
Eyapp
Comments are either Perl style, from # up to the end of line, or C style, enclosed between /* and */.
#
/*
*/
Two kind of symbols may appear inside a Parse::Eyapp program: Non-terminal symbols or syntactic variables, called also left-hand-side symbols and Terminal symbols, called also Tokens.
Tokens are the symbols the lexical analyzer function returns to the parser. There are two kinds of tokens: symbolic tokens and string literals.
Syntactic variables and symbolic tokens identifiers must conform to the regular expression [A-Za-z][A-Za-z0-9_]*.
[A-Za-z][A-Za-z0-9_]*
When building the syntax tree (i.e. when running under the %tree directive) symbolic tokens will be considered semantic tokens (see section "Syntactic and Semantic tokens"). Symbolic tokens yield nodes in the Abstract Syntax Tree.
%tree
String literals are enclosed in single quotes and can contain almost anything. They will be received by the parser as double-quoted strings. Any special character as '"', '$' and '@' is escaped. To have a single quote inside a literal, escape it with '\'.
'"'
'$'
'@'
When building the syntax tree (i.e. when running under the %tree directive) string literals will be considered syntactic tokens (see section "Syntactic and Semantic tokens"). Syntactic tokens do not produce nodes in the Abstract Syntax Tree.
The examples used along this document can be found in the directory examples/eyapplanguageref accompanying this distribution.
examples/eyapplanguageref
An Eyapp program has three parts called head, body and tail:
eyapp: head body tail ;
Each part is separated from the former by the symbol %%:
%%
head: headsec '%%' body: rulesec '%%'
The head section contains a list of declarations
headsec: decl *
There are different kinds of declarations.
This reference does not fully describes all the declarations that are shared with yacc and yapp.
In this and the next sections we will describe the basics of the Eyapp language using the file examples/eyapplanguageref/Calc.eyp that accompanies this distribution. This file implements a trivial calculator. Here is the header section:
examples/eyapplanguageref/Calc.eyp
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '1,11p' Calc.eyp | cat -n 1 # examples/eyapplanguageref/Calc.eyp 2 %right '=' 3 %left '-' '+' 4 %left '*' '/' 5 %left NEG 6 %right '^' 7 %{ 8 my %s; # symbol table 9 %} 10 11 %%
Lines 2-5 declare several tokens. The usual way to declare tokens is through the %token directive. The declarations %nonassoc, %left and %right not only declare the tokens but also associate a priority with them. Tokens declared in the same line have the same precedence. Tokens declared with these directives in lines below have more precedence than those declared above. Thus, in the example above we are saying that "+" and "-" have the same precedence but higher precedence than =. The final effect of "-" having greater precedence than = will be that an expression like:
%token
%nonassoc
%left
%right
"+"
"-"
a = 4 - 5
will be interpreted as
a = (4 - 5)
and not as
(a = 4) - 5
The use of the %left indicates that - in case of ambiguity and a match between precedences - the parser must build the tree corresponding to a left parenthesizing. Thus, the expression
4 - 5 - 9
(4 - 5) - 9
You can refer to the token end-of-input in the header section using the string '' (for example to give it some priority, see the example in examples/debuggingtut/typicalrrwithprec.eyp).
''
examples/debuggingtut/typicalrrwithprec.eyp
Perl code surrounded by %{ and %} can be inserted in the head section. Such code will be inserted in the module generated by eyapp near the beginning. Therefore, declarations like the one of the calculator symbol table %s
%{
%}
%s
7 %{ 8 my %s; # symbol table 9 %}
will be visible from almost any point in the file.
%start program declares program as the start symbol of the grammar. When %start is not used, the first rule in the body section will be used.
%start program
program
%start
The %expect #NUMBER directive works as in bison and suppress warnings when the number of Shift/Reduce conflicts is exactly #NUMBER.
%expect #NUMBER
bison
#NUMBER
The directive has been extended to be called with two numbers:
%expect NUMSHIFTRED NUMREDRED
no warnings will be emitted if the number of shift-reduce conflicts is exactly NUMSHIFTRED and the number of reduce-reduce conflicts is NUMREDRED.
NUMSHIFTRED
NUMREDRED
C oriented declarations like %type and %union are parsed but ignored.
%type
%union
%strict
By default, identifiers appearing in the rule section will be classified as terminal if they don't appear in the left hand side of any production rules.
The directive %strict forces the declaration of all tokens. The following eyapp program issues a warning:
pl@nereida:~/LEyapp/examples/eyapplanguageref$ cat -n bugyapp2.eyp 1 %strict 2 %% 3 expr: NUM; 4 %% pl@nereida:~/LEyapp/examples/eyapplanguageref$ eyapp bugyapp2.eyp Warning! Non declared token NUM at line 3 of bugyapp2.eyp
To keep silent the compiler declare all tokens using one of the token declaration directives (%token, %left, etc.)
pl@nereida:~/LEyapp/examples/eyapplanguageref$ cat -n bugyapp3.eyp 1 %strict 2 %token NUM 3 %% 4 expr: NUM; 5 %% pl@nereida:~/LEyapp/examples/eyapplanguageref$ eyapp bugyapp3.eyp pl@nereida:~/LEyapp/examples/eyapplanguageref$ ls -ltr | tail -1 -rw-r--r-- 1 pl users 2395 2008-10-02 09:41 bugyapp3.pm
It is a good practice to use %strict at the beginning of your grammar.
%prefix
The %prefix directive is equivalent to the use of the yyprefix. The node classes are prefixed with the specified prefix
yyprefix
%prefix Some::Prefix::
See the example in examples/eyapplanguageref/alias_and_yyprefix.pl. See also section "Grammar Reuse" in Parse::Eyapp::defaultactionsintro for an example that does not involve the %tree directive.
examples/eyapplanguageref/alias_and_yyprefix.pl
In Parse::Eyapp you can modify the default action using the %defaultaction { Perl code } directive. See section "DEFAULT ACTIONS". The examples examples/eyapplanguageref/Postfix.eyp and examples/eyapplanguageref/Lhs.eyp illustrate the use of the directive.
Parse::Eyapp
%defaultaction { Perl code }
examples/eyapplanguageref/Postfix.eyp
examples/eyapplanguageref/Lhs.eyp
Parse::Eyapp facilitates the construction of concrete syntax trees and abstract syntax trees (abbreviated AST from now on) through the %tree and %metatree directives. See sections "ABSTRACT SYNTAX TREES: %tree AND %name" and Parse::Eyapp::translationschemestut.
%metatree
The new token declaration directives %syntactic token and %semantic token can change the way eyapp builds the abstract syntax tree. See section "Syntactic and Semantic tokens".
%syntactic token
%semantic token
%nocompact
This directive influences the generation of the LALR tables. They will not be compacted and the tokens for the DEFAULT reduction will be explicitly set. It can be used to produce an .output file (option -v) with more information.
DEFAULT
.output
-v
The body section contains the rules describing the grammar:
body: rules * '%%' rules: IDENT ':' rhss ';' rhss: (optname rhs (prec epscode)?) <+ '|'>
A rule is made of a left-hand-side symbol (the syntactic variable), followed by a ':' and one or more right-hand-sides (or productions) separated by '|' and terminated by a ';' like in:
':'
'|'
';'
exp: exp '+' exp | exp '-' exp | NUM ;
A production (right hand side) may be empty:
input: /* empty */ | input line ;
The former two productions can be abbreviated as
input: line * ;
The operators *, + and ? are presented in section "LISTS AND OPTIONALS".
*
+
?
A syntactic variable cannot appear more than once as a rule name (This differs from yacc). So you can't write
thing: foo bar ; thing: foo baz ;
instead, write:
thing: foo bar | foo baz ;
In Parse::Eyapp a production rule
A -> X_1 X_2 ... X_n
can be followed by a semantic action:
A -> X_1 X_2 ... X_n { Perl Code }
Such semantic action is nothing but Perl code that will be treated as an anonymous subroutine. The semantic action associated with production rule A -> X_1 X_2 ... X_n is executed after any actions associated with the subtrees of X_1, X_2, ..., X_n. Eyapp parsers build the syntax tree using a left-right bottom-up traverse of the syntax tree. Each times the Parser visits the node associated with the production A -> X_1 X_2 ... X_n the associated semantic action is called. Associated with each symbol of a Parse::Eyapp grammar there is a scalar Semantic Value or Attribute. The semantic values of terminals are provided by the lexical analyzer. In the calculator example (see file examples/eyapplanguageref/Calc.yp in the distribution), the semantic value associated with an expression is its numeric value. Thus in the rule:
X_1
X_2
X_n
examples/eyapplanguageref/Calc.yp
exp '+' exp { $_[1] + $_[3] }
$_[1] refers to the attribute of the first exp, $_[2] is the attribute associated with '+', which is the second component of the pair provided by the lexical analyzer and $_[3] refers to the attribute of the second exp.
$_[1]
exp
$_[2]
'+'
$_[3]
When the semantic action/anonymous subroutine is called, the arguments are as follows:
$_[1] to $_[n] are the attributes of the symbols X_1, X_2, ..., X_n. Just as $1 to $n in yacc,
$_[n]
$1
$n
$_[0] is the parser object itself. Having $_[0] being the parser object itself allows you to call parser methods. Most yacc macros have been converted into parser methods. See section "METHODS AVAILABLE IN THE GENERATED CLASS" in Parse::Eyapp.
$_[0]
The returned value will be the attribute associated with the left hand side of the production.
Names can be given to the attributes using the dot notation (see file examples/eyapplanguageref/CalcSimple.eyp):
examples/eyapplanguageref/CalcSimple.eyp
exp.left '+' exp.right { $left + $right }
See section "NAMES FOR ATTRIBUTES" for more details about the dot and dollar notations.
If no action is specified and no %defaultaction is specified the default action
%defaultaction
{ $_[1] }
will be executed instead. See section "DEFAULT ACTIONS" to know more.
Actions can be inserted in the middle of a production like in:
block: '{'.bracket { $ids->begin_scope(); } declaration*.decs statement*.sts '}' { ... }
A middle production action is managed by inserting a new rule in the grammar and associating the semantic action with it:
Temp: /* empty */ { $ids->begin_scope(); }
Middle production actions can refer to the attributes on its left. They count as one of the components of the production. Thus the program:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '1,4p' intermediateaction2.yp %% S: 'a' { $_[1]x4 }.mid 'a' { print "$_[2], $mid, $_[3]\n"; } ; %%
The auxiliar syntactic variables are named @#position-#order where #position is the position of the action in the rhs and order is an ordinal number. See the .output file for the former example:
@#position-#order
#position
order
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ eyapp -v intermediateaction2.yp pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '1,5p' intermediateaction2.output Rules: ------ 0: $start -> S $end 1: S -> 'a' @1-1 'a' 2: @1-1 -> /* empty */
when given input aa the execution will produce as output aaaa, aaaa, a.
aa
aaaa, aaaa, a
Following with the calculator example, the body is:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '12,48p' Calc.eyp | cat -n 1 start: 2 input { \%s } 3 ; 4 5 input: line * 6 ; 7 8 line: 9 '\n' { undef } 10 | exp '\n' { print "$_[1]\n" if defined($_[1]); $_[1] } 11 | error '\n' 12 { 13 $_[0]->YYErrok; 14 undef 15 } 16 ; 17 18 exp: 19 NUM 20 | $VAR { $s{$VAR} } 21 | $VAR '=' $exp { $s{$VAR} = $exp } 22 | exp.left '+' exp.right { $left + $right } 23 | exp.left '-' exp.right { $left - $right } 24 | exp.left '*' exp.right { $left * $right } 25 | exp.left '/' exp.right 26 { 27 $_[3] and return($_[1] / $_[3]); 28 $_[0]->YYData->{ERRMSG} = "Illegal division by zero.\n"; 29 $_[0]->YYError; # Pretend that a syntactic error occurred: _Error will be called 30 undef 31 } 32 | '-' $exp %prec NEG { -$exp } 33 | exp.left '^' exp.right { $left ** $right } 34 | '(' $exp ')' { $exp } 35 ; 36 37 %%
This example does not uses any of the Eyapp extensions (with the exception of the star list at line 5) and the dot and dollar notations. Please, see the Parse::Yapp pages and elsewhere documentation on yacc and bison for more information.
When Eyapp analyzes a grammar like:
examples/eyapplanguageref$ cat -n ambiguities.eyp 1 %% 2 exp: 3 NUM 4 | exp '-' exp 5 ; 6 %%
it will produce a warning announcing the existence of shift-reduce conflicts:
examples/eyapplanguageref$ eyapp ambiguities.eyp 1 shift/reduce conflict (see .output file) State 5: reduce by rule 2: exp -> exp '-' exp (default action) State 5: shifts: to state 3 with '-'
when eyapp finds warnings automatically produces a .output file describing the conflict.
What the warning is saying is that an expression like exp '-' exp (rule 2) followed by a minus '-' can be parsed in more than one way. If we have an input like NUM - NUM - NUM the activity of a LALR(1) parser (the family of parsers to which Eyapp belongs) consists of a sequence of shift and reduce actions. A shift action has as consequence the reading of the next token. A reduce action is finding a production rule that matches and substituting the rhs of the production by the lhs. For input NUM - NUM - NUM the activity will be as follows (the dot is used to indicate where the next input token is):
exp '-' exp
'-'
NUM - NUM - NUM
.NUM - NUM - NUM # shift NUM.- NUM - NUM # reduce exp: NUM exp.- NUM - NUM # shift exp -.NUM - NUM # shift exp - NUM.- NUM # reduce exp: NUM exp - exp.- NUM # shift/reduce conflict
up this point two different decisions can be taken: the next description can be
exp.- NUM # reduce by exp: exp '-' exp (rule 2)
or:
exp - exp -.NUM # shift '-' (to state 3)
that is why it is called a shift-reduce conflict.
That is also the reason for the precedence declarations in the head section. Another kind of conflicts are reduce-reduce conflicts. They arise when more that rhs can be applied for a reduction action.
Eyapp solves the conflicts applying the following rules:
In a shift/reduce conflict, the default is the shift.
In a reduce/reduce conflict, the default is to reduce by the earlier grammar production (in the input sequence).
Precedences and associativities can be given to tokens in the declarations section. This is made by a sequence of lines beginning with one of the directives: %left, %right, or %nonassoc, followed by a list of tokens. All the tokens on the same line have the same precedence and associativity; the lines are listed in order of increasing precedence.
A precedence and associativity is associated with each grammar production; it is the precedence and associativity of the last token or literal in the right hand side of the production.
The %prec directive can be used when a rhs is involved in a conflict and has no tokens inside or it has but the precedence of the last token leads to an incorrect interpretation. A rhs can be followed by an optional %prec token directive giving the production the precedence of the token
%prec
%prec token
token
exp: '-' exp %prec NEG { -$_[1] }
If there is a shift/reduce conflict, and both the grammar production and the input token have precedence and associativity associated with them, then the conflict is solved in favor of the action (shift or reduce) associated with the higher precedence. If the precedences are the same, then the associativity is used; left associative implies reduce, right associative implies shift, and non associative implies error. The last is used to describe operators, like the operator .LT. in FORTRAN, that may not associate with themselves. That is, because
.LT.
A .LT. B .LT. C
is invalid in FORTRAN, .LT. would be described with the keyword %nonassoc in eyapp.
To solve a shift-reduce conflict between a production A --> SOMETHING and a token 'a' you can follow this procedure:
A --> SOMETHING
'a'
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '56,65p' ambiguities.output State 5: exp -> exp . '-' exp (Rule 2) exp -> exp '-' exp . (Rule 2) '-' shift, and go to state 3 '-' [reduce using rule 2 (exp)] $default reduce using rule 2 (exp)
A --> SOMETHING.
A --> prefix . suffix
MINUS(MINUS(NUM, NUM), NUM)
MINUS(NUM, MINUS(NUM, NUM))
exp - exp.- NUM
exp: exp '-' exp
%left '-'
The token name error is reserved for error handling. This name can be used in grammar productions; it suggests places where errors are expected, and recovery can take place:
error
line: '\n' { undef } | exp '\n' { print "$_[1]\n" if defined($_[1]); $_[1] } | error '\n' { $_[0]->YYErrok; undef }
The parser pops its stack until it enters a state where the token error is legal. It then shifts the token error and proceeds to discard tokens until finding one that is acceptable. In the example all the tokens until finding a '\n' will be skipped. If no special error productions have been specified, the processing will halt.
'\n'
In order to prevent a cascade of error messages, the parser, after detecting an error, remains in error state until three tokens have been successfully read and shifted. If an error is detected when the parser is already in error state, no message is given, and the input token is quietly deleted. The method YYErrok used in the example communicates to the parser that a satisfactory recovery has been reached and that it can safely emit new error messages.
YYErrok
You cannot have a literal 'error' in your grammar as it would confuse the driver with the error token. Use a symbolic token instead.
The tail section contains Perl code. Usually the lexical analyzer and the Error management subroutines go there. A better practice however is to isolate both subroutines in a module and use them in the grammar. An example of this is in files examples/eyapplanguageref/CalcUsingTail.eyp (the grammar), examples/eyapplanguageref/usecalcusingtail.pl (the client program), and examples/eyapplanguageref/Tail.pm (the module containing the lexical analyzer plus error handling and auxiliary subroutines).
examples/eyapplanguageref/CalcUsingTail.eyp
examples/eyapplanguageref/usecalcusingtail.pl
examples/eyapplanguageref/Tail.pm
The Lexical Analyzer is called each time the parser needs a new token. It is called with only one argument (the parser object) and returns a pair containing the next token and its associated attribute.
The fact that is a method of the parser object means that the parser methods are accessible inside the lexical analyzer. Specially interesting is the $_[0]->YYData method which provides access to the user data area.
$_[0]->YYData
When the lexical analyzer reaches the end of input, it must return the pair ('', undef)
('', undef)
See below how to write a lexical analyzer (file examples/eyapplanguageref/Calc.eyp):
1 sub make_lexer { 2 my $input = shift; 3 4 return sub { 5 my $parser = shift; 6 7 for ($$input) { 8 m{\G[ \t]*}gc; 9 m{\G([0-9]+(?:\.[0-9]+)?)}gc and return ('NUM',$1); 10 m{\G([A-Za-z][A-Za-z0-9_]*)}gc and return ('VAR',$1); 11 m{\G\n}gc and do { $lineno++; return ("\n", "\n") }; 12 m{\G(.)}gc and return ($1,$1); 13 14 return('',undef); 15 } 16 } 17 }
The subroutine make_lexer creates the lexical analyzer as a closure. The lexer returned by make_lexer is used by the YYParse method:
make_lexer
YYParse
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '90,97p' Calc.eyp | cat -n 1 sub Run { 2 my($self)=shift; 3 my $input = shift or die "No input given\n"; 4 5 return $self->YYParse( yylex => make_lexer($input), yyerror => \&_Error, 6 #yydebug =>0x1F 7 ); 8 }
The Error Report subroutine is also a parser method, and consequently receives as parameter the parser object.
See the error report subroutine for the example in examples/Calc.eyp:
examples/Calc.eyp
1 %% 2 3 my $lineno = 1; 4 5 sub _Error { 6 my $parser = shift; 7 8 exists $parser->YYData->{ERRMSG} 9 and do { 10 print $parser->YYData->{ERRMSG}; 11 delete $parser->YYData->{ERRMSG}; 12 return; 13 }; 14 my($token)=$parser->YYCurval; 15 my($what)= $token ? "input: '$token'" : "end of input"; 16 my @expected = $parser->YYExpect(); 17 local $" = ', '; 18 print << "ERRMSG"; 19 20 Syntax error near $what (lin num $lineno). 21 Expected one of these terminals: @expected 22 ERRMSG 23 }
See the Parse::Yapp pages and elsewhere documentation on yacc and bison for more information.
The following is an example of a program that uses the calculator explained in the two previous sections:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ cat -n usecalc.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Calc; 4 5 my $parser = Calc->new(); 6 my $input = <<'EOI'; 7 a = 2*3 8 d = 5/(a-6) 9 b = (a+1)/7 10 c=a*3+4)-5 11 a = a+1 12 EOI 13 my $t = $parser->Run(\$input); 14 print "========= Symbol Table ==============\n"; 15 print "$_ = $t->{$_}\n" for sort keys %$t;
The output for this program is (the input for each output appear as a Perl comment on the right):
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ eyapp Calc.eyp pl@nereida:~/src/perl/YappWithDefaultAction/examples$ usecalc.pl 6 # a = 2*3 Illegal division by zero. # d = 5/(a-6) 1 # b = (a+1)/7 Syntax error near input: ')' (lin num 4). # c=a*3+4)-5 Expected one of these terminals: -, /, ^, *, +, 7 # a = a+1 ========= Symbol Table ============== a = 7 b = 1 c = 22
The elements of the right hand side of a production (abbreviated rhs) can be one of these:
rhselt: symbol | code | '(' optname rhs ')' | rhselt STAR /* STAR is (%name\s*([A-Za-z_]\w*)\s*)?\* */ | rhselt '<' STAR symbol '>' | rhselt OPTION /* OPTION is (%name\s*([A-Za-z_]\w*)\s*)?\? */ | rhselt '<' PLUS symbol '>' | rhselt PLUS /* PLUS is (%name\s*([A-Za-z_]\w*)\s*)?\+ */
The STAR, OPTION and PLUS operators provide a simple mechanism to express lists:
STAR
OPTION
PLUS
In Eyapp the + operator indicates one or more repetitions of the element to the left of +, thus a rule like:
decls: decl +
is the same as:
decls: decls decl | decl
An additional symbol may be included to indicate lists of elements separated by such symbol. Thus
rhss: rule <+ '|'>
is equivalent to:
rhss: rhss '|' rule | rule
The operators * and ? have their usual meaning: 0 or more for * and optionality for ?. Is legal to parenthesize a rhs expression as in:
rhs
optname: (NAME IDENT)?
The grammar:
examples/eyapplanguageref$ head -14 List3.yp | cat -n 1 # List3.yp 2 %semantic token 'c' 3 %{ 4 use Data::Dumper; 5 %} 6 %% 7 S: 'c'+ 'd'+ 8 { 9 print Dumper($_[1]); 10 print Dumper($_[2]); 11 } 12 ; 13 14 %%
Is equivalent to:
examples/eyapplanguageref$ eyapp -v List3.yp | head -9 List3.output Rules: ------ 0: $start -> S $end 1: PLUS-1 -> PLUS-1 'c' 2: PLUS-1 -> 'c' 3: PLUS-2 -> PLUS-2 'd' 4: PLUS-2 -> 'd' 5: S -> PLUS-1 PLUS-2 By default, the semantic action associated with a C<+> returns the lists of attributes to which the C<+> applies: examples/eyapplanguageref$ use_list3.pl ccdd $VAR1 = [ 'c', 'c' ]; $VAR1 = [ 'd', 'd' ];
Observe that, in spite of 'd' being a syntactic token the actions related with the d+ element (i.e. the actions associated with the PLUS-2 productions) create the list of ds.
'd'
d+
PLUS-2
d
The semantic associated with a + changes when one of the tree creation directives is active (for instance %tree or %metatree) or it has been explicitly requested with a call to the YYBuildingTree method:
YYBuildingTree
$self->YYBuildingTree(1);
Other ways to change the associated semantic are to use the yybuildingtree option of YYParse:
yybuildingtree
$self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, yybuildingtree => 1, # yydebug => 0x1F );
In such case the associated semantic action creates a node labelled
_PLUS_LIST
whose children are the attributes associated with the items in the plus list. As it happens when using the %tree directive, syntactic tokens are skipped.
When executing the example above but under the %tree directive the output changes:
examples/eyapplanguageref$ head -3 List3_tree.yp; eyapp List3_tree.yp # List3.yp %semantic token 'c' %tree examples/eyapplanguageref$ diff List3.yp List3_tree.yp 2a3 > %tree
If we now run the client program with input ccdd we get a couple of syntax trees:
ccdd
examples/eyapplanguageref$ use_list3_tree.pl ccdd $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ), bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ) ] }, '_PLUS_LIST' ); $VAR1 = bless( { 'children' => [] }, '_PLUS_LIST' );
The node associated with the list of ds is empty since terminal d wasn't declared semantic.
When under the influence of the %tree directive the action associated with a list operator is to flat the children in a single list.
In the former example, the d nodes don't show up since 'd' is a syntactic token. However, it may happen that changing the status of 'd' to semantic will not suffice.
When inserting the children, the tree (%tree) node construction method (YYBuildAST) omits any attribute that is not a reference. Therefore, when inserting explicit actions, it is necessary to guarantee that the returned value is a reference or a semantic token to assure the presence of the value in the lists of children of the node. Certainly you can use this property to prune parts of the tree. Consider the following example:
YYBuildAST
examples/eyapplanguageref$ head -19 ListWithRefs1.eyp | cat -n 1 # ListWithRefs1.eyp 2 %semantic token 'c' 'd' 3 %{ 4 use Data::Dumper; 5 %} 6 %% 7 S: 'c'+ D+ 8 { 9 print Dumper($_[1]); 10 print $_[1]->str."\n"; 11 print Dumper($_[2]); 12 print $_[2]->str."\n"; 13 } 14 ; 15 16 D: 'd' 17 ; 18 19 %%
To activate the tree semantic for lists we use the yybuildingtree option of YYParse:
examples/eyapplanguageref$ tail -7 ListWithRefs1.eyp | cat -n 1 sub Run { 2 my($self)=shift; 3 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, 4 yybuildingtree => 1, 5 #, yydebug => 0x1F 6 ); 7 }
The execution gives an output like this:
examples/eyapplanguageref$ eyapp ListWithRefs1.eyp; use_listwithrefs1.pl ccdd $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ), bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ) ] }, '_PLUS_LIST' ); _PLUS_LIST(TERMINAL,TERMINAL) $VAR1 = bless( { 'children' => [] }, '_PLUS_LIST' ); _PLUS_LIST
Though 'd' was declared semantic the default action associated with the production D: 'd' in line 16 returns $_[1] (that is, the scalar 'd'). Since it is not a reference it won't be inserted in the list of children of _PLUS_LIST.
D: 'd'
The solution is to be sure that the attribute is a reference:
pl@nereida:~/LEyapp/examples$ head -22 ListWithRefs.eyp | cat -n 1 # ListWithRefs.eyp 2 %semantic token 'c' 3 %{ 4 use Data::Dumper; 5 %} 6 %% 7 S: 'c'+ D+ 8 { 9 print Dumper($_[1]); 10 print $_[1]->str."\n"; 11 print Dumper($_[2]); 12 print $_[2]->str."\n"; 13 } 14 ; 15 16 D: 'd' 17 { 18 bless { attr => $_[1], children =>[]}, 'DES'; 19 } 20 ; 21 22 %%
Now the attribute associated with D is a reference and appears in the list of children of _PLUS_LIST:
D
pl@nereida:~/LEyapp/examples$ eyapp ListWithRefs.eyp; use_listwithrefs.pl ccdd $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ), bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ) ] }, '_PLUS_LIST_1' ); _PLUS_LIST_1(TERMINAL,TERMINAL) $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'd' }, 'DES' ), bless( { 'children' => [], 'attr' => 'd' }, 'DES' ) ] }, '_PLUS_LIST_2' ); _PLUS_LIST_2(DES,DES)
Parse::Eyapp::Node->new
The former solution consisting on writing by hand the code to build the node may suffice when dealing with a single node. Writing by hand the code to build a node is a cumbersome task. Even worst: though the node built in the former example looks like a Parse::Eyapp node actually isn't. Parse::Eyapp nodes always inherit from Parse::Eyapp::Node and consequently have access to the methods in such package. The following execution using the debugger illustrates the point:
Parse::Eyapp::Node
pl@nereida:~/LEyapp/examples$ perl -wd use_listwithrefs.pl Loading DB routines from perl5db.pl version 1.28 Editor support available. Enter h or `h h' for help, or `man perldebug' for more help. main::(use_listwithrefs.pl:4): $parser = new ListWithRefs(); DB<1> f ListWithRefs.eyp 1 2 #line 3 "ListWithRefs.eyp" 3 4: use Data::Dumper; 5 6 #line 7 "ListWithRefs.eyp" 7 #line 8 "ListWithRefs.eyp" 8 9: print Dumper($_[1]); 10: print $_[1]->str."\n";
through the command f ListWithRefs.eyp we inform the debugger that subsequent commands will refer to such file. Next we execute the program up to the semantic action associated with the production rule S: 'c'+ D+ (line 9)
f ListWithRefs.eyp
S: 'c'+ D+
DB<2> c 9 # Continue up to line 9 of ListWithRefs.eyp ccdd ListWithRefs::CODE(0x84ebe5c)(ListWithRefs.eyp:9): 9: print Dumper($_[1]);
Now we are in condition to look at the contents of the arguments:
DB<3> x $_[2]->str 0 '_PLUS_LIST_2(DES,DES)' DB<4> x $_[2]->child(0) 0 DES=HASH(0x85c4568) 'attr' => 'd' 'children' => ARRAY(0x85c458c) empty array
the str method works with the object $_[2] since _PLUS_LIST_2 nodes inherit from Parse::Eyapp::Node. However, when we try with the DES node we get an error:
str
_PLUS_LIST_2
DES
DB<6> x $_[2]->child(0)->str Can't locate object method "str" via package "DES" at \ (eval 11)[/usr/share/perl/5.8/perl5db.pl:628] line 2, <STDIN> line 1. DB<7>
More robust than the former solution of building the node by hand is to use the constructor Parse::Eyapp::Node->new: The method Parse::Eyapp::Node->new is uset to build forests of syntactic trees.
It receives a list of terms describing the trees and - optionally - a reference to a subroutine used to set up the attributes of the just created nodes. After the creation of the trees the sub is called by Parse::Eyapp::Node->new with arguments the list of references to the nodes (in the order in which they appear in the terms, from left to right). Parse::Eyapp::Node->new returns a list of references to the just created nodes. In a scalar context returns a reference to the first of such trees. See an example:
pl@nereida:~/LEyapp/examples$ perl -MParse::Eyapp -MData::Dumper -wde 0 main::(-e:1): 0 DB<1> @t = Parse::Eyapp::Node->new('A(C,D) E(F)', sub { my $i = 0; $_->{n} = $i++ for @_ }) DB<2> $Data::Dumper::Indent = 0 DB<3> print Dumper($_)."\n" for @t $VAR1 = bless( {'n' => 0,'children' => [bless( {'n' => 1,'children' => []}, 'C' ), bless( {'n' => 2,'children' => []}, 'D' ) ] }, 'A' ); $VAR1 = bless( {'n' => 1,'children' => []}, 'C' ); $VAR1 = bless( {'n' => 2,'children' => []}, 'D' ); $VAR1 = bless( {'n' => 3,'children' => [bless( {'n' => 4,'children' => []}, 'F' )]}, 'E' ); $VAR1 = bless( {'n' => 4,'children' => []}, 'F' );
See the following example in which the nodes associated with 'd' are explicitly constructed:
pl@nereida:~/LEyapp/examples$ head -28 ListWithRefs2.eyp| cat -n 1 # ListWithRefs2.eyp 2 %semantic token 'c' 3 %{ 4 use Data::Dumper; 5 %} 6 %% 7 S: 'c'+ D+ 8 { 9 print Dumper($_[1]); 10 print $_[1]->str."\n"; 11 print Dumper($_[2]); 12 print $_[2]->str."\n"; 13 } 14 ; 15 16 D: 'd'.d 17 { 18 Parse::Eyapp::Node->new( 19 'DES(TERMINAL)', 20 sub { 21 my ($DES, $TERMINAL) = @_; 22 $TERMINAL->{attr} = $d; 23 } 24 ); 25 } 26 ; 27 28 %%
To know more about Parse::Eyapp::Node->new see the section for Parse::Eyapp::Node->new
When the former eyapp program is executed produces the following output:
pl@nereida:~/LEyapp/examples$ eyapp ListWithRefs2.eyp; use_listwithrefs2.pl ccdd $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ), bless( { 'children' => [], 'attr' => 'c', 'token' => 'c' }, 'TERMINAL' ) ] }, '_PLUS_LIST_1' ); _PLUS_LIST_1(TERMINAL,TERMINAL) $VAR1 = bless( { 'children' => [ bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'd' }, 'TERMINAL' ) ] }, 'DES' ), bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'd' }, 'TERMINAL' ) ] }, 'DES' ) ] }, '_PLUS_LIST_2' ); _PLUS_LIST_2(DES(TERMINAL),DES(TERMINAL))
Any list operator operates on the factor to its left. A list in the right hand side of a production rule counts as a single symbol.
Both operators * and + can be used with the format X <* Separator>. In such case they describe lists of Xs separated by separator. See an example:
X <* Separator>
X
separator
pl@nereida:~/LEyapp/examples$ head -25 CsBetweenCommansAndD.eyp | cat -n 1 # CsBetweenCommansAndD.eyp 2 3 %semantic token 'c' 'd' 4 5 %{ 6 sub TERMINAL::info { 7 $_[0]->attr; 8 } 9 %} 10 %tree 11 %% 12 S: 13 ('c' <* ','> 'd')* 14 { 15 print "\nNode\n"; 16 print $_[1]->str."\n"; 17 print "\nChild 0\n"; 18 print $_[1]->child(0)->str."\n"; 19 print "\nChild 1\n"; 20 print $_[1]->child(1)->str."\n"; 21 $_[1] 22 } 23 ; 24 25 %%
The rule
S: ('c' <* ','> 'd')*
has only two items in its right hand side: the (separated by commas) list of cs and the list of ds. The production rule is equivalent to:
c
pl@nereida:~/LEyapp/examples$ eyapp -v CsBetweenCommansAndD.eyp pl@nereida:~/LEyapp/examples$ head -11 CsBetweenCommansAndD.output | cat -n 1 Rules: 2 ------ 3 0: $start -> S $end 4 1: STAR-1 -> STAR-1 ',' 'c' 5 2: STAR-1 -> 'c' 6 3: STAR-2 -> STAR-1 7 4: STAR-2 -> /* empty */ 8 5: PAREN-3 -> STAR-2 'd' 9 6: STAR-4 -> STAR-4 PAREN-3 10 7: STAR-4 -> /* empty */ 11 8: S -> STAR-4
The semantic action associated with * is to return a reference to a list with the attributes of the matching items.
When working -as in the example - under a tree creation directive it returns a node belonging to a class named _STAR_LIST_#number whose children are the items in the list. The #number is the ordinal number of the production rule as it appears in the .output file. The attributes must be references or associated with semantic tokens to be included in the list. Notice -in the execution of the former example that follows - how the node for PAREN-3 has been eliminated from the tree. Parenthesis nodes are - generally - obviated:
_STAR_LIST_#number
#number
PAREN-3
pl@nereida:~/LEyapp/examples$ use_csbetweencommansandd.pl c,c,cd Node _STAR_LIST_4(_STAR_LIST_1(TERMINAL[c],TERMINAL[c],TERMINAL[c]),TERMINAL[d]) Child 0 _STAR_LIST_1(TERMINAL[c],TERMINAL[c],TERMINAL[c]) Child 1 TERMINAL[d]
Notice that the comma (since it is a syntactic token) has also been suppressed.
To set the name of the node associated with a list operator the %name directive must precede the operator as in the following example:
%name
pl@nereida:~/LEyapp/examples/eyapplanguageref$ sed -ne '1,27p' CsBetweenCommansAndDWithNames.eyp | cat -n 1 # CsBetweenCommansAndDWithNames.eyp 2 3 %semantic token 'c' 'd' 4 5 %{ 6 sub TERMINAL::info { 7 $_[0]->attr; 8 } 9 %} 10 %tree 11 %% 12 Start: S 13 ; 14 S: 15 ('c' <%name Cs * ','> 'd') %name Cs_and_d * 16 { 17 print "\nNode\n"; 18 print $_[1]->str."\n"; 19 print "\nChild 0\n"; 20 print $_[1]->child(0)->str."\n"; 21 print "\nChild 1\n"; 22 print $_[1]->child(1)->str."\n"; 23 $_[1] 24 } 25 ; 26 27 %%
The grammar describes the language of sequences
c,...,cd c,...,cd c,...,cd ....
The right hand side of the production has only one term which is a list, but the factor to which the star applies is itself a list. We are naming the term with the name Cs_and_d and the factor with the name Cs.
Cs_and_d
Cs
The execution shows the renamed nodes:
pl@nereida:~/LEyapp/examples/eyapplanguageref$ use_csbetweencommansanddwithnames.pl c,c,c,cd Node Cs_and_d(Cs(TERMINAL[c],TERMINAL[c],TERMINAL[c],TERMINAL[c]),TERMINAL[d]) Child 0 Cs(TERMINAL[c],TERMINAL[c],TERMINAL[c],TERMINAL[c]) Child 1 TERMINAL[d]
The X? operator stands for the presence or omission of X.
X?
pl@nereida:~/LEyapp/examples$ head -11 List5.yp | cat -n 1 %semantic token 'c' 2 %tree 3 %% 4 S: 'c' 'c'? 5 { 6 print $_[2]->str."\n"; 7 print $_[2]->child(0)->attr."\n" if $_[2]->children; 8 } 9 ; 10 11 %%
pl@nereida:~/LEyapp/examples$ eyapp -v List5 pl@nereida:~/LEyapp/examples$ head -7 List5.output Rules: ------ 0: $start -> S $end 1: OPTIONAL-1 -> 'c' 2: OPTIONAL-1 -> /* empty */ 3: S -> 'c' OPTIONAL-1
When yybuildingtree is false the associated attribute is a list that will be empty if CX> does not show up.
Under the %tree directive the action creates an _OPTIONAL node:
_OPTIONAL
pl@nereida:~/LEyapp/examples$ use_list5.pl cc _OPTIONAL_1(TERMINAL) c pl@nereida:~/LEyapp/examples$ use_list5.pl c _OPTIONAL_1
Any substring on the right hand side of a production rule can be grouped using a parenthesis. The introduction of a parenthesis implies the introduction of an additional syntactic variable whose only production is the sequence of symbols between the parenthesis. Thus the grammar:
pl@nereida:~/LEyapp/examples$ head -6 Parenthesis.eyp | cat -n 1 %% 2 S: 3 ('a' S ) 'b' { shift; [ @_ ] } 4 | 'c' 5 ; 6 %%
pl@nereida:~/LEyapp/examples$ eyapp -v Parenthesis.eyp; head -6 Parenthesis.output Rules: ------ 0: $start -> S $end 1: PAREN-1 -> 'a' S 2: S -> PAREN-1 'b' 3: S -> 'c'
By default the semantic rule associated with a parenthesis returns an anonymous list with the attributes of the symbols between the parenthesis:
pl@nereida:~/LEyapp/examples$ cat -n use_parenthesis.pl 1 #!/usr/bin/perl -w 2 use Parenthesis; 3 use Data::Dumper; 4 5 $Data::Dumper::Indent = 1; 6 $parser = Parenthesis->new(); 7 print Dumper($parser->Run); pl@nereida:~/LEyapp/examples$ use_parenthesis.pl acb $VAR1 = [ [ 'a', 'c' ], 'b' ]; pl@nereida:~/LEyapp/examples$ use_parenthesis.pl aacbb $VAR1 = [ [ 'a', [ [ 'a', 'c' ], 'b' ] ], 'b' ];
when working under a tree directive or when the attribute buildingtree is set via the YYBuildingtree method the semantic action returns a node with children the attributes of the symbols between parenthesis. As usual attributes which aren't references will be skipped from the list of children. See an example:
buildingtree
YYBuildingtree
pl@nereida:~/LEyapp/examples$ head -23 List2.yp | cat -n 1 %{ 2 use Data::Dumper; 3 %} 4 %semantic token 'a' 'b' 'c' 5 %tree 6 %% 7 S: 8 (%name AS 'a' S )'b' 9 { 10 print "S -> ('a' S )'b'\n"; 11 print "Attribute of the first symbol:\n".Dumper($_[1]); 12 print "Attribute of the second symbol: $_[2]\n"; 13 $_[0]->YYBuildAST(@_[1..$#_]); 14 } 15 | 'c' 16 { 17 print "S -> 'c'\n"; 18 my $r = Parse::Eyapp::Node->new(qw(C(TERMINAL)), sub { $_[1]->attr('c') }) ; 19 print Dumper($r); 20 $r; 21 } 22 ; 23 %%
The example shows (line 8) how to rename a _PAREN node. The %name CLASSNAME goes after the opening parenthesis.
_PAREN
%name CLASSNAME
The call to YYBuildAST at line 13 with argumetns the attributes of the symbols on the right hand side returns the node describing the current production rule. Notice that line 13 can be rewritten as:
goto &Parse::Eyapp::Driver::YYBuildAST;
At line 18 the node for the rule is explicitly created using Parse::Eyapp::Node->new. The handler passed as second argument is responsible for setting the value of the atribute attr of the just created TERMINAL node.
attr
TERMINAL
Let us see an execution:
pl@nereida:~/LEyapp/examples$ use_list2.pl aacbb S -> 'c' $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c' }, 'TERMINAL' ) ] }, 'C' );
the first reduction occurs by the non recursive rule. The execution shows the tree built by the call to Parse::Eyapp::Node-new> at line 18.
Parse::Eyapp::Node-
The execution continues with the reduction or reverse derivation by the rule S -> ('a' S )'b'. The action at lines 9-14 dumps the attribute associated with ('a' S) - or, in other words, the attribute associated with the variable PAREN-1. It also dumps the attribute of 'b':
S -> ('a' S )'b'
('a' S)
PAREN-1
'b'
S -> ('a' S )'b' Attribute of the first symbol: $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'a', 'token' => 'a' }, 'TERMINAL' ), bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c' }, 'TERMINAL' ) ] }, 'C' ) ] }, 'AS' ); Attribute of the second symbol: b
The last reduction shown is by the rule: S -> ('a' S )'b':
S -> ('a' S )'b' Attribute of the first symbol: $VAR1 = bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'a', 'token' => 'a' }, 'TERMINAL' ), bless( { 'children' => [ bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'a', 'token' => 'a' }, 'TERMINAL' ), bless( { 'children' => [ bless( { 'children' => [], 'attr' => 'c' }, 'TERMINAL' ) ] }, 'C' ) ] }, 'AS' ), bless( { 'children' => [], 'attr' => 'b', 'token' => 'b' }, 'TERMINAL' ) ] }, 'S_2' ) ] }, 'AS' ); Attribute of the second symbol: b
Though is a practice to avoid, since it clutters the code, it is certainly permitted to introduce actions between the parenthesis, as in the example below:
pl@nereida:~/LEyapp/examples$ head -16 ListAndAction.eyp | cat -n 1 # ListAndAction.eyp 2 %{ 3 my $num = 0; 4 %} 5 6 %% 7 S: 'c' 8 { 9 print "S -> c\n" 10 } 11 | ('a' {$num++; print "Seen <$num> 'a's\n"; $_[1] }) S 'b' 12 { 13 print "S -> (a ) S b\n" 14 } 15 ; 16 %%
This is the output when executing this program with input aaacbbb:
aaacbbb
pl@nereida:~/LEyapp/examples$ use_listandaction.pl aaacbbb Seen <1> 'a's Seen <2> 'a's Seen <3> 'a's S -> c S -> (a ) S b S -> (a ) S b S -> (a ) S b
Attributes can be referenced by meaningful names using the dot notation instead of using the classic error-prone positional approach:
rhs: rhseltwithid * rhseltwithid : rhselt '.' IDENT | '$' rhselt | rhselt
for example:
exp : exp.left '-' exp.right { $left - $right }
By qualifying the first appearance of the syntactic variable exp with the notation exp.left we can later refer inside the actions to the associated attribute using the lexical variable $left.
exp.left
$left
The dollar notation $A can be used as an abbreviation of A.A.
$A
A.A
When no action is specified both yapp and eyapp implicitly insert the semantic action { $_[1] }. In Parse::Eyapp you can modify such behavior using the %defaultaction { Perl code } directive. The { Perl code } clause that follows the %defaultaction directive is executed when reducing by any production for which no explicit action was specified.
{ Perl code }
See an example that translates an infix expression like a=b*-3 into a postfix expression like a b 3 NEG * = :
a=b*-3
a b 3 NEG * =
# File Postfix.eyp (See the examples/ directory) %right '=' %left '-' '+' %left '*' '/' %left NEG %defaultaction { return "$left $right $op"; } %% line: $exp { print "$exp\n" } ; exp: $NUM { $NUM } | $VAR { $VAR } | VAR.left '='.op exp.right | exp.left '+'.op exp.right | exp.left '-'.op exp.right | exp.left '*'.op exp.right | exp.left '/'.op exp.right | '-' $exp %prec NEG { "$exp NEG" } | '(' $exp ')' { $exp } ; %% # Support subroutines as in the Synopsis example ...
The file containing the Eyapp program must be compiled with eyapp:
nereida:~/src/perl/YappWithDefaultAction/examples> eyapp Postfix.eyp
Next, you have to write a client program:
nereida:~/src/perl/YappWithDefaultAction/examples> cat -n usepostfix.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Postfix; 4 5 my $parser = new Postfix(); 6 $parser->Run;
Now we can run the client program:
nereida:~/src/perl/YappWithDefaultAction/examples> usepostfix.pl Write an expression: -(2*a-b*-3) 2 a * b 3 NEG * - NEG
YYName
In eyapp each production rule has a name. The name of a rule can be explicitly given by the programmer using the %name directive. For example, in the piece of code that follows the name ASSIGN is given to the rule exp: VAR '=' exp.
ASSIGN
exp: VAR '=' exp
When no explicit name is given the rule has an implicit name. The implicit name of a rule is shaped by concatenating the name of the syntactic variable on its left, an underscore and the ordinal number of the production rule Lhs_# as it appears in the .output file. Avoid giving names matching such pattern to production rules. The patterns /${lhs}_\d+$/ where ${lhs} is the name of the syntactic variable are reserved for internal use by eyapp.
Lhs_#
/${lhs}_\d+$/
${lhs}
pl@nereida:~/LEyapp/examples$ cat -n Lhs.eyp 1 # Lhs.eyp 2 3 %right '=' 4 %left '-' '+' 5 %left '*' '/' 6 %left NEG 7 8 %defaultaction { 9 my $self = shift; 10 my $name = $self->YYName(); 11 bless { children => [ grep {ref($_)} @_] }, $name; 12 } 13 14 %% 15 input: 16 /* empty */ 17 { [] } 18 | input line 19 { 20 push @{$_[1]}, $_[2] if defined($_[2]); 21 $_[1] 22 } 23 ; 24 25 line: '\n' { } 26 | exp '\n' { $_[1] } 27 ; 28 29 exp: 30 NUM { $_[1] } 31 | VAR { $_[1] } 32 | %name ASSIGN 33 VAR '=' exp 34 | %name PLUS 35 exp '+' exp 36 | %name MINUS 37 exp '-' exp 38 | %name TIMES 39 exp '*' exp 40 | %name DIV 41 exp '/' exp 42 | %name UMINUS 43 '-' exp %prec NEG 44 | '(' exp ')' { $_[2] } 45 ;
Inside a semantic action the name of the current rule can be recovered using the method YYName of the parser object.
The default action (lines 8-12) computes as attribute of the left hand side a reference to an object blessed in the name of the rule. The object has an attribute children which is a reference to the list of children of the node. The call to grep
children
grep
11 bless { children => [ grep {ref($_)} @_] }, $name;
excludes children that aren't references. Notice that the lexical analyzer only returns references for the NUM and VAR terminals:
NUM
VAR
59 sub _Lexer { 60 my($parser)=shift; 61 62 for ($parser->YYData->{INPUT}) { 63 s/^[ \t]+//; 64 return('',undef) unless $_; 65 s/^([0-9]+(?:\.[0-9]+)?)// 66 and return('NUM', bless { attr => $1}, 'NUM'); 67 s/^([A-Za-z][A-Za-z0-9_]*)// 68 and return('VAR',bless {attr => $1}, 'VAR'); 69 s/^(.)//s 70 and return($1, $1); 71 } 72 return('',undef); 73 }
follows the client program:
pl@nereida:~/LEyapp/examples$ cat -n uselhs.pl 1 #!/usr/bin/perl -w 2 use Lhs; 3 use Data::Dumper; 4 5 $parser = new Lhs(); 6 my $tree = $parser->Run; 7 $Data::Dumper::Indent = 1; 8 if (defined($tree)) { print Dumper($tree); } 9 else { print "Cadena no válida\n"; }
When executed with input a=(2+3)*b the parser produces the following tree:
a=(2+3)*b
ASSIGN(TIMES(PLUS(NUM[2],NUM[3]), VAR[b]))
See the result of an execution:
pl@nereida:~/LEyapp/examples$ uselhs.pl a=(2+3)*b $VAR1 = [ bless( { 'children' => [ bless( { 'attr' => 'a' }, 'VAR' ), bless( { 'children' => [ bless( { 'children' => [ bless( { 'attr' => '2' }, 'NUM' ), bless( { 'attr' => '3' }, 'NUM' ) ] }, 'PLUS' ), bless( { 'attr' => 'b' }, 'VAR' ) ] }, 'TIMES' ) ] }, 'ASSIGN' ) ];
The name of a production rule can be changed at execution time. See the following example:
$ sed -n '29,50p' YYNameDynamic.eyp | cat -n 1 exp: 2 NUM { $_[1] } 3 | VAR { $_[1] } 4 | %name ASSIGN 5 VAR '=' exp 6 | %name PLUS 7 exp '+' exp 8 | %name MINUS 9 exp '-' exp 10 { 11 my $self = shift; 12 $self->YYName('SUBTRACT'); # rename it 13 $self->YYBuildAST(@_); # build the node 14 } 15 | %name TIMES 16 exp '*' exp 17 | %name DIV 18 exp '/' exp 19 | %name UMINUS 20 '-' exp %prec NEG 21 | '(' exp ')' { $_[2] } 22 ;
When the client program is executed we can see the presence of the SUBTRACT nodes:
SUBTRACT
pl@nereida:~/LEyapp/examples$ useyynamedynamic.pl 2-b $VAR1 = [ bless( { 'children' => [ bless( { 'attr' => '2' }, 'NUM' ), bless( { 'attr' => 'b' }, 'VAR' ) ] }, 'SUBTRACT' ) ];
An method to reuse a grammar is via inheritance. The client inherits the generated parser module and expands it with methods that inherit or overwrite the actions. Here is an example. Initially we have this Eyapp grammar:
pl@europa:~/LEyapp/examples/recycle$ cat -n NoacInh.eyp 1 %left '+' 2 %left '*' 3 4 %defaultaction { 5 my $self = shift; 6 7 my $action = $self->YYName; 8 9 $self->$action(@_); 10 } 11 12 %% 13 exp: %name NUM 14 NUM 15 | %name PLUS 16 exp '+' exp 17 | %name TIMES 18 exp '*' exp 19 | '(' exp ')' 20 { $_[2] } 21 ; 22 23 %% 24 25 sub _Error { 26 my($token)=$_[0]->YYCurval; 27 my($what)= $token ? "input: '$token'" : "end of input"; 28 my @expected = $_[0]->YYExpect(); 29 30 local $" = ', '; 31 die "Syntax error near $what. Expected one of these tokens: @expected\n"; 32 } 33 34 35 my $x = ''; 36 37 sub _Lexer { 38 my($parser)=shift; 39 40 for ($x) { 41 s/^\s+//; 42 $_ eq '' and return('',undef); 43 44 s/^([0-9]+(?:\.[0-9]+)?)// and return('NUM',$1); 45 s/^([A-Za-z][A-Za-z0-9_]*)// and return('VAR',$1); 46 s/^(.)//s and return($1,$1); 47 } 48 } 49 50 sub Run { 51 my($self)=shift; 52 $x = shift; 53 my $debug = shift; 54 55 $self->YYParse( 56 yylex => \&_Lexer, 57 yyerror => \&_Error, 58 yydebug => $debug, 59 ); 60 }
The following program defines two classes: CalcActions that implements the actions for the calculator and package PostActions that implements the actions for the infix to postfix translation. This way we have an example that reuses the former grammar twice:
CalcActions
PostActions
pl@europa:~/LEyapp/examples/recycle$ cat -n icalcu_and_ipost.pl 1 #!/usr/bin/perl -w 2 package CalcActions; 3 use strict; 4 use base qw{NoacInh}; 5 6 sub NUM { 7 return $_[1]; 8 } 9 10 sub PLUS { 11 $_[1]+$_[3]; 12 } 13 14 sub TIMES { 15 $_[1]*$_[3]; 16 } 17 18 package PostActions; 19 use strict; 20 use base qw{NoacInh}; 21 22 sub NUM { 23 return $_[1]; 24 } 25 26 sub PLUS { 27 "$_[1] $_[3] +"; 28 } 29 30 sub TIMES { 31 "$_[1] $_[3] *"; 32 } 33 34 package main; 35 use strict; 36 37 my $calcparser = CalcActions->new(); 38 print "Write an expression: "; 39 my $x = <STDIN>; 40 my $e = $calcparser->Run($x); 41 42 print "$e\n"; 43 44 my $postparser = PostActions->new(); 45 my $p = $postparser->Run($x); 46 47 print "$p\n";
The subroutine used as default action in NoacInh.eyp is so useful that is packed as the Parse::Eyapp::Driver method YYDelegateaction.
NoacInh.eyp
YYDelegateaction
See files examples/recycle/NoacYYDelegateaction.eyp and examples/recycle/icalcu_and_ipost_yydel.pl for an example of use of YYDelegateaction.
examples/recycle/NoacYYDelegateaction.eyp
examples/recycle/icalcu_and_ipost_yydel.pl
The methods YYSetaction and YYAction of the parser object provide a way to selectively substitute some actions of a given grammar. Let us consider once more a postfix to infix translator:
YYSetaction
YYAction
pl@europa:~/LEyapp/examples/recycle$ cat -n PostfixWithActions.eyp 1 # File PostfixWithActions.eyp 2 %right '=' 3 %left '-' '+' 4 %left '*' '/' 5 %left NEG 6 7 %% 8 line: $exp { print "$exp\n" } 9 ; 10 11 exp: $NUM 12 { $NUM } 13 | $VAR 14 { $VAR } 15 | %name ASSIGN 16 VAR.left '='exp.right 17 { "$_[3] &$_[1] ASSIGN"; } 18 | %name PLUS 19 exp.left '+'exp.right 20 { "$_[1] $_[3] PLUS"; } 21 | %name MINUS 22 exp.left '-'exp.right 23 { "$_[1] $_[3] MINUS"; } 24 | %name TIMES 25 exp.left '*'exp.right 26 { "$_[1] $_[3] TIMES"; } 27 | %name DIV 28 exp.left '/'exp.right 29 { "$_[1] $_[3] DIV"; } 30 | %name NEG '-' $exp %prec NEG 31 { "$exp NEG" } 32 | '(' $exp ')' 33 { $exp } 34 ; 35 36 %% 37 38 sub _Error { 39 my($token)=$_[0]->YYCurval; 40 my($what)= $token ? "input: '$token'" : "end of input"; 41 my @expected = $_[0]->YYExpect(); 42 43 local $" = ', '; 44 die "Syntax error near $what. Expected one of these tokens: @expected\n"; 45 } 46 47 my $x; 48 49 sub _Lexer { 50 my($parser)=shift; 51 52 for ($x) { 53 s/^\s+//; 54 $_ eq '' and return('',undef); 55 56 s/^([0-9]+(?:\.[0-9]+)?)// and return('NUM',$1); 57 s/^([A-Za-z][A-Za-z0-9_]*)// and return('VAR',$1); 58 s/^(.)//s and return($1,$1); 59 } 60 } 61 62 sub Run { 63 my($self)=shift; 64 $x = shift; 65 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, 66 #yydebug => 0xFF 67 ); 68 }
The program rewritepostfixwithactions.pl uses the former grammar to translate infix expressions to postfix expressions. It also implements a calculator reusing the grammar in PostfixWithActions.eyp. It does so using the YYSetaction method. The semantic actions for the productions named
rewritepostfixwithactions.pl
PostfixWithActions.eyp
TIMES
DIV
NEG
are selectively substituted by the appropriate actions, while the other semantic actions remain unchanged:
pl@europa:~/LEyapp/examples/recycle$ cat -n rewritepostfixwithactions.pl 1 #!/usr/bin/perl 2 use warnings; 3 use PostfixWithActions; 4 5 my $debug = shift || 0; 6 my $pparser = PostfixWithActions->new(); 7 print "Write an expression: "; 8 my $x = <STDIN>; 9 10 # First, trasnlate to postfix ... 11 $pparser->Run($x, $debug); 12 13 # And then selectively substitute 14 # some semantic actions 15 # to obtain an infix calculator ... 16 my %s; # symbol table 17 $pparser->YYSetaction( 18 ASSIGN => sub { $s{$_[1]} = $_[3] }, 19 PLUS => sub { $_[1] + $_[3] }, 20 TIMES => sub { $_[1] * $_[3] }, 21 DIV => sub { $_[1] / $_[3] }, 22 NEG => sub { -$_[2] }, 23 ); 24 25 $pparser->Run($x, $debug);
When running this program the output is:
examples/recycle$ ./rewritepostfixwithactions.pl Write an expression: 2*3+4 2 3 TIMES 4 PLUS 10 examples/recycle$ rewritepostfixwithactions.pl Write an expression: a = 2*(b = 3+5) 2 3 5 PLUS &b ASSIGN TIMES &a ASSIGN 16
Parse::Eyapp facilitates the construction of concrete syntax trees and abstract syntax trees (abbreviated AST from now on) through the %tree directive. Actually, the %tree directive is equivalent to a call to the YYBuildAST method of the parser object.
Any production production rule A->XYZ can be named using a directive %name someclass.
A->XYZ
%name someclass
When reducing by a production rule A->XYZ the %tree directive (i.e., the YYBuildAST method) builds an anonymous hash blessed in someclass. The hash has an attribute children containing the references to the AST trees associated with the symbols in the right hand side X, C>Y>, etc.
someclass
If no explicit name was given to the production rule, YYBuildAST blesses the node in the class name resulting from the concatenation of the left hand side and the production number. The production number is the ordinal number of the production as they appear in the associated .output file (see option -v of eyapp). For example, given the grammar:
pl@europa:~/LEyapp/examples/eyapplanguageref$ sed -ne '8,27p' treewithoutnames.pl my $grammar = q{ %right '=' # Lowest precedence %left '-' '+' # + and - have more precedence than = Disambiguate a-b-c as (a-b)-c %left '*' '/' # * and / have more precedence than + Disambiguate a/b/c as (a/b)/c %left NEG # Disambiguate -a-b as (-a)-b and not as -(a-b) %tree # Let us build an abstract syntax tree ... %% line: exp <+ ';'> { $_[1] } /* list of expressions separated by ';' */ ; exp: NUM | VAR | VAR '=' exp | exp '+' exp | exp '-' exp | exp '*' exp | exp '/' exp | '-' exp %prec NEG | '(' exp ')' { $_[2] } ; %%
The tree produced by the parser when feed with input a=2*b is:
a=2*b
pl@europa:~/LEyapp/examples/eyapplanguageref$ ./treewithoutnames.pl ************ _PLUS_LIST(exp_6(TERMINAL[a],exp_9(exp_4(TERMINAL[2]),exp_5(TERMINAL[b])))) ************
If we want to see the correspondence between names and rules we can generate and check the corresponding file .output setting the outputfile of Parse::Eyapp:
outputfile
Parse::Eyapp->new_grammar( # Create the parser package/class input=>$grammar, classname=>'Calc', # The name of the package containing the parser firstline=>9, # String $grammar starts at line 9 (for error diagnostics) outputfile=>'treewithoutnames' );
The grammar with the expanded rules appears in the .output file:
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/eyapplanguageref$ sed -ne '28,42p' treewithoutnames.output Rules: ------ 0: $start -> line $end 1: PLUS-1 -> PLUS-1 ';' exp 2: PLUS-1 -> exp 3: line -> PLUS-1 4: exp -> NUM 5: exp -> VAR 6: exp -> VAR '=' exp 7: exp -> exp '+' exp 8: exp -> exp '-' exp 9: exp -> exp '*' exp 10: exp -> exp '/' exp 11: exp -> '-' exp 12: exp -> '(' exp ')'
We can see now that the node exp_9 corresponds to the production exp -> exp '*' exp. Observe also that the Eyapp production:
exp_9
exp -> exp '*' exp
line: exp <+ ';'> actually produces the productions: 1: PLUS-1 -> PLUS-1 ';' exp 2: PLUS-1 -> exp
and that the name of the class associated with the non empty list is _PLUS_LIST.
A production rule can be named using the %name IDENTIFIER directive. For each production rule a namespace/package is created. The IDENTIFIER is the name of the associated package. Therefore, by modifying the former grammar with additional %name directives:
%name IDENTIFIER
IDENTIFIER
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/eyapplanguageref$ sed -ne '8,26p' treewithnames.pl my $grammar = q{ %right '=' # Lowest precedence %left '-' '+' # + and - have more precedence than = Disambiguate a-b-c as (a-b)-c %left '*' '/' # * and / have more precedence than + Disambiguate a/b/c as (a/b)/c %left NEG # Disambiguate -a-b as (-a)-b and not as -(a-b) %tree # Let us build an abstract syntax tree ... %% line: exp <%name EXPS + ';'> { $_[1] } /* list of expressions separated by ';' */ ; exp: %name NUM NUM | %name VAR VAR | %name ASSIGN VAR '=' exp | %name PLUS exp '+' exp | %name MINUS exp '-' exp | %name TIMES exp '*' exp | %name DIV exp '/' exp | %name UMINUS '-' exp %prec NEG | '(' exp ')' { $_[2] } ;
we are explicitly naming the productions. Thus, all the node instances corresponding to the production exp: VAR '=' exp will belong to the class ASSIGN. Now the tree for a=2*b becomes:
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/eyapplanguageref$ ./treewithnames.pl ************ EXPS(ASSIGN(TERMINAL[a],TIMES(NUM(TERMINAL[2]),VAR(TERMINAL[b])))) ************
Observe how the list has been named EXPS. The %name directive prefixes the list operator ([+*?]).
EXPS
[+*?]
Nodes named TERMINAL are built from the tokens provided by the lexical analyzer. Parse::Eyapp follows the same protocol than Parse::Yapp for communication between the parser and the lexical analyzer: A couple ($token, $attribute) is returned by the lexical analyzer. These values are stored under the keys token and attr. TERMINAL nodes as all Parse::Eyapp::Node nodes also have the attribute children but is - almost always - empty.
($token, $attribute)
Explicit actions can be specified by the programmer like in this line from the Parse::Eyapp SYNOPSIS example:
| '(' exp ')' { $_[2] } /* Let us simplify a bit the tree */
Explicit actions receive as arguments the references to the children nodes already built. The programmer can influence the shape of the tree by inserting these explicit actions. In this example the programmer has decided to simplify the syntax tree: the nodes associated with the parenthesis are discarded and the reference to the subtree containing the proper expression is returned. Such manoeuvre is called bypassing. See section "The bypass clause and the %no bypass directive" to know more about automatic bypassing
Sometimes the best time to decorate a node with some attributes is just after being built. In such cases the programmer can take manual control building the node with YYBuildAST to inmediately proceed to decorate it.
The following example illustrates the situation (see file lib/Simple/Types.eyp inside examples/typechecking/Simple-Types-XXX.tar.gz):
lib/Simple/Types.eyp
examples/typechecking/Simple-Types-XXX.tar.gz
$ sed -n '397,408p' lib/Simple/Types.eyp Variable: %name VAR ID | %name VARARRAY $ID ('[' binary ']') <%name INDEXSPEC +> { my $self = shift; my $node = $self->YYBuildAST(@_); $node->{line} = $ID->[1];# $_[1]->[1] return $node; } ;
This production rule defines the expression to access an array element as an identifier followed by a non empty list of binary expressions Variable: ID ('[' binary ']')+. Furthermore, the node corresponding to the list of indices has been named INDEXSPEC.
Variable: ID ('[' binary ']')+
INDEXSPEC
When no explicit action is inserted a binary node will be built having as first child the node corresponding to the identifier $ID and as second child the reference to the list of binary expressions. The children corresponding to '[' and ']' are discarded since they are -by default- syntactic tokens (see section "Syntactic and Semantic tokens"). However, the programmer wants to decorate the node being built with a line attribute holding the line number in the source code where the identifier being used appears. The call to the Parse::Eyapp::Driver method YYBuildAST does the job of building the node. After that the node can be decorated and returned.
$ID
'['
']'
line
Parse::Eyapp::Driver
Actually, the %tree directive is semantically equivalent to:
%default action { goto &Parse::Eyapp::Driver::YYBuildAST }
When a explicit user action returns s.t. that is not a reference no node will be inserted. This fact can be used to suppress nodes in the AST being built. See the following example (file examples/returnnonode.yp):
examples/returnnonode.yp
$ sed -ne '1,17p' returnnonode.yp | cat -n 1 %tree 2 %semantic token 'a' 'b' 3 %% 4 S: %name EMPTY 5 /* empty */ 6 | %name AES 7 S A 8 | %name BES 9 S B 10 ; 11 A : %name A 12 'a' 13 ; 14 B : %name B 15 'b' { } 16 ; 17 %%
since the action at line 15 returns undef the B : 'b' subtree will not be inserted in the AST:
undef
B : 'b'
$ usereturnnonode.pl ababa AES(BES(AES(BES(AES(EMPTY,A(TERMINAL[a]))),A(TERMINAL[a]))),A(TERMINAL[a]))
Observe the absence of Bs and 'b's.
B
Intermediate actions can be used to change the shape of the AST (prune it, decorate it, etc.) but the value returned by them is ignored. The grammar below has two intermediate actions. They modify the attributes of the node to its left and return a reference $f to such node (lines 5 and 6):
$f
$ sed -ne '1,15p' intermediateactiontree.yp | cat -n 1 %semantic token 'a' 'b' 2 %tree bypass 3 %% 4 S: %name EMPTY 5 /* empty */ 6 | %name SA 7 S A.f { $f->{attr} = "A"; $f; } A 8 | %name SB 9 S B.f { $f->{attr} = "B"; $f; } B 10 ; 11 A : %name A 'a' 12 ; 13 B : %name B 'b' 14 ; 15 %%
See the client program:
nereida:~/src/perl/YappWithDefaultAction/examples> cat -n useintermediateactiontree.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Parse::Eyapp; 4 use intermediateactiontree; 5 6 { no warnings; 7 *A::info = *B::info = sub { $_[0]{attr} }; 8 } 9 10 my $parser = intermediateactiontree->new(); 11 my $t = $parser->Run; 12 print $t->str,"\n";
When it runs produces this output:
$ useintermediateactiontree.pl aabbaa SA(SB(SA(EMPTY,A[A],A[a]),B[B],B[b]),A[A],A[a])
The attributes of left As have been effectively changed by the intermediate actions from 'a' to 'A'. However no further children have been inserted.
A
'A'
Parse::Eyapp differences between syntactic tokens and semantic tokens. By default all tokens declared using string notation (i.e. between quotes like '+', '=') are considered syntactic tokens. Tokens declared by an identifier (like NUM or VAR) are by default considered semantic tokens. Syntactic tokens do not yield to nodes in the syntactic tree. Thus, the first print in the section Parse::Eyapp SYNOPSIS example:
syntactic tokens
semantic tokens
'='
$ cat -n synopsis.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Parse::Eyapp; 4 use Parse::Eyapp::Treeregexp; 5 6 sub TERMINAL::info { 7 $_[0]{attr} 8 } 9 10 my $grammar = q{ 11 %right '=' # Lowest precedence 12 %left '-' '+' # + and - have more precedence than = Disambiguate a-b-c as (a-b)-c 13 %left '*' '/' # * and / have more precedence than + Disambiguate a/b/c as (a/b)/c 14 %left NEG # Disambiguate -a-b as (-a)-b and not as -(a-b) 15 %tree # Let us build an abstract syntax tree ... 16 17 %% 18 line: 19 exp <%name EXPRESSION_LIST + ';'> 20 { $_[1] } /* list of expressions separated by ';' */ 21 ; 22 23 /* The %name directive defines the name of the class */ 24 exp: 25 %name NUM 26 NUM 27 | %name VAR 28 VAR 29 | %name ASSIGN 30 VAR '=' exp 31 | %name PLUS 32 exp '+' exp 33 | %name MINUS 34 exp '-' exp 35 | %name TIMES 36 exp '*' exp 37 | %name DIV 38 exp '/' exp 39 | %name UMINUS 40 '-' exp %prec NEG 41 | '(' exp ')' 42 { $_[2] } /* Let us simplify a bit the tree */ 43 ; 44 45 %% 46 sub _Error { die "Syntax error near ".($_[0]->YYCurval?$_[0]->YYCurval:"end of file")."\n" } 47 48 sub _Lexer { 49 my($parser)=shift; # The parser object 50 51 for ($parser->YYData->{INPUT}) { # Topicalize 52 m{\G\s+}gc; 53 $_ eq '' and return('',undef); 54 m{\G([0-9]+(?:\.[0-9]+)?)}gc and return('NUM',$1); 55 m{\G([A-Za-z][A-Za-z0-9_]*)}gc and return('VAR',$1); 56 m{\G(.)}gcs and return($1,$1); 57 } 58 return('',undef); 59 } 60 61 sub Run { 62 my($self)=shift; 63 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, ); 64 } 65 }; # end grammar 66 67 our (@all, $uminus); 68 69 Parse::Eyapp->new_grammar( # Create the parser package/class 70 input=>$grammar, 71 classname=>'Calc', # The name of the package containing the parser 72 firstline=>7 # String $grammar starts at line 7 (for error diagnostics) 73 ); 74 my $parser = Calc->new(); # Create a parser 75 $parser->YYData->{INPUT} = "2*-3+b*0;--2\n"; # Set the input 76 my $t = $parser->Run; # Parse it! 77 local $Parse::Eyapp::Node::INDENT=2; 78 print "Syntax Tree:",$t->str; 79 80 # Let us transform the tree. Define the tree-regular expressions .. 81 my $p = Parse::Eyapp::Treeregexp->new( STRING => q{ 82 { # Example of support code 83 my %Op = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/'); 84 } 85 constantfold: /TIMES|PLUS|DIV|MINUS/:bin(NUM($x), NUM($y)) 86 => { 87 my $op = $Op{ref($bin)}; 88 $x->{attr} = eval "$x->{attr} $op $y->{attr}"; 89 $_[0] = $NUM[0]; 90 } 91 uminus: UMINUS(NUM($x)) => { $x->{attr} = -$x->{attr}; $_[0] = $NUM } 92 zero_times_whatever: TIMES(NUM($x), .) and { $x->{attr} == 0 } => { $_[0] = $NUM } 93 whatever_times_zero: TIMES(., NUM($x)) and { $x->{attr} == 0 } => { $_[0] = $NUM } 94 }, 95 OUTPUTFILE=> 'main.pm' 96 ); 97 $p->generate(); # Create the tranformations 98 99 $t->s($uminus); # Transform UMINUS nodes 100 $t->s(@all); # constant folding and mult. by zero 101 102 local $Parse::Eyapp::Node::INDENT=0; 103 print "\nSyntax Tree after transformations:\n",$t->str,"\n";
gives as result the following output:
nereida:~/src/perl/YappWithDefaultAction/examples> synopsis.pl Syntax Tree: EXPRESSION_LIST( PLUS( TIMES( NUM( TERMINAL[2] ), UMINUS( NUM( TERMINAL[3] ) ) # UMINUS ) # TIMES, TIMES( VAR( TERMINAL[b] ), NUM( TERMINAL[0] ) ) # TIMES ) # PLUS, UMINUS( UMINUS( NUM( TERMINAL[2] ) ) # UMINUS ) # UMINUS ) # EXPRESSION_LIST
TERMINAL nodes corresponding to tokens that were defined by strings like '=', '-', '+', '/', '*', '(' and ')' do not appear in the tree. TERMINAL nodes corresponding to tokens that were defined using an identifier, like NUM or VAR are, by default, semantic tokens and appear in the AST.
'/'
'*'
'('
')'
The new token declaration directives %syntactic token and %semantic token can change the status of a token. For example (file 15treewithsyntactictoken.pl in the examples/ directory), given the grammar:
15treewithsyntactictoken.pl
examples/
%syntactic token b %semantic token 'a' 'c' %tree %% S: %name ABC A B C | %name BC B C ; A: %name A 'a' ; B: %name B b ; C: %name C 'c' ; %%
the tree build for input abc will be ABC(A(TERMINAL[a]),B,C(TERMINAL[c])).
abc
ABC(A(TERMINAL[a]),B,C(TERMINAL[c]))
The reason for the adjective %syntactic applied to a token is to state that the token influences the shape of the syntax tree but carries no other information. When the syntax tree is built the node corresponding to the token is discarded.
%syntactic
Sometimes the difference between syntactic and semantic tokens is blurred. For example the line number associated with an instance of the syntactic token '+' can be used later -say during type checking- to emit a more accurate error diagnostic. But if the node was discarded the information about that line number is no longer available. When building the syntax tree Parse::Eyapp (namely the method Parse::Eyapp::YYBuildAST) checks if the method TERMINAL::save_attributes exists and if so it will be called when dealing with a syntactic token. The method receives as argument - additionally to the reference to the attribute of the token as it is returned by the lexical analyzer - a reference to the node associated with the left hand side of the production. Here is an example (file lib/Simple/Types.eyp in examples/typechecking/Simple-Types-XXX.tar.gz) of use:
Parse::Eyapp::YYBuildAST
TERMINAL::save_attributes
sub TERMINAL::save_attributes { # $_[0] is a syntactic terminal # $_[1] is the father. push @{$_[1]->{lines}}, $_[0]->[1]; # save the line number }
bypass
%no bypass
The shape of the tree can be also modified using some %tree clauses as %tree bypass which will produce an automatic bypass of any node with only one child at tree-construction-time.
%tree bypass
A bypass operation consists in returning the only child of the node being visited to the father of the node and re-typing (re-blessing) the node in the name of the production (if a name was provided).
A node may have only one child at tree-construction-time for one of two reasons.
The first occurs when the right hand side of the production was already unary like in:
exp: %name NUM NUM
Here - if the bypass clause is used - the NUM node will be bypassed and the child TERMINAL built from the information provided by the lexical analyzer will be renamed/reblessed as NUM.
Another reason for a node to be bypassed is the fact that though the right hand side of the production may have more than one symbol, only one of them is not a syntactic token like in:
exp: '(' exp ')'
A consequence of the global scope application of %tree bypass is that undesired bypasses may occur like in
exp : %name UMINUS '-' $exp %prec NEG
though the right hand side has two symbols, token '-' is a syntactic token and therefore only exp is left. The bypass operation will be applied when building this node. This bypass can be avoided applying the no bypass ID directive to the corresponding production:
no bypass ID
exp : %no bypass UMINUS '-' $exp %prec NEG
The following example (file examples/bypass.pl) is the equivalent of the Parse::Eyapp SYNOPSIS example but using the bypass clause instead:
examples/bypass.pl
use Parse::Eyapp; use Parse::Eyapp::Treeregexp; sub TERMINAL::info { $_[0]{attr} } { no warnings; *VAR::info = *NUM::info = \&TERMINAL::info; } my $grammar = q{ %right '=' # Lowest precedence %left '-' '+' %left '*' '/' %left NEG # Disambiguate -a-b as (-a)-b and not as -(a-b) %tree bypass # Let us build an abstract syntax tree ... %% line: exp <%name EXPRESSION_LIST + ';'> { $_[1] } ; exp: %name NUM NUM | %name VAR VAR | %name ASSIGN VAR '=' exp | %name PLUS exp '+' exp | %name MINUS exp '-' exp | %name TIMES exp '*' exp | %name DIV exp '/' exp | %no bypass UMINUS '-' $exp %prec NEG | '(' exp ')' ; %% # sub _Error, _Lexer and Run like in the synopsis example # ... }; # end grammar our (@all, $uminus); Parse::Eyapp->new_grammar( # Create the parser package/class input=>$grammar, classname=>'Calc', # The name of the package containing the parser firstline=>7 # String $grammar starts at line 7 (for error diagnostics) ); my $parser = Calc->new(); # Create a parser $parser->YYData->{INPUT} = "a=2*-3+b*0\n"; # Set the input my $t = $parser->Run; # Parse it! print "\n************\n".$t->str."\n************\n"; # Let us transform the tree. Define the tree-regular expressions .. my $p = Parse::Eyapp::Treeregexp->new( STRING => q{ { # Example of support code my %Op = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/'); } constantfold: /TIMES|PLUS|DIV|MINUS/:bin(NUM, NUM) => { my $op = $Op{ref($_[0])}; $NUM[0]->{attr} = eval "$NUM[0]->{attr} $op $NUM[1]->{attr}"; $_[0] = $NUM[0]; } zero_times_whatever: TIMES(NUM, .) and { $NUM->{attr} == 0 } => { $_[0] = $NUM } whatever_times_zero: TIMES(., NUM) and { $NUM->{attr} == 0 } => { $_[0] = $NUM } uminus: UMINUS(NUM) => { $NUM->{attr} = -$NUM->{attr}; $_[0] = $NUM } }, OUTPUTFILE=> 'main.pm' ); $p->generate(); # Create the tranformations $t->s(@all); # constant folding and mult. by zero print $t->str,"\n";
when running this example with input "a=2*-3+b*0\n" we obtain the following output:
"a=2*-3+b*0\n"
nereida:~/src/perl/YappWithDefaultAction/examples> bypass.pl ************ EXPRESSION_LIST(ASSIGN(TERMINAL[a],PLUS(TIMES(NUM[2],UMINUS(NUM[3])),TIMES(VAR[b],NUM[0])))) ************ EXPRESSION_LIST(ASSIGN(TERMINAL[a],NUM[-6]))
As you can see the trees are more compact when using the bypass directive.
alias
Access to children in Parse::Eyapp is made through the child and children methods. There are occasions however where access by name to the children may be preferable. The use of the alias clause with the %tree directive creates accessors to the children with names specified by the programmer. The dot and dollar notations are used for this. When dealing with a production like:
child
A: %name A_Node Node B.bum N.pum $Chip
methods bum, pum and Chip will be created for the class A_Node. Those methods will provide access to the respective child (first, second and third in the example). The methods are build at compile-time and therefore later transformations of the AST modifying the order of the children may invalidate the use of these getter-setters.
bum
pum
Chip
A_Node
The %prefix directive used in line 7 of the following example is equivalent to the use of the yyprefix. The node classes are prefixed with the specified prefix: R::S:: in this example.
R::S::
cat -n alias_and_yyprefix.pl 1 #!/usr/local/bin/perl 2 use warnings; 3 use strict; 4 use Parse::Eyapp; 5 6 my $grammar = q{ 7 %prefix R::S:: 8 9 %right '=' 10 %left '-' '+' 11 %left '*' '/' 12 %left NEG 13 %tree bypass alias 14 15 %% 16 line: $exp { $_[1] } 17 ; 18 19 exp: 20 %name NUM 21 $NUM 22 | %name VAR 23 $VAR 24 | %name ASSIGN 25 $VAR '=' $exp 26 | %name PLUS 27 exp.left '+' exp.right 28 | %name MINUS 29 exp.left '-' exp.right 30 | %name TIMES 31 exp.left '*' exp.right 32 | %name DIV 33 exp.left '/' exp.right 34 | %no bypass UMINUS 35 '-' $exp %prec NEG 36 | '(' exp ')' { $_[2] } /* Let us simplify a bit the tree */ 37 ; 38 39 %% .. .... 76 }; # end grammar 77 78 79 Parse::Eyapp->new_grammar( 80 input=>$grammar, 81 classname=>'Alias', 82 firstline =>7, 83 outputfile => 'main', 84 ); 85 my $parser = Alias->new(); 86 $parser->YYData->{INPUT} = "a = -(2*3+5-1)\n"; 87 my $t = $parser->Run; 88 $Parse::Eyapp::Node::INDENT=0; 89 print $t->VAR->str."\n"; # a 90 print "***************\n"; 91 print $t->exp->exp->left->str."\n"; # 2*3+5 92 print "***************\n"; 93 print $t->exp->exp->right->str."\n"; # 1
The tree $t for the expression "a = -(2*3+5-1)\n" is:
$t
"a = -(2*3+5-1)\n"
R::S::ASSIGN( R::S::TERMINAL, R::S::UMINUS( R::S::MINUS( R::S::PLUS(R::S::TIMES(R::S::NUM,R::S::NUM),R::S::NUM), R::S::NUM ) ) )
The R::S::ASSIGN class has methods VAR (see line 89 above) and exp (see lines 91 and 93) to refer to its two children. The result of the execution is:
R::S::ASSIGN
$ alias_and_yyprefix.pl R::S::TERMINAL *************** R::S::PLUS(R::S::TIMES(R::S::NUM,R::S::NUM),R::S::NUM) *************** R::S::NUM
As a second example of the use of %alias, the CPAN module Language::AttributeGrammar provides AST decorators from an attribute grammar specification of the AST. To work Language::AttributeGrammar requires named access to the children of the AST nodes. Follows an example (file examples/CalcwithAttributeGrammar.pl) of a small calculator:
%alias
examples/CalcwithAttributeGrammar.pl
pl@nereida:~/LEyapp/examples$ cat -n CalcwithAttributeGrammar.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Parse::Eyapp; 4 use Data::Dumper; 5 use Language::AttributeGrammar; 6 7 my $grammar = q{ 8 %{ 9 # use Data::Dumper; 10 %} 11 %right '=' 12 %left '-' '+' 13 %left '*' '/' 14 %left NEG 15 %tree bypass alias 16 17 %% 18 line: $exp { $_[1] } 19 ; 20 21 exp: 22 %name NUM 23 $NUM 24 | %name VAR 25 $VAR 26 | %name ASSIGN 27 $VAR '=' $exp 28 | %name PLUS 29 exp.left '+' exp.right 30 | %name MINUS 31 exp.left '-' exp.right 32 | %name TIMES 33 exp.left '*' exp.right 34 | %name DIV 35 exp.left '/' exp.right 36 | %no bypass UMINUS 37 '-' $exp %prec NEG 38 | '(' $exp ')' { $_[2] } /* Let us simplify a bit the tree */ 39 ; 40 41 %% 42 43 sub _Error { 44 exists $_[0]->YYData->{ERRMSG} 45 and do { 46 print $_[0]->YYData->{ERRMSG}; 47 delete $_[0]->YYData->{ERRMSG}; 48 return; 49 }; 50 print "Syntax error.\n"; 51 } 52 53 sub _Lexer { 54 my($parser)=shift; 55 56 $parser->YYData->{INPUT} 57 or $parser->YYData->{INPUT} = <STDIN> 58 or return('',undef); 59 60 $parser->YYData->{INPUT}=~s/^\s+//; 61 62 for ($parser->YYData->{INPUT}) { 63 s/^([0-9]+(?:\.[0-9]+)?)// 64 and return('NUM',$1); 65 s/^([A-Za-z][A-Za-z0-9_]*)// 66 and return('VAR',$1); 67 s/^(.)//s 68 and return($1,$1); 69 } 70 } 71 72 sub Run { 73 my($self)=shift; 74 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, 75 #yydebug =>0xFF 76 ); 77 } 78 }; # end grammar 79 80 81 $Data::Dumper::Indent = 1; 82 Parse::Eyapp->new_grammar( 83 input=>$grammar, 84 classname=>'Rule6', 85 firstline =>7, 86 outputfile => 'Calc.pm', 87 ); 88 my $parser = Rule6->new(); 89 $parser->YYData->{INPUT} = "a = -(2*3+5-1)\n"; 90 my $t = $parser->Run; 91 print "\n***** Before ******\n"; 92 print Dumper($t); 93 94 my $attgram = new Language::AttributeGrammar <<'EOG'; 95 96 # Compute the expression 97 NUM: $/.val = { $<attr> } 98 TIMES: $/.val = { $<left>.val * $<right>.val } 99 PLUS: $/.val = { $<left>.val + $<right>.val } 100 MINUS: $/.val = { $<left>.val - $<right>.val } 101 UMINUS: $/.val = { -$<exp>.val } 102 ASSIGN: $/.val = { $<exp>.val } 103 EOG 104 105 my $res = $attgram->apply($t, 'val'); 106 107 $Data::Dumper::Indent = 1; 108 print "\n***** After ******\n"; 109 print Dumper($t); 110 print Dumper($res); CalcwithAttributeGrammar.pl
The program computes the tree for expression for expression a = -(2*3+5-1) which is:
a = -(2*3+5-1)
ASSIGN(TERMINAL,UMINUS(MINUS(PLUS(TIMES(NUM,NUM),NUM),NUM)))
The children of the binary nodes can be accessed through the left and right methods.
left
right
There is no encapsulation of nodes. The user/client knows that they are hashes that can be decorated with new keys/attributes. All nodes in the AST created by %tree are Parse::Eyapp::Node nodes. The only reserved field is children which is a reference to the array of children. You can always create a Node class by hand by inheriting from Parse::Eyapp::Node.
Node
Yacc-like parser generators provide ways to solve shift-reduce mechanims based on token precedence. No mechanisms are provided for the resolution of reduce-reduce conflicts. The solution for such kind of conflicts is to modify the grammar. The strategy I present here provides a way to broach conflicts that can't be solved using static precedences.
The C++ syntax does not disambiguate between expression statements and declaration statements. The ambiguity arises when an expression statement has a function-style cast as its left-most subexpression. (Since C does not support function-style casts, this ambiguity does not occur in C programs.)
For example,
int (x) = y+z;
parses as either an expr or a stmt.
expr
stmt
If the statement can be interpreted both as a declaration and as an expression, the statement is interpreted as a declaration statement.
The following expressions disambiguate into expression statements because the declarator is followed by an operator different from the assignment operator.
type_spec(i)++; // expression statement type_spec(i,3)<<d; // expression statement type_spec(i)->l=24; // expression statement
Where type_spec stands for a type specifier.
type_spec
In the following examples, the interpretation as declaration works, and consequently the statements are interpreted as declarations:
type_spec(*i)(int); // declaration type_spec(j)[5]; // declaration type_spec(m) = { 1, 2 }; // declaration type_spec(a); // declaration type_spec(*b)(); // declaration type_spec(c)=23; // declaration type_spec(d),e,f,g=0; // declaration type_spec(h)(e,3); // declaration
The simple grammar in examples/debuggingtut/SimplifiedCplusplusAmbiguity.eyp illustrates the problem of parsing C++:
examples/debuggingtut/SimplifiedCplusplusAmbiguity.eyp
examples/debuggingtut$ eyapp -c SimplifiedCplusplusAmbiguity.eyp %strict %token ID INT NUM %right '=' %left '+' %% prog: /* empty */ | prog stmt ; stmt: expr ';' | decl ; expr: ID | NUM | INT '(' expr ')' /* typecast */ | expr '+' expr | expr '=' expr ; decl: INT declarator ';' | INT declarator '=' expr ';' ; declarator: ID | '(' declarator ')' ; %%
The grammar is ambiguous since an input like:
int (x) = 4;
can be interpreted as a decl or an expr.
decl
The eyapp compiler warn us of the presence of reduce/reduce conflict:
examples/debuggingtut$ eyapp -v SimplifiedCplusplusAmbiguity.eyp 1 reduce/reduce conflict
when we look at the .output file we see that the reduce-reduce conflict is at state 18:
examples/debuggingtut$ head -12 SimplifiedCplusplusAmbiguity.output Warnings: --------- 1 reduce/reduce conflict Conflicts: ---------- Conflict in state 15 between rule 8 and token '+' resolved as reduce. Conflict in state 15 between rule 8 and token '=' resolved as reduce. Conflict in state 17 between rule 9 and token '+' resolved as shift. Conflict in state 17 between rule 9 and token '=' resolved as shift. State 18 contains 1 reduce/reduce conflict
When we look at the description of the involved state, we see the reasons for the conflict:
examples/debuggingtut$ sed -ne '/^State 18:/,/^State/p' SimplifiedCplusplusAmbiguity.output State 18: expr -> ID . (Rule 5) declarator -> ID . (Rule 12) ')' [reduce using rule 12 (declarator)] $default reduce using rule 5 (expr) State 19:
The conflict means that once the parser has seen the ID and is in the presence of the closing parenthesis ')', it is incapable to decide whether to reduce by rule 12 or rule 5.
ID
As we said, the C++ disambiguation rule is: take it as a declaration if it looks as a declaration, otherwise is an expression. But we see that interpretation as decl will succeed if declarator is followed - after the sequence of closing parenthesis - by one of the two tokens a ; or =. That can be traced during parsing time. Parse::Eyapp provides the mechanisms to change the parsing actions at parsing time.
;
=
The postponed conflict strategy presented here can be used whenever there is a shift-reduce or reduce-reduce conflict that can not be solved using static precedences but that can be solved using information obtained at parsing time.
Let us assume we have a reduce-reduce conflict between to productions
A -> alpha . B -> beta .
for some token @. Let also assume that production
@
A -> alpha
has name ruleA and production
ruleA
B -> beta
has name ruleB.
ruleB
The postponed conflict resolution strategy consists in modifying the conflictive grammar by adding at the points of conflict, in this case at then end of the involved productions a new syntactic variable IsAorB:
IsAorB
A -> alpha IsAorB . B -> beta IsAorB .
The reduce-reduce conflict is now postponed after IsAorB is seen. The new syntactic variable IsAorB will be called the conflict name. The new introduced syntactic variable has only one empty production:
IsAorB -> /* empty */
The programmer associates with that production a semantic action whose mission is to solve the conflict by dynamically changing the parsing table like this:
IsAorB -> /* empty */ { my $self = shift; if (looks_like_A($self)) { $self->YYSetReduce('@', 'ruleA' ); } else { $self->YYSetReduce('@', 'ruleB' ); } }
The semantic action associated with the conflict name receives the name of conflict handler.
The Eyapp program examples/debuggingtut/Cplusplus.eyp solves the conflicts in examples/debuggingtut/SimplifiedCplusplusAmbiguity.eyp by using the Postponed Conflict Resolution strategy:
examples/debuggingtut/Cplusplus.eyp
examples/debuggingtut$ head -67 Cplusplus.eyp | cat -n 1 # See http://www.gnu.org/software/bison/manual/html_mono/bison.html#GLR-Parsers 2 %strict 3 %token ID INT NUM 4 5 %right '=' 6 %left '+' 7 8 %tree bypass 9 10 %expect 0 1 # 0 shift-reduce conflicts, 1 reduce-reduce conflict 11 12 %% 13 prog: 14 %name EMPTY 15 /* empty */ 16 | %name PROG 17 prog stmt 18 ; 19 20 stmt: 21 %name EXP 22 expr ';' 23 | %name DECL 24 decl 25 ; 26 27 expr: 28 %name EXPID 29 ID decexpconflict 30 | %name NUM 31 NUM 32 | %name TYPECAST 33 INT '(' expr ')' /* typecast */ 34 | %name PLUS 35 expr '+' expr 36 | %name ASSIGN 37 expr '=' expr 38 ; 39 40 decl: 41 %name DECLARATOR 42 INT declarator ';' 43 | %name DECLARATORINIT 44 INT declarator '=' expr ';' 45 ; 46 47 declarator: 48 %name DECID 49 ID decexpconflict 50 | '(' declarator ')' 51 ; 52 53 decexpconflict: 54 /* empty. Just to solve the reduce-reduce conflict */ 55 { 56 my $self = shift; 57 58 if ($self->{INPUT} =~ m{^[)\s]*[;=]\s*}) { 59 $self->YYSetReduce(')', 'DECID' ); 60 } 61 else { 62 $self->YYSetReduce(')', 'EXPID' ); 63 } 64 } 65 ; 66 67 %%
The aforementioned conflict in examples/debuggingtut/SimplifiedCplusplusAmbiguity.eyp:
is now delayed just after seen decexpconflict:
decexpconflict
examples/debuggingtut$ eyapp -vb '' Cplusplus.eyp 1 reduce/reduce conflict examples/debuggingtut$ sed -ne '/^State 27:/,/^State/p' Cplusplus.output State 27: expr -> ID decexpconflict . (Rule 5) declarator -> ID decexpconflict . (Rule 12) ')' [reduce using rule 12 (declarator)] $default reduce using rule 5 (expr) State 28:
The call
$self->YYSetReduce(')', 'DECID' );
inside the semantic action associated with decexpconflict sets the parsing action to reduce by the production with name DECID, i.e.
DECID
declarator -> ID
when the incoming input is followed by a semicolon or an equal. Otherwise the action taken is to reduce by
expr -> ID
The program now successfully solves the ambiguity:
examples/debuggingtut$ Cplusplus.pm int(x)+2; PROG(EMPTY,EXP(TYPECAST(TERMINAL[int],EXPID[x]),NUM[2])) debuggingtut$ Cplusplus.pm int(x)=2; PROG(EMPTY,DECL(TERMINAL[int],DECID[x],NUM[2])) examples/debuggingtut$ Cplusplus.pm int(x); PROG(EMPTY,DECL(TERMINAL[int],DECID[x]))
YYSetLRAction
YYNextState
The modulino glrexpressions.eyp illustrates an alternative way to apply the postponed conflict resolution strategy to the aforementioned problem:
glrexpressions.eyp
examples/debuggingtut$ head -69 glrexpressions.eyp | cat -n 1 # See http://www.gnu.org/software/bison/manual/html_mono/bison.html#GLR-Parsers 2 %strict 3 %token ID INT NUM 4 5 %right '=' 6 %left '+' 7 8 %{ 9 my $input; 10 %} 11 12 %tree bypass 13 %% 14 prog: 15 %name EMPTY 16 /* empty */ 17 | %name PROG 18 prog stmt 19 ; 20 21 stmt: 22 %name EXP 23 expr ';' 24 | %name DECL 25 decl 26 ; 27 28 expr: 29 %name EXPID 30 ID decexpconflict 31 | %name NUM 32 NUM 33 | %name TYPECAST 34 INT '(' expr ')' /* typecast */ 35 | %name PLUS 36 expr '+' expr 37 | %name ASSIGN 38 expr '=' expr 39 ; 40 41 decl: 42 %name DECLARATOR 43 INT declarator ';' 44 | %name DECLARATORINIT 45 INT declarator '=' expr ';' 46 ; 47 48 declarator: 49 %name DECID 50 ID decexpconflict 51 | '(' declarator ')' 52 ; 53 54 decexpconflict: 55 /* empty. Just for hacking the LALR tables */ 56 { 57 my $self = shift; 58 59 my $conflictstate = $self->YYNextState(); 60 if ($input =~ m{^[)\s]*[;=]\s*}) { 61 $self->YYSetLRAction($conflictstate, ')', 'DECID' ); 62 } 63 else { 64 $self->YYSetLRAction($conflictstate, ')', 'EXPID' ); 65 } 66 } 67 ; 68 69 %%
Line 59 uses the method YYNextState to compute the state after the reduction for the production rule
decexpconflict -> /* empty */
which is precisely the conflict state. If the incoming input is a sequence of parenthesis followed by either a semicolon or an equal we call to the method YYSetLRAction to set a reduction by the rule
for that state and token ')', otherwise we indicate a reduction by the rule:
The program in examples/debuggingtut/DynamicallyChangingTheParser2.eyp illustrates how the postponed conflict strategy is used for shift-reduce conflicts. This is an extension of the grammar in examples/debuggingtut/Debug.eyp. The generated language is constituted by sequences like:
examples/debuggingtut/DynamicallyChangingTheParser2.eyp
examples/debuggingtut/Debug.eyp
{ D; D; S; S; S; } {D; S} { S }
As you remember the conflict was:
examples/debuggingtut$ sed -ne '/^State 13:/,/^State/p' DynamicallyChangingTheParser2.output State 13: ds -> D conflict . ';' ds (Rule 6) ds -> D conflict . (Rule 7) ';' shift, and go to state 16 ';' [reduce using rule 7 (ds)] State 14:
The conflict handler (lines 50-56 below) sets the LR action to reduce by the production with name LAST_D
LAST_D
ds -> D
in the presence of token ';' if indeed is the last 'D'. The semantic action associated with the production LAST_D (lines 34-37) restores the former shift action and proceeds to call Parse::Eyapp::Driver::YYBuildAST in order to continue with the building of the abstract syntax tree:
'D'
Parse::Eyapp::Driver::YYBuildAST
examples/debuggingtut$ cat -n DynamicallyChangingTheParser2.eyp 1 # See section 'Hacking the Parsing Tables: ACTION and GOTOs' in 2 # http://search.cpan.org/perldoc?Parse::Eyapp::debuggingtut 3 # 4 # See also: Debug.eyp Debug1.eyp Debug2.eyp LookForward.eyp 5 # DynamicallyChangingTheParser.eyp This example illustrates how to dynamically 6 # change the behavior of the parser 7 8 %token D S 9 10 %{ 11 our $VERSION = '0.01'; 12 %} 13 14 %tree bypass 15 16 %% 17 p: %name PROG 18 block + 19 ; 20 21 block: 22 %name BLOCK 23 '{' ds ';' ss '}' 24 | %name SS 25 '{' ss '}' 26 ; 27 28 ds: 29 %name MORE_Ds 30 D conflict ';' ds 31 | %name LAST_D 32 D conflict 33 { 34 # Recover former LALR action 35 $_[0]->YYRestoreLRAction('conflict', ';'); 36 # Do whatever you want 37 goto &Parse::Eyapp::Driver::YYBuildAST; 38 } 39 ; 40 41 ss: 42 %name SS 43 S ';' ss 44 | %name S 45 S 46 ; 47 48 conflict: 49 /* empty. Just for dynamic precedence */ 50 { 51 my $self = shift; 52 53 $self->YYSetReduce(';', 'LAST_D' ) if ($self->{INPUT} =~ m{^;\s*S}); 54 55 undef; # skip this node in the AST 56 } 57 ; 58 59 %% 60 61 sub _Error { 62 my $parser = shift; 63 64 my ($token) = $parser->YYCurval; 65 my ($what) = $token ? "input: '$token'" : "end of input"; 66 warn "Syntax error near $what\n"; 67 } 68 69 sub _Lexer { 70 my $self = shift; 71 72 for ($self->{INPUT}) { 73 s{^(\s*)}{}; 74 75 return ('',undef) unless $_; 76 77 return ($1,$1) if s/^(.)//; 78 } 79 return ('',undef); 80 } 81 82 sub Run { 83 my $debug = shift || 0; 84 $debug = 0x1F if $debug; 85 86 my $self = __PACKAGE__->new(); 87 $self->{INPUT} = <STDIN>; 88 89 print 90 $self->YYParse( 91 yylex => \&_Lexer, 92 yyerror => \&_Error, 93 yydebug => $debug, 94 )->str."\n"; 95 } 96 97 Run(@ARGV) unless caller;
Explicit names can be given to grammar productions via the %name directive. An alternative to explicitly gave names to rules is to define a naming scheme via the Eyapp directive %namingscheme. This can be helpful when you inherit a large grammar and want to quickly build a parser. The ANSI C parser in examples/languages/C/ansic.eyp is a good example. Another example is the Pascal parser in examples/languages/pascal/pascal.eyp.
%namingscheme
examples/languages/C/ansic.eyp
examples/languages/pascal/pascal.eyp
The Eyapp directive %namingscheme is followed by some Perl code. Such Perl code must return a reference to a subroutine that will be called each time a new production right hand side is parsed. The subroutine returns the name for the production.
The Perl code defining the handler receives a Parse::Eyapp object that describes the grammar. The code after the %namingscheme directive is evaluated during the early phases of the compilation of the input grammar. As an example of how to set a naming scheme, see lines 22-38 below (you can find this example and others in the directory examples/naming of the accompanying distribution):
examples/naming
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/naming$ cat -n GiveNamesToCalc.eyp 1 # GiveNamesToCalc.eyp 2 %right '=' 3 %left '-' '+' 4 %left '*' '/' 5 %left NEG 6 %right '^' 7 8 %tree bypass 9 10 %{ 11 use base q{Tail}; 12 13 sub exp_is_NUM::info { 14 my $self = shift; 15 16 $self->{attr}[0]; 17 } 18 19 *exp_is_VAR::info = *var_is_VAR::info = \&exp_is_NUM::info; 20 %} 21 22 %namingscheme { 23 #Receives a Parse::Eyapp object describing the grammar 24 my $self = shift; 25 26 $self->tokennames( 27 '=' => 'ASSIGN', 28 '+' => 'PLUS', 29 '*' => 'TIMES', 30 '-' => 'MINUS', 31 '/' => 'DIV', 32 '^' => 'EXP', 33 ); 34 35 # returns the handler that will give names 36 # to the right hand sides 37 \&give_token_name; 38 } 39 %% 40 41 line: 42 exp 43 ; 44 45 exp: 46 NUM 47 | VAR 48 | var '=' exp 49 | exp '+' exp 50 | exp '-' exp 51 | exp '*' exp 52 | exp '/' exp 53 | %no bypass exp_is_NEG 54 '-' exp %prec NEG 55 | exp '^' exp 56 | '(' exp ')' 57 ; 58 59 var: 60 VAR 61 ; 62 %% 63 64 unless (caller) { 65 my $t = __PACKAGE__->main(@ARGV); 66 print $t->str."\n"; 67 }
The example uses a naming scheme that is provided by Parse::Eyapp: Parse::Eyapp::Grammar::give_token_name. The current provided naming schemes handlers are:
Parse::Eyapp::Grammar::give_token_name
give_default_name: The name of the production is the name of the Left Hand Side of the Production Rule concatenated with an underscore and the index of the production
give_default_name
give_lhs_name: The name of the production is the name of the Left Hand Side of the Production Rule (this is the naming scheme used by the %tree directive when no explicit name was given)
give_lhs_name
give_token_name: The name of the production is the Left Hand Side of the Production Rule followed by the word _is_ followed by the concatenation of the names of the tokens in the right and side (separated by underscores).
give_token_name
_is_
All of these handlers are implemented inside the class Parse::Eyapp::Grammar. There is no need at line 37 to explicit the class name prefix since the naming scheme code is evaluated inside such class:
Parse::Eyapp::Grammar
22 %namingscheme { 23 #Receives a Parse::Eyapp object describing the grammar 24 my $self = shift; 25 26 $self->tokennames( 27 '=' => 'ASSIGN', 28 '+' => 'PLUS', 29 '*' => 'TIMES', 30 '-' => 'MINUS', 31 '/' => 'DIV', 32 '^' => 'EXP', 33 ); 34 35 # returns the handler that will give names 36 # to the right hand sides 37 \&give_token_name; 38 }
As it is illustrated in this example, the method tokennames of Parse::Eyapp objects provide a way to give identifier names to tokens that are defined by strings. When we execute the former module/program (modulino) with input a=2*-3 we got the following output:
tokennames
a=2*-3
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/naming$ eyapp -b '' GiveNamesToCalc.eyp lusasoft@LusaSoft:~/src/perl/Eyapp/examples/naming$ ./GiveNamesToCalc.pm Expressions. Press CTRL-D (Unix) or CTRL-Z (Windows) to finish: a=2*-3 line_is_exp(var_is_VAR[a],exp_is_TIMES(exp_is_NUM[2],exp_is_NEG(exp_is_NUM[3])))
For each production rule the handler is called with arguments:
the Parse::Eyapp object,
the production index (inside the grammar),
the left hand side symbol and a reference to a list with the symbols in the right hand side.
The following code of some version of give_token_name exemplifies how a naming scheme handler can be written:
lusasoft@LusaSoft:~/src/perl/Eyapp$ sed -ne '101,132p' lib/Parse/Eyapp/Grammar.pm | cat -n 1 sub give_token_name { 2 my ($self, $index, $lhs, $rhs) = @_; 3 4 my @rhs = @$rhs; 5 $rhs = ''; 6 7 unless (@rhs) { # Empty RHS 8 return $lhs.'_is_empty'; 9 } 10 11 my $names = $self->{GRAMMAR}{TOKENNAMES} || {}; 12 for (@rhs) { 13 if ($self->is_token($_)) { 14 s/^'(.*)'$/$1/; 15 my $name = $names->{$_} || ''; 16 unless ($name) { 17 $name = $_ if /^\w+$/; 18 } 19 $rhs .= "_$name" if $name; 20 } 21 } 22 23 unless ($rhs) { # no 'word' tokens in the RHS 24 for (@rhs) { 25 $rhs .= "_$_" if /^\w+$/; 26 } 27 } 28 29 # check if another production with such name exists? 30 my $name = $lhs.'_is'.$rhs; 31 return $name; 32 }
A Parse::Eyapp object holds the information about the Eyapp input grammar: parsing tables, conflicts, semantic actions, etc.
To translate an Eyapp grammar you must use either the eyapp script or call the class constructor new_grammar. The Parse::Eyapp method Parse::Eyapp->new_grammar(input=>$grammar) creates a package containing the code that implements a LALR parser for the input grammar:
new_grammar
Parse::Eyapp->new_grammar(input=>$grammar)
my $p = Parse::Eyapp->new_grammar( input=>$translationscheme, classname=>'Grammar', firstline => 6, outputfile => 'main' ); die $p->Warnings if $p->Warnings; my $new_parser_for_grammar = Grammar->new();
The method returns a Parse::Eyapp object.
You can check the object to see if there were problems during the construction of the parser for your grammar:
die $p->qtables() if $p->Warnings;
The method Warnings returns the warnings produced during the parsing. The absence of warnings indicates the correctness of the input program.
Warnings
The call to Parse::Eyapp->new_grammar generates a class/package containing the parser for your input grammar. Such package lives in the namespace determined by the classname argument of new_grammar. To create a parser for the grammar you call the constructor new of the just created class:
Parse::Eyapp->new_grammar
classname
new
my $new_parser_for_grammar = Grammar->new();
The meaning of the arguments of Parse::Eyapp->new_grammar is:
The string containing the input
The name of the package that will held the code for the LALR parser. The package of the caller will be used as default if none is specified.
For error diagnostics. The line where the definition of the Eyapp grammar starts.
Include/not include # line directives in the generated code
# line directives
If defined the generated code fill be dumped in the specified filename (with extension .pm) and the LALR information ambiguities and conflicts) in the specified filename with extension .output.
Returns a string containing information on warnings, ambiguities, conflicts, rules and the generated DFA tables. Is the same information in file.output when using the command eyapp -v file.eyp.
file.output
eyapp -v file.eyp
my $p = Parse::Eyapp->new_grammar( input=>$eyappprogram, classname=>'SimpleC', outputfile => 'SimpleC.pm', firstline=>12, ); print $p->qtables() if $p->Warnings;
It receives two arguments
$eyapp->outputtables($path, $base)
Similar to qtables but prints the information on warnings, conflicts and rules to the specified $path/$base.
qtables
$path/$base
Returns the warnings resulting from compiling the grammar:
my $p = Parse::Eyapp->new_grammar( input=>$translationscheme, classname=>'main', firstline => 6, outputfile => 'main' ); die $p->Warnings if $p->Warnings;
Returns the empty string if there were no conflicts.
Returns a string with the information about the LALR generated DFA.
Returns a string with summary information about the compilation of the grammar. No arguments.
Returns a string with summary information about the conflicts that arose when compiling the grammar. No arguments.
Returns a string with the parsing tables
Used when defining a naming scheme to associate identifiers with (usually string) tokens. See section NAMING SCHEMES of Parse::Eyapp
CLASS
See the documentation for Parse::Eyapp::Driver
Parse::Eyapp::Parse
The parser for the Eyapp language was written and generated using Parse::Eyapp and the eyapp compiler (actually the first version was bootstrapped using the yapp compiler). The Eyapp program parsing the Eyapp language is in the file Parse/Eyapp/Parse.yp in the Parse::Eyapp distribution. Therefore Parse::Eyapp::Parse objects have all the methods in Parse::Eyapp::Driver.
Parse/Eyapp/Parse.yp
A Parse::Eyapp::Parse is nothing but a particular kind of Parse::Eyapp parser: the one that parses Eyapp grammars.
See the documentation for Parse::Eyapp::translationschemestut
See the documentation for Parse::Eyapp::Treeregexp
See the documentation for Parse::Eyapp::Node
See the documentation for Parse::Eyapp::YATW
treereg
A Treeregexp program can be isolated in a file an compiled with the program treereg. The default extension is .trg. See the following example:
.trg
pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ cat -n Shift.trg 1 # File: Shift.trg 2 { 3 sub log2 { 4 my $n = shift; 5 return log($n)/log(2); 6 } 7 8 my $power; 9 } 10 mult2shift: TIMES($e, NUM($m)) 11 and { $power = log2($m->{attr}); (1 << $power) == $m->{attr} } => { 12 $_[0]->delete(1); 13 $_[0]->{shift} = $power; 14 $_[0]->type('SHIFTLEFT'); 15 }
Note that auxiliary support code can be inserted at any point between transformations (lines 2-9). The code will be inserted (without the defining curly brackets) at that point. Note also that the lexical variable $power is visible inside the definition of the mult2shift transformation.
$power
mult2shift
A treeregexp like $e matches any node (line 10). A reference to the node is saved in the lexical variable $e. The scope of the variable $e is the current tree transformation, i.e. mult2shift. Such kind of treeregexps are called scalar treeregexps.
$e
The call to the delete method at line 12 deletes the second child of the node being visited (i.e. NUM($m)).
delete
NUM($m)
The call to type at line 14 retypes the node as a SHIFTLEFT node.
type
SHIFTLEFT
The program is compiled using the script treereg:
pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ eyapp Rule5 pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ treereg Shift pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ ls -ltr | tail -2 -rw-r--r-- 1 pl users 6439 2008-09-02 08:59 Rule5.pm -rw-r--r-- 1 pl users 1424 2008-09-02 08:59 Shift.pm
The Grammar Rule5.yp is similar to the one in the "SYNOPSIS" section. Module Rule5.pm contains the parser. The module Shift.pm contains the code implementing the tree transformations.
Rule5.yp
Rule5.pm
Shift.pm
The client program follows:
pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ cat -n useruleandshift.pl 1 #!/usr/bin/perl -w 2 use strict; 3 use Rule5; 4 use Parse::Eyapp::Base qw(insert_function); 5 use Shift; 6 7 sub SHIFTLEFT::info { $_[0]{shift} } 8 insert_function('TERMINAL::info', \&TERMINAL::attr); 9 10 my $parser = new Rule5(); 11 my $t = $parser->Run; 12 print "***********\n",$t->str,"\n"; 13 $t->s(@Shift::all); 14 print "***********\n",$t->str,"\n";
Lines 7 and 8 provide the node classes TERMINAL and SHIFTLEFT of info methods to be used during the calls to the str method (lines 12 and 14).
info
Multiplications by a power of two are substituted by the corresponding shifts:
pl@nereida:~/src/perl/YappWithDefaultAction/examples/Eyapp$ useruleandshift.pl a=b*8 *********** ASSIGN(TERMINAL[a],TIMES(VAR(TERMINAL[b]),NUM(TERMINAL[8]))) *********** ASSIGN(TERMINAL[a],SHIFTLEFT[3](VAR(TERMINAL[b])))
See files Rule9.yp, Transform4.trg and foldand0rule9_4.pl in the examples directory for a more detailed vision of this example. File Rule9.yp is very much like the grammar in the "SYNOPSIS" example. To compile the grammar Rule9.yp and the treeregexp file Transform4.trg use the commands:
Rule9.yp
Transform4.trg
foldand0rule9_4.pl
eyapp -m 'Calc' Rule9.yp
That will produce a file Calc.pm containing a package Calc that implements the LALR parser. Then the command:
Calc.pm
Calc
treereg -o T.pm -p 'R::' -m T Transform4
produces a file T.pm containing a package T that implements the tree transformation program. The -p option announces that node classes are prefixed by 'R::'.
T.pm
T
-p
'R::'
With such parameters the client program uses the generated modules as follows:
nereida:~/src/perl/YappWithDefaultAction/examples> cat -n foldand0rule9_4.pl 1 #!/usr/bin/perl -w 2 # File: foldand0rule9_4.pl. Compile it with 3 # eyapp -m 'Calc' Rule9.yp; treereg -o T.pm -p 'R::' -m T Transform4 4 use strict; 5 use Calc; 6 use T; 7 8 sub R::TERMINAL::info { $_[0]{attr} } 9 my $parser = new Calc(yyprefix => "R::"); 10 my $t = $parser->YYParse( yylex => \&Calc::Lexer, yyerror => \&Calc::Error); 11 print "\n***** Before ******\n"; 12 print $t->str."\n"; 13 $t->s(@T::all); 14 print "\n***** After ******\n"; 15 print $t->str."\n";
running the program produces the following output:
nereida:~/src/perl/YappWithDefaultAction/examples> foldand0rule9_4.pl 2*3 ***** Before ****** R::TIMES(R::NUM(R::TERMINAL[2]),R::TERMINAL[*],R::NUM(R::TERMINAL[3])) ***** After ****** R::NUM(R::TERMINAL[6])
See the documentation for Parse::Eyapp::Scope
See the documentation in Parse::Eyapp::Base
Remember to set the environment variable PERL5LIB if you decide to install Parse::Eyapp at a location other than the standard. For example, on a bash or sh:
PERL5LIB
export PERL5LIB=/home/user/wherever_it_is/lib/:$PERL5LIB
on a csh or tcsh
csh
tcsh
setenv PERL5LIB /home/user/wherever_it_is/lib/:$PERL5LIB
Be sure the scripts eyapp and treereg are in the execution PATH.
This distribution depends on the following modules:
List::Util
Data::Dumper
Pod::Usage
It seems that List::Util is in the core of Perl distributions since version 5.73:
> perl -MModule::CoreList -e 'print Module::CoreList->first_release("List::Util")' 5.007003
and Data::Dumper is also in the core since 5.5:
> perl -MModule::CoreList -e 'print Module::CoreList->first_release("Data::Dumper")' 5.005
and Pod::Usage is also in the core since 5.6:
> perl -MModule::CoreList -e 'print Module::CoreList->first_release("Pod::Usage")' 5.006
I also recommend the following modules:
Test::Pod
Test::Warn
Test::Exception
The dependence on Test::Warn, Test::Pod and Test::Exception is merely for the execution of tests. If the modules aren't installed the tests depending on them will be skipped.
To install it, follow the traditional mantra:
perl Makefile.PL make make test make install
Also:
Make a local copy of the examples/ directory in this distribution. They contain the examples used in the tutorials
The way Parse::Eyapp parses Perl code is verbatim the way it does Parse::Yapp 1.05. Quoting Francois Desarmenien Parse::Yapp documentation:
"Be aware that matching braces in Perl is much more difficult than in C: inside strings they don't need to match. While in C it is very easy to detect the beginning of a string construct, or a single character, it is much more difficult in Perl, as there are so many ways of writing such literals. So there is no check for that today. If you need a brace in a double-quoted string, just quote it (\{ or \}). For single-quoted strings, you will need to make a comment matching it in the right order. Sorry for the inconvenience.
\{
\}
{ "{ My string block }". "\{ My other string block \}". qq/ My unmatched brace \} /. # Force the match: { q/ for my closing brace } / q/ My opening brace { / # must be closed: } }
All of these constructs should work."
Alternative exact solutions were tried but resulted in much slower code. Therefore, until something faster is found, I rather prefer for Parse::Eyapp to live with this limitation.
The same limitation may appear inside header code (code between %{ and %})
The project home is at http://code.google.com/p/parse-eyapp/. Use a subversion client to anonymously check out the latest project source code:
svn checkout http://parse-eyapp.googlecode.com/svn/trunk/ parse-eyapp-read-only
Parse::Eyapp, Parse::Eyapp::eyapplanguageref, Parse::Eyapp::debuggingtut, Parse::Eyapp::defaultactionsintro, Parse::Eyapp::translationschemestut, Parse::Eyapp::Driver, Parse::Eyapp::Node, Parse::Eyapp::YATW, Parse::Eyapp::Treeregexp, Parse::Eyapp::Scope, Parse::Eyapp::Base, Parse::Eyapp::datagenerationtut
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/languageintro.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/debuggingtut.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/eyapplanguageref.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Treeregexp.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Node.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/YATW.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Eyapp.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Base.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/translationschemestut.pdf
The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/MatchingTrees.pdf
The tutorial Parsing Strings and Trees with Parse::Eyapp (An Introduction to Compiler Construction in seven pages) in http://nereida.deioc.ull.es/~pl/eyapsimple/
perldoc eyapp,
perldoc treereg,
perldoc vgg,
The Syntax Highlight file for vim at http://www.vim.org/scripts/script.php?script_id=2453 and http://nereida.deioc.ull.es/~vim/
Analisis Lexico y Sintactico, (Notes for a course in compiler construction) by Casiano Rodriguez-Leon. Available at http://nereida.deioc.ull.es/~pl/perlexamples/ Is the more complete and reliable source for Parse::Eyapp. However is in Spanish.
Parse::Yapp,
Man pages of yacc(1) and bison(1), http://www.delorie.com/gnu/docs/bison/bison.html
Language::AttributeGrammar
Parse::RecDescent.
HOP::Parser
HOP::Lexer
ocamlyacc tutorial at http://plus.kaist.ac.kr/~shoh/ocaml/ocamllex-ocamlyacc/ocamlyacc-tutorial/ocamlyacc-tutorial.html
The classic Dragon's book Compilers: Principles, Techniques, and Tools by Alfred V. Aho, Ravi Sethi and Jeffrey D. Ullman (Addison-Wesley 1986)
CS2121: The Implementation and Power of Programming Languages (See http://www.cs.man.ac.uk/~pjj, http://www.cs.man.ac.uk/~pjj/complang/g2lr.html and http://www.cs.man.ac.uk/~pjj/cs2121/ho/ho.html) by Pete Jinks
Hal Finkel http://www.halssoftware.com/
G. Williams http://kasei.us/
Thomas L. Shinnick http://search.cpan.org/~tshinnic/
Casiano Rodriguez-Leon (casiano@ull.es)
This work has been supported by CEE (FEDER) and the Spanish Ministry of Educacion y Ciencia through Plan Nacional I+D+I number TIN2005-08818-C04-04 (ULL::OPLINK project http://www.oplink.ull.es/). Support from Gobierno de Canarias was through GC02210601 (Grupos Consolidados). The University of La Laguna has also supported my work in many ways and for many years.
A large percentage of code is verbatim taken from Parse::Yapp 1.05. The author of Parse::Yapp is Francois Desarmenien.
I wish to thank Francois Desarmenien for his Parse::Yapp module, to my students at La Laguna and to the Perl Community. Thanks to the people who have contributed to improve the module (see "CONTRIBUTORS" in Parse::Eyapp). Thanks to Larry Wall for giving us Perl. Special thanks to Juana.
Copyright (c) 2006-2008 Casiano Rodriguez-Leon (casiano@ull.es). All rights reserved.
Parse::Yapp copyright is of Francois Desarmenien, all rights reserved. 1998-2001
These modules are free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
1 POD Error
The following errors were encountered while parsing the POD:
Non-ASCII character seen before =encoding in 'válida\n";'. Assuming CP1252
To install Parse::Eyapp, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Parse::Eyapp
CPAN shell
perl -MCPAN -e shell install Parse::Eyapp
For more information on module installation, please visit the detailed CPAN module installation guide.