NAME
Parse::Eyapp::eyapplanguageref - The Eyapp language reference manual
THE EYAPP LANGUAGE
Eyapp Grammar
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.
lusasoft@LusaSoft:~/src/perl/Eyapp$ eyapp -c lib/Parse/Eyapp/Parse.yp
%token ASSOC /* is %(left|right|nonassoc) */
%token BEGINCODE /* is %begin { Perl code ... } */
%token CODE /* is { Perl code ... } */
%token DEFAULTACTION /* is %defaultaction */
%token EXPECT /* is %expect */
%token HEADCODE /* is %{ Perl code ... %} */
%token IDENT /* is [A-Za-z_][A-Za-z0-9_]* */
%token LITERAL /* is a string literal like 'hello' */
%token METATREE /* is %metatree */
%token NAME /* is %name */
%token NUMBER /* is \d+ */
%token OPTION /* is (%name\s*([A-Za-z_]\w*)\s*)?\? */
%token PLUS /* is (%name\s*([A-Za-z_]\w*)\s*)?\+ */
%token PREC /* is %prec */
%token SEMANTIC /* is %semantic\s+token */
%token STAR /* is (%name\s*([A-Za-z_]\w*)\s*)?\* */
%token START /* is %start */
%token SYNTACTIC /* is %syntactic\s+token */
%token TAILCODE /* is { Perl code ... } */
%token TOKEN /* is %token */
%token TREE /* is %tree */
%token TYPE /* is %type */
%token UNION /* is %union */
%start eyapp
%%
eyapp:
head body tail
;
symbol:
LITERAL
| ident
;
ident:
IDENT
;
# Head section:
head:
headsec '%%'
;
headsec:
#empty
| decls
;
decls:
decls decl
| decl
;
decl:
'\n'
| SEMANTIC typedecl symlist '\n'
| SYNTACTIC typedecl symlist '\n'
| TOKEN typedecl symlist '\n'
| ASSOC typedecl symlist '\n'
| START ident '\n'
| PREFIX '\n'
| NAMINGSCHEME CODE '\n'
| HEADCODE '\n'
| UNION CODE '\n' #ignore
| DEFAULTACTION CODE '\n'
| TREE '\n'
| METATREE '\n'
| STRICT '\n'
| TYPE typedecl identlist '\n'
| EXPECT NUMBER '\n'
| error '\n'
;
typedecl:
#empty
| '<' IDENT '>'
;
symlist:
symlist symbol
| symbol
;
identlist:
identlist ident
| ident
;
# Rule section
body:
rulesec '%%'
| '%%'
;
rulesec:
rulesec rules
| startrules
;
startrules:
IDENT ':' rhss ';'
| error ';'
;
rules:
IDENT ':' rhss ';'
| error ';'
;
rhss:
rhss '|' rule
| rule
;
rule:
optname rhs prec epscode
| optname rhs
;
rhs:
#empty
| rhselts
;
rhselts:
rhselts rhseltwithid
| rhseltwithid
;
rhseltwithid:
rhselt '.' IDENT
| '$' rhselt
| '$' error
| rhselt
;
rhselt:
symbol
| code
| '(' optname rhs ')'
| rhselt STAR
| rhselt '<' STAR symbol '>'
| rhselt OPTION
| rhselt '<' PLUS symbol '>'
| rhselt PLUS
;
optname:
/* empty */
| NAME IDENT
;
prec:
PREC symbol
;
epscode:
| code
;
code:
CODE
| BEGINCODE
;
# Tail section:
tail:
/*empty*/
| TAILCODE
;
%%The semantic of Eyapp agrees with the semantic of yacc and yapp for all the common constructions.
Comments
Comments are either Perl style, from # up to the end of line, or C style, enclosed between /* and */.
Syntactic Variables, Symbolic Tokens and String Literals
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_]*.
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.
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 '"', '$' and '@' is escaped. To have a single quote inside a literal, escape it with '\'.
When building the syntax tree (i.e. when running under the %tree directive) string literals will be considered syntactic tokens (see section "Syntactic and Semantic tokens"). Syntactic tokens do not produce nodes in the Abstract Syntax Tree.
The examples used along this document can be found in the directory examples/eyapplanguageref accompanying this distribution.
Parts of an eyapp Program
An Eyapp program has three parts called head, body and tail:
eyapp: head body tail ;Each part is separated from the former by the symbol %%:
head: headsec '%%'
body: rulesec '%%'THE HEAD SECTION
The head section contains a list of declarations
headsec: decl *There are different kinds of declarations.
This reference does not fully describes all the declarations that are shared with yacc and yapp.
Example of Head Section
In this and the next sections we will describe the basics of the Eyapp language using the file examples/eyapplanguageref/Calc.eyp that accompanies this distribution. This file implements a trivial calculator. Here is the header section:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 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 %%Declarations and Precedence
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:
a = 4 - 5will be interpreted as
a = (4 - 5)and not as
(a = 4) - 5The 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 parenthesization. Thus, the expression
4 - 5 - 9will be interpreted as
(4 - 5) - 9Header Code
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
7 %{
8 my %s; # symbol table
9 %}will be visible from almost any point in the file.
The Start Symbol of the Grammar
%start IDENT declares IDENT as the start symbol of the grammar. When %start is not used, the first rule in the body section will be used.
Expect
The %expect #NUMBER directive works as in bison and suppress warnings when the number of Shift/Reduce conflicts is exactly #NUMBER. See section "Solving Ambiguities and Conflicts" to know more about Shift/Reduce conflicts.
Type and Union
C oriented declarations like %type and %union are parsed but ignored.
The %strict Directive
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.eypTo 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.pmIt is a good practice to use %strict at the beginning of your grammar.
The %prefix Directive
The %prefix directive is equivalent to the use of the yyprefix. The node classes are prefixed with the specified prefix
%prefix Some::Prefix::See the example in examples/eyapplanguageref/alias_and_yyprefix.pl
Default Action Directive
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.
Tree Construction Directives
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.
Syntactic and Semantic Tokens
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".
THE BODY
The body section contains the rules describing the grammar:
body: rules * '%%'
rules: IDENT ':' rhss ';'
rhss: (optname rhs (prec epscode)?) <+ '|'> Rules
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).
Semantic Values and Semantic Actions
In Parse::Eyapp a production rule
A -> X_1 X_2 ... X_ncan 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. Asociated 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:
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.
When the semantic action/anonymous subroutine is called, the arguments are as follows:
$_[1]to$_[n]are the attributes of the symbolsX_1,X_2, ...,X_n. Just as$1to$ninyacc,$_[0]is the parser object itself. Having$_[0]beeing the parser object itself allows you to call parser methods. Mostyaccmacros have been converted into parser methods. See section "METHODS AVAILABLE IN THE GENERATED CLASS" in Parse::Eyapp.
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):
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
{ $_[1] }will be executed instead. See section "DEFAULT ACTIONS" to know more.
Actions in Mid-Rule
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, $_[3]\n"; }
;
%%The auxiliar syntactic variables are named @#position-#order where #position is the position of the action in the rhs and order is an ordinal number. See the .output file for the former example:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ eyapp -v intermediateaction2.yp
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ sed -ne '1,5p' intermediateaction2.output
Rules:
------
0: $start -> S $end
1: S -> 'a' @1-1 'a'
2: @1-1 -> /* empty */when given input aa the execution will produce as output aaaa, aaaa, a.
Example of Body Section
Following with the calculator example, the body is:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 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 ocurred: _Error will be called
30 undef
31 }
32 | '-' $exp %prec NEG { -$exp }
33 | exp.left '^' exp.right { $left ** $right }
34 | '(' $exp ')' { $exp }
35 ;
36
37 %%This example does not uses any of the Eyapp extensions (with the exception of the star list at line 5) and the dot and dollar notations. Please, see the Parse::Yapp pages and elsewhere documentation on yacc and bison for more information.
Solving Ambiguities and Conflicts
When Eyapp analizes a grammar like:
examples/eyapplanguageref$ 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):
.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 conflictup 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).
The precedences and associativities are associated with 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
%precdirective 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 tokendirective giving the production the precedence of thetokenexp: '-' exp %prec NEG { -$_[1] }If there is a shift/reduce conflict, and both the grammar production and the input character have precedence and associativity associated with them, then the conflict is solved in favor of the action (shift or reduce) associated with the higher precedence. If the precedences are the same, then the associativity is used; left associative implies reduce, right associative implies shift, and nonassociating implies error.
To solve a shift-reduce conflict between a production A --> SOMETHING and a token 'a' you can follow this procedure:
- 1. Edit the
.outputfile - 2. Search for the state where the conflict between the production and the token is. In our example it looks like:
-
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 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)] $default reduce using rule 2 (exp) - 3. Inside the state there has to be a production of the type
A --> SOMETHING.(with the dot at the end) indicating that a reduction must take place. There has to be also another production of the formA --> prefix . suffix, where suffix can start with the involved token'a'. - 4. Decide what action shift or reduce matches the kind of trees you want. In this example we want
NUM - NUM - NUMto produce a tree likeMINUS(MINUS(NUM, NUM), NUM)and notMINUS(NUM, MINUS(NUM, NUM)). We want the conflict inexp - exp.- NUMto be solved in favor of the reduction byexp: exp '-' exp. This is achieved by declaring%left '-'.
Error Recovery
The token name error is reserved for error handling. This name can be used in grammar productions; it suggests places where errors are expected, and recovery can take place:
line:
'\n' { undef }
| exp '\n' { print "$_[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.
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.
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
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 analizer plus error handling and auxiliary subroutines).
The Lexical Analyzer
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 accesible inside the lexical analyzer. Specially interesting is the $_[0]->YYData method which provides access to the user data area.
When the lexical analyzer reaches the end of input, it must return the pair ('', 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,$1);
13
14 return('',undef);
15 }
16 }
17 }The subroutine make_lexer creates the lexical analyzer as a closure. The lexer returned by make_lexer is used by the YYParse method:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 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
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:
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 $lineno).
21 Expected one of these terminals: @expected
22 ERRMSG
23 }See the Parse::Yapp pages and elsewhere documentation on yacc and bison for more information.
USING AN EYAPP GRAMMAR
The following is an example of a program that uses the calculator explained in the two previous sections:
pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 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 = 22LISTS AND OPTIONALS
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:
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 | declAn additional symbol may be included to indicate lists of elements separated by such symbol. Thus
rhss: rule <+ '|'>is equivalent to:
rhss: rhss '|' rule | ruleThe operators
*and?have their usual meaning: 0 or more for*and optionality for?. Is legal to parenthesize arhsexpression as in:optname: (NAME IDENT)?
The + operator
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 action related with the d+ element creates the list of ds.
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:
$self->YYBuildingTree(1);Other ways to change the associated semantic are to use the yybuildingtree option of YYParse:
$self->YYParse( yylex => \&_Lexer, yyerror => \&_Error,
yybuildingtree => 1,
# yydebug => 0x1F
);In such case the associated semantic action creates a node labelled
_PLUS_LISTwhose children are the attributes associated with the items in the plus list. As it happens when using the %tree directive, syntactic tokens are skipped.
When executing the example above but under the %tree directive the ouput changes:
examples/eyapplanguageref$ head -3 List3_tree.yp; eyapp List3_tree.yp
# List3.yp
%semantic token 'c'
%tree
examples/eyapplanguageref$ diff List3.yp List3_tree.yp
2a3
> %treeIf we now run the client program with input ccdd we get a couple of syntax trees:
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' );
$VAR1 = bless( {
'children' => []
}, '_PLUS_LIST' );The node associated with the list of ds is empty since terminal d wasn't declared semantic.
When Nodes Dissappear from Lists
When under the influence of the %tree directive the action associated with a list operator is to flat the children in a single list.
In the former example, the d nodes don't show up since 'd' is a syntactic token. However, it may happen that changing the status of 'd' to semantic will not suffice.
When inserting the children, the tree (%tree) node construction method (YYBuildAST) omits any attribute that is not a reference. Therefore, when inserting explicit actions, it is necessary to guarantee that the returned value is a reference or a semantic token to assure the presence of the value in the lists of children of the node. Certainly you can use this property to prune parts of the tree. Consider the following example:
examples/eyapplanguageref$ 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 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error,
4 yybuildingtree => 1,
5 #, yydebug => 0x1F
6 );
7 }The execution gives an ouput like this:
examples/eyapplanguageref$ 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_LISTThough 'd' was declared semantic the default action associated with the production D: 'd' in line 16 returns $_[1] (that is, the scalar 'd'). Since it is not a reference it won't be inserted in the list of children of _PLUS_LIST.
Recovering the Missing Nodes
The solution is to be sure that the attribute is a reference:
pl@nereida:~/LEyapp/examples$ 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 => $_[1], children =>[]}, 'DES';
19 }
20 ;
21
22 %%Now the attribute associated with D is a reference and appears in the list of children of _PLUS_LIST:
pl@nereida:~/LEyapp/examples$ 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)
$VAR1 = bless( {
'children' => [
bless( {
'children' => [],
'attr' => 'd'
}, 'DES' ),
bless( {
'children' => [],
'attr' => 'd'
}, 'DES' )
]
}, '_PLUS_LIST_2' );
_PLUS_LIST_2(DES,DES)Building a Tree with Parse::Eyapp::Node->new
The former solution consisting on writing by hand the code to build the node may suffice when dealing with a single node. Writing by hand the code to build a node is a cumbersome task. Even worst: though the node built in the former example looks like a Parse::Eyapp node actually isn't. Parse::Eyapp nodes always inherit from Parse::Eyapp::Node and consequently have access to the methods in such package. Thefollowing execution using the debugger illustrates the point:
pl@nereida:~/LEyapp/examples$ 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 $_[1]->str."\n";through the command f ListWithRefs.eyp we inform the debugger that subsequent commands will refer to such file. Next we execute the program up to the semantic action associated with the production rule S: 'c'+ D+ (line 9)
DB<2> c 9 # Continue up to line 9 of ListWithRefs.eyp
ccdd
ListWithRefs::CODE(0x84ebe5c)(ListWithRefs.eyp:9):
9: print Dumper($_[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 arraythe 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:
DB<6> x $_[2]->child(0)->str
Can't locate object method "str" via package "DES" at \
(eval 11)[/usr/share/perl/5.8/perl5db.pl:628] line 2, <STDIN> line 1.
DB<7> More robust than the former solution of building the node by hand is to use the constructor Parse::Eyapp::Node->new: The method Parse::Eyapp::Node->new is uset to build forests of syntactic trees.
It receives a list of terms describing the trees and - optionally - a reference to a subroutine used to set up the attributes of the just created nodes. After the creation of the trees the sub is called by Parse::Eyapp::Node->new with arguments the list of references to the nodes (in the order in which they appear in the terms, from left to right). Parse::Eyapp::Node->new returns a list of references to the jsut created nodes. In a scalar context returns a reference to the first of such trees. See an example:
pl@nereida:~/LEyapp/examples$ 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' );
$VAR1 = bless( {'n' => 4,'children' => []}, 'F' );See the following example in which the nodes associated with 'd' are explictly constructed:
pl@nereida:~/LEyapp/examples$ 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)
$VAR1 = bless( {
'children' => [
bless( {
'children' => [
bless( { 'children' => [], 'attr' => 'd' }, 'TERMINAL' )
]
}, 'DES' ),
bless( {
'children' => [
bless( { 'children' => [], 'attr' => 'd' }, 'TERMINAL' )
]
}, 'DES' )
]
}, '_PLUS_LIST_2' );
_PLUS_LIST_2(DES(TERMINAL),DES(TERMINAL))The * operator
Any list operator operates on the factor to its left. A list in the right hand side of a production rule counts as a single symbol.
Both operators * and + can be used with the format X <* Separator>. In such case they describe lists of Xs separated by separator. See an example:
pl@nereida:~/LEyapp/examples$ 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 $_[1]
22 }
23 ;
24
25 %%The rule
S: ('c' <* ','> 'd')*has only two items in its right hand side: the (separated by commas) list of cs and the list of ds. The production rule is equivalent to:
pl@nereida:~/LEyapp/examples$ eyapp -v CsBetweenCommansAndD.eyp
pl@nereida:~/LEyapp/examples$ head -11 CsBetweenCommansAndD.output | cat -n
1 Rules:
2 ------
3 0: $start -> S $end
4 1: STAR-1 -> STAR-1 ',' 'c'
5 2: STAR-1 -> 'c'
6 3: STAR-2 -> STAR-1
7 4: STAR-2 -> /* empty */
8 5: PAREN-3 -> STAR-2 'd'
9 6: STAR-4 -> STAR-4 PAREN-3
10 7: STAR-4 -> /* empty */
11 8: S -> STAR-4The semantic action associated with * is to return a reference to a list with the attributes of the matching items.
When working -as in the example - under a tree creation directive it returns a node belonging to a class named _STAR_LIST_#number whose children are the items in the list. The #number is the ordinal number of the production rule as it appears in the .output file. The attributes must be references or associated with semantic tokens to be included in the list. Notice -in the execution of the former example that follows - how the node for PAREN-3 has been eliminated from the tree. Parenthesis nodes are - generally - obivated:
pl@nereida:~/LEyapp/examples$ 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 supressed.
Giving Names to Lists
To set the name of the node associated with a list operator the %name directive must precede the operator as in the following example:
pl@nereida:~/LEyapp/examples$ 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 $_[1]
24 }
25 ;
26
27 %%The execution shows the renamed nodes:
pl@nereida:~/LEyapp/examples$ 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]Optionals
The X? operator stands for the presence or omission of X.
The grammar:
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 $_[2]->children;
8 }
9 ;
10
11 %%is equivalent to:
pl@nereida:~/LEyapp/examples$ eyapp -v List5
pl@nereida:~/LEyapp/examples$ head -7 List5.output
Rules:
------
0: $start -> S $end
1: OPTIONAL-1 -> 'c'
2: OPTIONAL-1 -> /* empty */
3: S -> 'c' OPTIONAL-1When yybuildingtree is false the associated attribute is a list that will be empty if CX> does not show up.
Under the %tree directive the action creates an _OPTIONAL node:
pl@nereida:~/LEyapp/examples$ use_list5.pl
cc
_OPTIONAL_1(TERMINAL)
c
pl@nereida:~/LEyapp/examples$ use_list5.pl
c
_OPTIONAL_1Parenthesis
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 %%is equivalent to:
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
$VAR1 = [
[
'a',
[ [ 'a', 'c' ], 'b' ]
],
'b'
];when working under a tree directive or when the attribute buildingtree is set via theYYBuildingtree method the semantic action returns a node with children the attributes of the symbols between parenthesis. As usual attributes which aren't references will be skipped from the list of children. See an example:
pl@nereida:~/LEyapp/examples$ 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 $r;
21 }
22 ;
23 %%The example shows (line 8) how to rename a _PAREN node. The %name CLASSNAME goes after the opening parenthesis.
The call to YYBuildAST at line 13 with argumetns the attributes of the symbols on the right hand side returns the node describing the current production rule. Notice that line 13 can be rewritten as:
goto &Parse::Eyapp::Driver::YYBuildAST;At line 18 the node for the rule is explictly created using Parse::Eyapp::Node-new>. The handler passed as second argument is responsible for setting the value of the atribute attr of the just created TERMINAL node.
Let us see an execution:
pl@nereida:~/LEyapp/examples$ 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.
The execution continues with the reduction or antiderivation 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'
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: bThe last reduction shown is by the rule: S -> ('a' S )'b':
S -> ('a' S )'b'
Attribute of the first symbol:
$VAR1 = bless( {
'children' => [
bless( { 'children' => [], 'attr' => 'a', 'token' => 'a' }, 'TERMINAL' ),
bless( {
'children' => [
bless( {
'children' => [
bless( { 'children' => [], 'attr' => 'a', 'token' => 'a' }, 'TERMINAL' ),
bless( {
'children' => [
bless( { 'children' => [], 'attr' => 'c' }, 'TERMINAL' )
]
}, 'C' )
]
}, 'AS' ),
bless( { 'children' => [], 'attr' => 'b', 'token' => 'b' }, 'TERMINAL' )
]
}, 'S_2' )
]
}, 'AS' );
Attribute of the second symbol: bActions Inside Parenthesis
Though is a practice to avoid, since it clutters the code, it is certainly permitted to introduce actions between the parenthesis, as in the example below:
pl@nereida:~/LEyapp/examples$ 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"; $_[1] }) S 'b'
12 {
13 print "S -> (a ) S b\n"
14 }
15 ;
16 %%This is the output when executing this program with input aaacbbb:
pl@nereida:~/LEyapp/examples$ 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 bNAMES FOR ATTRIBUTES
Attributes can be referenced by meaningful names instead of the classic error-prone positional approach using the dot notation:
rhs: rhseltwithid *
rhseltwithid :
rhselt '.' IDENT
| '$' rhselt
| rhseltfor 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.
The dolar notation $A can be used as an abbreviation of A.A.
DEFAULT ACTIONS
When no action is specified both yapp and eyapp implicitly insert the semantic action { $_[1] }. In Parse::Eyapp you can modify such behavior using the %defaultaction { Perl code } directive. The { Perl code } clause that follows the %defaultaction directive is executed when reducing by any production for which no explicit action was specified.
An Example of Default Action: Translator from Infix to Postfix
See an example that translates an infix expression like a=b*-3 into a postfix expression like a b 3 NEG * = :
# File Postfix.eyp (See the examples/ directory)
%right '='
%left '-' '+'
%left '*' '/'
%left NEG
%defaultaction { return "$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.eypNext, 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 $parser->Run;Now we can run the client program:
nereida:~/src/perl/YappWithDefaultAction/examples> usepostfix.pl
Write an expression: -(2*a-b*-3)
2 a * b 3 NEG * - NEGDefault Actions, %name and YYName
In eyapp each production rule has a name. The name of a rule can be explicitly given by the programmer using the %name directive. For example, in the piece of code that follows the name ASSIGN is given to the rule exp: VAR '=' exp.
When no explicit name is given the rule has an implicit name. The implicit name of a rule is shaped by concatenating the name of the syntactic variable on its left, an underscore and the ordinal number of the production rule Lhs_# as it appears in the .output file. Avoid giving names matching such pattern to production rules. The patterns /${lhs}_\d+$/ where ${lhs} is the name of the syntactic variable are reserved for internal use by eyapp.
pl@nereida:~/LEyapp/examples$ 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 ')' { $_[2] }
45 ;Inside a semantic action the name of the current rule can be recovered using the method YYName of the parser object.
The default action (lines 8-12) computes as attribute of the left hand side a reference to an object blessed in the name of the rule. The object has an attribute children which is a reference to the list of children of the node. The call to grep
11 bless { children => [ grep {ref($_)} @_] }, $name;excludes children that aren't references. Notice that the lexical analyzer only returns references for the NUM and VAR terminals:
59 sub _Lexer {
60 my($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($tree); }
9 else { print "Cadena no válida\n"; }When executed with input a=(2+3)*b the parser produces the following tree:
ASSIGN(TIMES(PLUS(NUM[2],NUM[3]), VAR[b]))See the result of an execution:
pl@nereida:~/LEyapp/examples$ 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('SUBSTRACT'); # 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 ')' { $_[2] }
22 ;When the client program is executed we can see the presence of the SUBSTRACT nodes:
pl@nereida:~/LEyapp/examples$ useyynamedynamic.pl
2-b
$VAR1 = [
bless( {
'children' => [
bless( {
'attr' => '2'
}, 'NUM' ),
bless( {
'attr' => 'b'
}, 'VAR' )
]
}, 'SUBSTRACT' )
];GRAMMAR REUSE
Reusing Grammars Using Inheritance
An alternative method is to use inheritance. The client inherits the grammar and expands it with methods that implement the actions. Here is the grammar:
pl@europa:~/LEyapp/examples/recycle$ 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 => $debug,
59 );
60 }The following program defines two classes: CalcActions that implements the actions for the calculator and package PostActions that implements the actions for the infix to postfix translation:
pl@europa:~/LEyapp/examples/recycle$ 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 "$p\n";The subroutine used as default action in NoacInh.eyp is so useful that is packed as the Parse::Eyapp::Driver method YYDelegateaction. See files examples/recycle/NoacYYDelegateaction.eyp and examples/recycle/icalcu_and_ipost_yydel.pl for an example of use of YYDelegateaction.
Reusing Grammars by Dynamic Substitution of Semantic Actions
The methods YYSetaction and YYAction of the parser object provide a way to selectively substitute some actions of a given grammar. Let us consider once more a postfix to infix translator:
pl@europa:~/LEyapp/examples/recycle$ 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 $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error,
66 #yydebug => 0xFF
67 );
68 }The program rewritepostfixwithactions.pl uses the former grammar to translate infix expressions to postfix expressions. It also implements a calculator reusing the grammar in PostfixWithActions.eyp. It does so using the YYSetaction method. The semantic actions for the productions named
ASSIGN
PLUS
TIMES
DIV
NEG
are selectively substituted by the appropriate actions, while the other semantic actions remain unchanged:
pl@europa:~/LEyapp/examples/recycle$ 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, $debug);When running this program the output is:
pl@europa:~/LEyapp/examples/recycle$ ./rewritepostfixwithactions.pl
Write an expression: 2*3+4
2 3 TIMES 4 PLUS
10ABSTRACT SYNTAX TREES: %tree AND %name
%tree Default Names
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.
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.
If no explict 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 ')' { $_[2] }
;
%%The tree produced by the parser when feed with input a=2*b is:
pl@europa:~/LEyapp/examples/eyapplanguageref$ ./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:
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 $end
1: PLUS-1 -> PLUS-1 ';' exp
2: PLUS-1 -> exp
3: line -> PLUS-1
4: exp -> NUM
5: exp -> VAR
6: exp -> VAR '=' exp
7: exp -> exp '+' exp
8: exp -> exp '-' exp
9: exp -> exp '*' exp
10: exp -> exp '/' exp
11: exp -> '-' exp
12: exp -> '(' exp ')'We can see now that the node exp_9 corresponds to the production exp -> exp '*' exp. Observe also that the Eyapp production:
line: exp <+ ';'>
actually produces the productions:
1: PLUS-1 -> PLUS-1 ';' exp
2: PLUS-1 -> expand that the name of the class associated with the non empty list is _PLUS_LIST.
%tree Giving Explicit Names
A production rule can be named using the %name IDENTIFIER directive. For each production rule a namespace/package is created. The IDENTIFIER is the name of the associated package. Therefore, by modifying the former grammar with additional %name directives:
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/eyapplanguageref$ 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 ')' { $_[2] }
;we are explictly naming the productions. Thus, all the node instances corresponding to the production exp: VAR '=' exp will belong to the class ASSIGN. Now the tree for a=2*b becomes:
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/eyapplanguageref$ ./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 ([+*?]).
TERMINAL Nodes
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.
Explicit Actions Inside %tree
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
Explicitly Building Nodes With YYBuildAST
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):
$ 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.
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.
Actually, the %tree directive is semantically equivalent to:
%default action { goto &Parse::Eyapp::Driver::YYBuildAST }Returning non References Under %tree
When a explicit user action returns s.t. that is not a reference no node will be inserted. This fact can be used to supress nodes in the AST being built. See the following example (file 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:
$ 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.
Intermediate actions and %tree
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):
$ 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.
Syntactic and Semantic tokens
Parse::Eyapp diferences 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:
$ 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 EXPRESION_LIST + ';'>
20 { $_[1] } /* list of expressions separated by ';' */
21 ;
22
23 /* The %name directive defines the name of the class to which the node being built belongs */
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:
EXPRESION_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
) # EXPRESION_LISTTERMINAL 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 identifer, like NUM or VAR are, by default, semantic tokens and appear in the AST.
Changing the Status of a Token
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:
%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])).
Saving the Information of Syntactic Tokens in their Father
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.
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:
sub TERMINAL::save_attributes {
# $_[0] is a syntactic terminal
# $_[1] is the father.
push @{$_[1]->{lines}}, $_[0]->[1]; # save the line number
}The bypass clause and the %no bypass directive
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.
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 NUMHere - if the
bypassclause is used - theNUMnode will be bypassed and the childTERMINALbuilt from the information provided by the lexical analyzer will be renamed/reblessed asNUM.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 NEGthough 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:
exp : %no bypass UMINUS
'-' $exp %prec NEGThe following example (file examples/bypass.pl) is the equivalent of the Parse::Eyapp SYNOPSIS example but using the bypass clause instead:
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 EXPRESION_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:
nereida:~/src/perl/YappWithDefaultAction/examples> bypass.pl
************
EXPRESION_LIST(ASSIGN(TERMINAL[a],PLUS(TIMES(NUM[2],UMINUS(NUM[3])),TIMES(VAR[b],NUM[0]))))
************
EXPRESION_LIST(ASSIGN(TERMINAL[a],NUM[-6]))As you can see the trees are more compact when using the bypass directive.
The alias clause of the %tree directive
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 dolar notations are used for this. When dealing with a production like:
A:
%name A_Node
Node B.bum N.pum $Chipmethods bum, pum and Chip will be created for the class A_Node. Those methods wil 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.
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.
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"; # 1The tree $t for the expression "a = -(2*3+5-1)\n" is:
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:
$ alias_and_yyprefix.pl
R::S::TERMINAL
***************
R::S::PLUS(R::S::TIMES(R::S::NUM,R::S::NUM),R::S::NUM)
***************
R::S::NUMAs 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:
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($res);
CalcwithAttributeGrammar.plThe program computes the tree for expression for expression a = -(2*3+5-1) which is:
ASSIGN(TERMINAL,UMINUS(MINUS(PLUS(TIMES(NUM,NUM),NUM),NUM)))The children of the binary nodes can be accessed through the left and right methods.
About the Encapsulation of Nodes
There is no encapsulation of nodes. The user/client knows that they are hashes that can be decorated with new keys/attributes. All nodes in the AST created by %tree are Parse::Eyapp::Node nodes. The only reserved field is children which is a reference to the array of children. You can always create a Node class by hand by inheriting from Parse::Eyapp::Node.
NAMING SCHEMES
Explict names can be given to grammar productions via the %name directive. An alternative to explictly gave names to rules is to define a naming scheme via the Eyapp directive %namingscheme.
The Eyapp directive %namingscheme is followed by some Perl code. Such Perl code must return a reference to a subroutine that will be called each time a new production right hand side is parsed. This Perl code receives a Parse::Eyapp object that describes the grammar. Observe that the code after the %namingscheme directive is evaluated during the early phases of the compilation of the input grammar. As an example, see lines 22-38 below (the example can be found in the directory examples/naming of the accompanying distribution):
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/naming$ 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:
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 productiongive_lhs_name: The name of the production is the name of the Left Hand Side of the Production Rulegive_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).
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:
22 %namingscheme {
23 #Receives a Parse::Eyapp object describing the grammar
24 my $self = shift;
25
26 $self->tokennames(
27 '=' => 'ASSIGN',
28 '+' => 'PLUS',
29 '*' => 'TIMES',
30 '-' => 'MINUS',
31 '/' => 'DIV',
32 '^' => 'EXP',
33 );
34
35 # returns the handler that will give names
36 # to the right hand sides
37 \&give_token_name;
38 }As it is illustrated in the example, the method tokennames of Parse::Eyapp objects provide a way to give identifier names to tokens that are defined by strings. When we execute the former program with input a=2*-3 we got the following output:
lusasoft@LusaSoft:~/src/perl/Eyapp/examples/naming$ 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 }SEE ALSO
Parse::Eyapp, Parse::Eyapp::eyapplanguageref, Parse::Eyapp::debugingtut, Parse::Eyapp::defaultactionsintro, Parse::Eyapp::translationschemestut, Parse::Eyapp::Driver, Parse::Eyapp::Node, Parse::Eyapp::YATW, Parse::Eyapp::Treeregexp, Parse::Eyapp::Scope, Parse::Eyapp::Base,
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.
Man pages of yacc(1),
Man pages of bison(1),
ocamlyacc tutorial at http://plus.kaist.ac.kr/~shoh/ocaml/ocamllex-ocamlyacc/ocamlyacc-tutorial/ocamlyacc-tutorial.html
REFERENCES
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
AUTHOR
Casiano Rodriguez-Leon (casiano@ull.es)
ACKNOWLEDGMENTS
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. Special thanks to my family and Larry Wall.
LICENCE AND COPYRIGHT
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:
- Around line 1959:
Non-ASCII character seen before =encoding in 'válida\n";'. Assuming CP1252