NAME

XS::Parse::Keyword - XS functions to assist in parsing keyword syntax

DESCRIPTION

This module provides some XS functions to assist in writing syntax modules that provide new perl-visible syntax, primarily for authors of keyword plugins using the PL_keyword_plugin hook mechanism. It is unlikely to be of much use to anyone else; and highly unlikely to be any use when writing perl code using these. Unless you are writing a keyword plugin using XS, this module is not for you.

This module is also currently experimental, and the design is still evolving and subject to change. Later versions may break ABI compatibility, requiring changes or at least a rebuild of any module that depends on it.

XS FUNCTIONS

boot_xs_parse_keyword

   void boot_xs_parse_keyword(double ver);

Call this function from your BOOT section in order to initialise the module and parsing hooks.

ver should either be 0 or a decimal number for the module version requirement; e.g.

   boot_xs_parse_keyword(0.14);

register_xs_parse_keyword

   void register_xs_parse_keyword(const char *keyword,
     const struct XSParseKeywordHooks *hooks, void *hookdata);

This function installs a set of parsing hooks to be associated with the given keyword. Such a keyword will then be handled automatically by a keyword parser installed by XS::Parse::Keyword itself.

PARSE HOOKS

The XSParseKeywordHooks structure provides the following hook stages, which are invoked in the given order.

flags

The following flags are defined:

XPK_FLAG_EXPR

The parse or build function is expected to return KEYWORD_PLUGIN_EXPR.

XPK_FLAG_STMT

The parse or build function is expected to return KEYWORD_PLUGIN_STMT.

These two flags are largely for the benefit of giving static information at registration time to assist static parsing or other related tasks to know what kind of grammatical element this keyword will produce.

XPK_FLAG_AUTOSEMI

The syntax forms a complete statement, which should be followed by a statement separator semicolon (;). This semicolon is optional at the end of a block.

The semicolon, if present, will be consumed automatically.

XPK_FLAG_BLOCKSCOPE

The entire parse and build process will be wrapped in a pair of block_start() and block_end() calls. This ensures that, for example, any newly-introduced lexical variables do not escape from the scope of the syntax created by the keyword.

The `permit` Stage

   const char *permit_hintkey;
   bool (*permit) (pTHX_ void *hookdata);

Called by the installed keyword parser hook which is used to handle keywords registered by "register_xs_parse_keyword".

As a shortcut for the common case, the permit_hintkey may point to a string to look up from the hints hash. If the given key name is not found in the hints hash then the keyword is not permitted. If the key is present then the permit function is invoked as normal.

If not rejected by a hint key that was not found in the hints hash, the function part of the stage is called next and should inspect whether the keyword is permitted at this time perhaps by inspecting other lexical clues, and return true only if the keyword is permitted.

Both the string and the function are optional. Either or both may be present. If neither is present then the keyword is always permitted - which is likely not what you wanted to do.

The `check` Stage

   void (*check)(pTHX_ void *hookdata);

Invoked once the keyword has been permitted. If present, this hook function can check the surrounding lexical context, state, or other information and throw an exception if it is unhappy that the keyword should apply in this position.

The `parse` Stage

This stage is invoked once the keyword has been checked, and actually parses the incoming text into an optree. It is implemented by calling the first of the following function pointers which is not NULL. The invoked function may optionally build an optree to represent the parsed syntax, and place it into the variable addressed by out. If it does not, then a simple OP_NULL will be constructed in its place.

lex_read_space() is called both before and after this stage is invoked, so in many simple cases the hook function itself does not need to bother with it.

   int (*parse)(pTHX_ OP **out, void *hookdata);

If present, this should consume text from the parser buffer by invoking lex_* or parse_* functions and eventually return a KEYWORD_PLUGIN_* result value.

This is the most generic and powerful of the options, but requires the most amount of implementation work.

   int (*build)(pTHX_ OP **out, XSParseKeywordPiece *args[], size_t nargs, void *hookdata);

If parse is not present, this is called instead after parsing a sequence of arguments, of types given by the pieces field; which should be a zero- terminated array of piece types.

This alternative is somewhat less generic and powerful than providing parse yourself, but involves much less parsing work and is shorter and easier to implement.

   int (*build1)(pTHX_ OP **out, XSParseKeywordPiece *arg0, void *hookdata);

If neither parse nor build are present, this is called as a simpler variant of build when only a single argument is required. It takes its type from the piece1 field instead.

PIECES AND PIECE TYPES

When using the build or build1 alternatives for the parse phase, the actual syntax is parsed automatically by this module, according to the specification given by the pieces or piece1 field. The result of that parsing step is placed into the args or arg0 parameter to the invoked function, using a struct type consisting of the following fields:

   typedef struct
      union {
         OP *op;
         CV *cv;
         SV *sv;
         int i;
         struct {
            SV *name;
            SV *value;
         } attr;
         PADOFFSET padix;
         struct XSParseInfixInfo *infix;
      };
      int line;
   } XSParseKeywordPiece;

Which field of the anonymous union is set depends on the type of the piece. The line field contains the line number of the source file where parsing of that piece began.

Some piece types are "atomic", whose definition is self-contained. Others are structural, defined in terms of inner pieces. Together these form an entire tree-shaped definition of the syntax that the keyword expects to find.

Atomic types generally provide exactly one argument into the list of args (with the exception of literal matches, which do not provide anything). Structural types may provide an initial argument themselves, followed by a list of the values of each sub-piece they contained inside them. Thus, while the data structure defining the syntax shape is a tree, the argument values it parses into is passed as a flat array to the build function.

Some structural types need to be able to determine whether or not syntax relating some optional part of them is present in the incoming source text. In this case, the pieces relating to those optional parts must support "probing". This ability is also noted below.

Many of the atomic piece types have a variant which is optional; if the given input does not look like the expected syntax for the piece type then an _OPT-suffixed version of the type will instead yield NULL in its result pointer.

The type of each piece should be one of the following macro values.

XPK_BLOCK

atomic, can probe, emits op.

   XPK_BLOCK

A brace-delimited block of code is expected, passed as an optree in the op field. This will be parsed as a block within the current function scope.

This can be probed by checking for the presence of an open-brace ({) character.

Be careful defining grammars with this because an open-brace is also a valid character to start a term expression, for example. Given a choice between XPK_BLOCK and XPK_TERMEXPR, either of them could try to consume such code as

   { 123, 456 }

XPK_BLOCK_VOIDCTX, XPK_BLOCK_SCALARCTX, XPK_BLOCK_LISTCTX

Variants of XPK_BLOCK which wrap a void, scalar or list-context scope around the block.

XPK_PREFIXED_BLOCK

structural, emits op.

   XPK_PREFIXED_BLOCK(pieces ...)

Some pieces are expected, followed by a brace-delimited block of code, which is passed as an optree in the op field. The prefix pieces are parsed first, and their results are passed before the block itself.

The entire sequence, including the prefix items, is contained within a pair of block_start() / block_end() calls. This permits the prefix pieces to introduce new items into the lexical scope of the block - for example by the use of XPK_LEXVAR_MY.

A call to intro_my() is automatically made at the end of the prefix pieces, before the block itself is parsed, ensuring any new lexical variables are now visible.

In addition, the following extra piece types are recognised here:

XPK_SETUP

   void setup(pTHX_ void *hookdata);

   XPK_SETUP(&setup)

atomic, emits nothing.

This piece type runs a function given by pointer. Typically this function may be used to introduce new lexical state into the parser, or in some other way have some side-effect on the parsing context of the block to be parsed.

XPK_PREFIXED_BLOCK_ENTERLEAVE

A variant of XPK_PREFIXED_BLOCK which additionally wraps the entire parsing operation, including the block_start(), block_end() and any calls to XPK_SETUP functions, within a ENTER/LEAVE pair.

This should not make a difference to the standard parser pieces provided here, but may be useful behaviour for the code in the setup function, especially if it wishes to modify parser state and use the savestack to ensure it is restored again when parsing has finished.

XPK_ANONSUB

atomic, emits cv.

A brace-delimited block of code is expected, and assembled into the body of a new anonymous subroutine. This will be passed as a protosub CV in the cv field.

XPK_STAGED_ANONSUB

   XPK_STAGED_ANONSUB(stages ...)

structural, emits cv.

A variant of XPK_ANONSUB which accepts additional function pointers to be invoked at various points during parsing and compilation. These can be used to interrupt the normal parsing in a manner similar to XS::Parse::Sublike, though currently somewhat less flexibly.

The stages list may contain elements of the following types. Not every stage must be present, but any that are present must be in the following order. Multiple copies of each stage are permitted; they are invoked in the written order, with parser code happening inbetween.

XPK_ANONSUB_PREPARE

   XPK_ANONSUB_PREPARE(&callback)

atomic, emits nothing.

Invokes the callback before start_subparse().

XPK_ANONSUB_START

   XPK_ANONSUB_START(&callback)

atomic, emits nothing.

Invokes the callback after block_start() but before parsing the actual block contents.

XPK_ANONSUB_END

   OP *op_wrapper_callback(pTHX_ OP *o, void *hookdata);

   XPK_ANONSUB_END(&op_wrapper_callback)

atomic, emits nothing.

Invokes the callback after parsing the block contents but before calling block_end(). The callback may modify the optree if required and return a new one.

XPK_ANONSUB_WRAP

   XPK_ANONSUB_WRAP(&op_wrapper_callback)

atomic, emits nothing.

Invokes the callback after block_end() but before passing the optree to newATTRSUB(). The callback may modify the optree if required and return a new one.

XPK_ARITHEXPR. XPK_ARITHEXPR_OPT

atomic, emits op.

   XPK_ARITHEXPR

An arithmetic expression is expected, parsed using parse_arithexpr(), and passed as an optree in the op field.

XPK_ARITHEXPR_VOIDCTX, XPK_ARITHEXPR_OPT

XPK_ARITHEXPR_SCALARCTX, XPK_ARITHEXPR_SCALARCTX_OPT

Variants of XPK_ARITHEXPR which puts the expression in void or scalar context.

XPK_TERMEXPR, XPK_TERMEXPR_OPT

atomic, emits op.

   XPK_TERMEXPR

A term expression is expected, parsed using parse_termexpr(), and passed as an optree in the op field.

XPK_TERMEXPR_VOIDCTX, XPK_TERMEXPR_VOIDCTX

XPK_TERMEXPR_SCALARCTX, XPK_TERMEXPR_SCALARCTX_OPT

Variants of XPK_TERMEXPR which puts the expression in void or scalar context.

XPK_PREFIXED_TERMEXPR_ENTERLEAVE

   XPK_PREFIXED_TERMEXPR_ENTERLEAVE(pieces ...)

A variant of XPK_TERMEXPR which expects a sequence pieces first before it parses a term expression, similar to how XPK_PREFIXED_BLOCK_ENTERLEAVE works. The entire operation is wrapped in an ENTER/LEAVE pair.

This is intended just for use of XPK_SETUP pieces as prefixes. Any other pieces which actually parse real input are likely to cause overly-complex, subtle, or outright ambiguous grammars, and should be avoided.

XPK_LISTEXPR, XPK_LISTEXPR_OPT

atomic, emits op.

   XPK_LISTEXPR

A list expression is expected, parsed using parse_listexpr(), and passed as an optree in the op field.

XPK_LISTEXPR_LISTCTX, XPK_LISTEXPR_LISTCTX_OPT

Variant of XPK_LISTEXPR which puts the expression in list context.

XPK_IDENT, XPK_IDENT_OPT

atomic, can probe, emits sv.

A bareword identifier name is expected, and passed as an SV containing a PV in the sv field. An identifier is not permitted to contain a double colon (::).

XPK_PACKAGENAME, XPK_PACKAGENAME_OPT

atomic, can probe, emits sv.

A bareword package name is expected, and passed as an SV containing a PV in the sv field. A package name is similar to an identifier, except it permits double colons in the middle.

XPK_LEXVARNAME

atomic, emits sv.

   XPK_LEXVARNAME(kind)

A lexical variable name is expected, and passed as an SV containing a PV in the sv field. The kind argument specifies what kinds of variable are permitted, and should be a bitmask of one or more bits from XPK_LEXVAR_SCALAR, XPK_LEXVAR_ARRAY and XPK_LEXVAR_HASH. A convenient shortcut XPK_LEXVAR_ANY permits all three.

XPK_ATTRIBUTES

atomic, emits i followed by more args.

A list of :-prefixed attributes is expected, in the same format as sub or variable attributes. An optional leading : indicates the presence of attributes, then one or more of them are parsed. Attributes may be optionally separated by additional :s, but this is not required.

Each attribute is expected to be an identifier name, followed by an optional value wrapped in parentheses. Whitespace is NOT permitted between the name and value, as per standard Perl parsing rules.

   :attrname
   :attrname(value)

The i field indicates how many attributes were found. That number of additional arguments are then passed, each containing two SVs in the attr.name and attr.value fields. This number may be zero.

It is not an error for there to be no attributes present, or for the optional colon to be missing. In this case i will be set to zero.

XPK_VSTRING, XPK_VSTRING_OPT

atomic, can probe, emits sv.

A version string is expected, of the form v1.234 including the leading v character. It is passed as a version SV object in the sv field.

XPK_LEXVAR

atomic, emits padix.

   XPK_LEXVAR(kind)

A lexical variable name is expected and looked up from the current pad. The resulting pad index is passed in the padix field. No error happens if the variable is not found; the value NOT_IN_PAD is passed instead.

The kind argument specifies what kinds of variable are permitted, as per XPK_LEXVARNAME.

XPK_LEXVAR_MY

atomic, emits padix.

   XPK_LEXVAR_MY(kind)

A lexical variable name is expected, added to the current pad as if specified in a my expression, and passed as the pad index in the padix field.

The kind argument specifies what kinds of variable are permitted, as per XPK_LEXVARNAME.

XPK_COMMA, XPK_COLON, XPK_EQUALS

atomic, can probe, emits nothing.

A literal character (,, : or =) is expected. No argument value is passed.

XPK_AUTOSEMI

atomic, emits nothing.

A literal semicolon (;) as a statement terminator is optionally expected. If the next token is a closing brace to indicate the end of a block, then a semicolon is not required. If anything else is encountered an error will be raised.

This piece type is the same as specifying the XPK_FLAG_AUTOSEMI. It is useful to put at the end of a sequence that forms part of a choice of syntax, where some forms indicate a statement ending in a semicolon, whereas others may end in a full block that does not need one.

XPK_INFIX_*

atomic, can probe, emits infix.

An infix operator as recognised by XS::Parse::Infix. The returned pointer points to a structure allocated by XS::Parse::Infix describing the operator.

Various versions of the macro are provided, each using a different selection filter to choose certain available infix operators:

   XPK_INFIX_RELATION         # any relational operator
   XPK_INFIX_EQUALITY         # an equality operator like `==` or `eq`
   XPK_INFIX_MATCH_NOSMART    # any sort of "match"-like operator, except smartmatch
   XPK_INFIX_MATCH_SMART      # XPK_INFIX_MATCH_NOSMART plus smartmatch

XPK_LITERAL

atomic, can probe, emits nothing.

   XPK_LITERAL("literal")

A literal string match is expected. No argument value is passed.

This form should generally be avoided if at all possible, because it is very easy to abuse to make syntaxes which confuse humans and code tools alike. Generally it is best reserved just for the first component of a XPK_OPTIONAL or XPK_REPEATED sequence, to provide a "secondary keyword" that such a repeated item can look out for.

XPK_KEYWORD

atomic, can probe, emits nothing.

   XPK_KEYWORD("keyword")

A literal string match is expected. No argument value is passed.

This is similar to XPK_LITERAL except that it additionally checks that the following character is not an identifier character. This ensures that the expected keyword-like behaviour is preserved. For example, given the input "keyword", the piece XPK_LITERAL("key") would match it, whereas XPK_KEYWORD("key") would not because of the subsequent "w" character.

XPK_INTRO_MY

atomic, emits nothing.

Calls the core perl intro_my() function immediately. No input is consumed and no output value is generated. This is often useful after XPK_LEXVAR_MY.

XPK_WARNING

atomic, emits nothing.

   XPK_WARNING("message here")

Emits a warning by calling the core perl warn() function on the given string literal. This is equivalent to simply calling warn() from the build function, except that it is emitted immediately at parse time, so line numbering will be more accurate. Also, by placing it as part of an optional or choice sequence, the warning will only be emitted conditionally if that part of the grammar structure is encountered.

XPK_WARNING_...

Several variants of XPK_WARNING exist that are conditional on particular warning categories being enabled. These are ones that are likely to be useful at parse time:

   XPK_WARNING_AMBIGUOUS
   XPK_WARNING_DEPRECATED
   XPK_WARNING_EXPERIMENTAL
   XPK_WARNING_PRECEDENCE
   XPK_WARNING_SYNTAX

XPK_SEQUENCE

structural, might support probe, emits nothing.

   XPK_SEQUENCE(pieces ...)

A structural type which contains a number of pieces. This is normally equivalent to simply placing the pieces in sequence inside their own container, but it is useful inside XPK_CHOICE or XPK_TAGGEDCHOICE.

An XPK_SEQUENCE supports probe if its first contained piece does; i.e. is transparent to probing.

XPK_OPTIONAL

structural, emits i.

   XPK_OPTIONAL(pieces ...)

A structural type which may expects to find its contained pieces, or is happy not to. This will pass an argument whose i field contains either 1 or 0, depending whether the contents were found. The first piece type within must support probe.

XPK_REPEATED

structural, emits i.

   XPK_REPEATED(pieces ...)

A structural type which expects to find zero or more repeats of its contained pieces. This will pass an argument whose i field contains the count of the number of repeats it found. The first piece type within must support probe.

XPK_CHOICE

structural, can probe, emits i.

   XPK_CHOICE(options ...)

A structural type which expects to find one of a number of alternative options. An ordered list of types is provided, all of which must support probe. This will pass an argument whose i field gives the index of the first choice that was accepted. The first option takes the value 0.

As each of the options is interpreted as an alternative, not a sequence, you should use XPK_SEQUENCE if a sequence of multiple items should be considered as a single alternative.

It is not an error if no choice matches. At that point, the i field will be set to -1.

If you require a failure message in this case, set the final choice to be of type XPK_FAILURE. This will cause an error message to be printed instead.

   XPK_FAILURE("message string")

XPK_TAGGEDCHOICE

structural, can probe, emits i.

   XPK_TAGGEDCHOICE(choice, tag, ...)

A structural type similar to XPK_CHOICE, except that each choice type is followed by an element of type XPK_TAG which gives an integer. It is that integer value, rather than the positional index of the choice within the list, which is passed in the i field.

   XPK_TAG(value)

As each of the options is interpreted as an alternative, not a sequence, you should use XPK_SEQUENCE if a sequence of multiple items should be considered as a single alternative.

XPK_COMMALIST

structural, might support probe, emits i.

   XPK_COMMALIST(pieces ...)

A structural type which expects to find one or more repeats of its contained pieces, separated by literal comma (,) characters. This is somewhat similar to XPK_REPEATED, except that it needs at least one copy, needs commas between its items, but does not require that the first contained piece support probe (the comma itself is sufficient to indicate a repeat).

An XPK_COMMALIST supports probe if its first contained piece does; i.e. is transparent to probing.

XPK_PARENS

structural, can probe, emits nothing.

   XPK_PARENS(pieces ...)

A structural type which expects to find a sequence of pieces, all contained in parentheses as ( ... ). This will pass no extra arguments.

XPK_ARGS

structural, emits nothing.

   XPK_ARGS(pieces ...)

A structural type similar to XPK_PARENS, except that the parentheses themselves are optional; much like Perl's parsing of calls to known functions.

If parentheses are encountered in the input, they will be consumed by this piece and it will behave identically to XPK_PARENS. If there is no open parenthesis, this piece will behave like XPK_SEQUENCE and consume all the pieces inside it, without expecting a closing parenthesis.

XPK_BRACKETS

structural, can probe, emits nothing.

   XPK_BRACKETS(pieces ...)

A structural type which expects to find a sequence of pieces, all contained in square brackets as [ ... ]. This will pass no extra arguments.

XPK_BRACES

structural, can probe, emits nothing.

   XPK_BRACES(pieces ...)

A structural type which expects to find a sequence of pieces, all contained in braces as { ... }. This will pass no extra arguments.

Note that this is not necessary to use with XPK_BLOCK or XPK_ANONSUB; those will already consume a set of braces. This is intended for special constrained syntax that should not just accept an arbitrary block.

XPK_CHEVRONS

structural, can probe, emits nothing.

   XPK_CHEVRONS(pieces ...)

A structural type which expects to find a sequence of pieces, all contained in angle brackets as < ... >. This will pass no extra arguments.

Remember that expressions like a > b are valid term expressions, so the contents of this scope shouldn't allow arbitrary expressions or the closing bracket will be ambiguous.

XPK_PARENS_OPT, XPK_BRACKETS_OPT, XPK_BRACES_OPT, XPK_CHEVRONS_OPT

structural, can probe, emits i.

   XPK_PARENS_OPT(pieces ...)
   XPK_BRACKETS_OPT(pieces ...)
   XPK_BRACES_OPT(pieces ...)
   XPK_CHEVERONS_OPT(pieces ...)

Each of the four contained structure macros above has an optional variant, whose name is suffixed by _OPT. These pass an argument whose i field is either true or false, indicating whether the scope was found, followed by the values from the scope itself.

This is a convenient shortcut to nesting the scope within a XPK_OPTIONAL macro.

XPK_..._pieces

   XPK_SEQUENCE_pieces(ptr)
   XPK_OPTIONAL_pieces(ptr)
   ...

For each of the XPK_... macros that takes a variable-length list of pieces, there is a variant whose name ends with ..._pieces, taking a single pointer argument directly. This must point at a const XSParseKeywordPieceType [] array whose final element is the zero element.

Normally hand-written C code of a fixed grammar would be unlikely to use these forms, but they may be useful in dynamically-generated cases.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

To install XS::Parse::Keyword, copy and paste the appropriate command in to your terminal.

cpanm

cpanm XS::Parse::Keyword

CPAN shell

perl -MCPAN -e shell
install XS::Parse::Keyword

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

DESCRIPTION

XS FUNCTIONS

boot_xs_parse_keyword

register_xs_parse_keyword

PARSE HOOKS

flags

The permit Stage

The check Stage

The parse Stage

PIECES AND PIECE TYPES

XPK_BLOCK

XPK_BLOCK_VOIDCTX, XPK_BLOCK_SCALARCTX, XPK_BLOCK_LISTCTX

XPK_PREFIXED_BLOCK

XPK_PREFIXED_BLOCK_ENTERLEAVE

XPK_ANONSUB

XPK_STAGED_ANONSUB

XPK_ARITHEXPR. XPK_ARITHEXPR_OPT

XPK_ARITHEXPR_VOIDCTX, XPK_ARITHEXPR_OPT

XPK_ARITHEXPR_SCALARCTX, XPK_ARITHEXPR_SCALARCTX_OPT

XPK_TERMEXPR, XPK_TERMEXPR_OPT

XPK_TERMEXPR_VOIDCTX, XPK_TERMEXPR_VOIDCTX

XPK_TERMEXPR_SCALARCTX, XPK_TERMEXPR_SCALARCTX_OPT

XPK_PREFIXED_TERMEXPR_ENTERLEAVE

XPK_LISTEXPR, XPK_LISTEXPR_OPT

XPK_LISTEXPR_LISTCTX, XPK_LISTEXPR_LISTCTX_OPT

XPK_IDENT, XPK_IDENT_OPT

XPK_PACKAGENAME, XPK_PACKAGENAME_OPT

XPK_LEXVARNAME

XPK_ATTRIBUTES

XPK_VSTRING, XPK_VSTRING_OPT

XPK_LEXVAR

XPK_LEXVAR_MY

XPK_COMMA, XPK_COLON, XPK_EQUALS

XPK_AUTOSEMI

XPK_INFIX_*

XPK_LITERAL

XPK_KEYWORD

XPK_INTRO_MY

XPK_WARNING

XPK_WARNING_...

XPK_SEQUENCE

XPK_OPTIONAL

XPK_REPEATED

XPK_CHOICE

XPK_TAGGEDCHOICE

XPK_COMMALIST

XPK_PARENS

XPK_ARGS

XPK_BRACKETS

XPK_BRACES

XPK_CHEVRONS

XPK_PARENS_OPT, XPK_BRACKETS_OPT, XPK_BRACES_OPT, XPK_CHEVRONS_OPT

XPK_..._pieces

AUTHOR

Module Install Instructions

The `permit` Stage

The `check` Stage

The `parse` Stage