XS::Parse::Sublike(3pm)

NAME
"XS::Parse::Sublike" - XS functions to assist in parsing "sub"-like syntax

DESCRIPTION
This module provides some XS functions to assist in writing parsers for "sub"-like 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_sublike
  void boot_xs_parse_sublike(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_sublike(0.04);

  xs_parse_sublike
   int xs_parse_sublike(const struct XSParseSublikeHooks *hooks, void *hookdata, OP **op_ptr)

This function performs the actual parsing of a "sub"-like keyword. It expects the lexer to be at a position just after the introduction keyword has been consumed, and will proceed to parse an optional name, list of attributes, signature (if enabled by "use feature 'signatures'"), and code body. The return value and "op_ptr" can be used directly from the keyword plugin function. It is intended this function be invoked from it, and the result returned directly.

For a more automated handling of keywords, see "register_xs_parse_sublike".

hooks should be a structure that can provide optional function pointers used to customise the parsing process at various stages. hookdata is an opaque pointer which is passed through to each of the hook stage functions.

  register_xs_parse_sublike
   void register_xs_parse_sublike(const char *keyword,
     const struct XSParseSublikeHooks *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::Sublike" itself.

When the keyword is encountered, the hook's "permit" function is first tested to see if the keyword is permitted at this point. If the function returns true then the keyword is consumed and parsed as per "xs_parse_sublike".

hookdata is an opaque pointer which is passed through to each of the hook stage functions when they are invoked.

  xs_parse_sublike_any
   int xs_parse_sublike_any(const struct XSParseSublikeHooks *hooks, void *hookdata,
     OP **op_ptr)

This function expects to consume an introduction keyword at the lexer position which is either "sub" or the name of another "sub"-like keyword, which has been previously registered using "register_xs_parse_sublike". It then proceeds to parse the subsequent syntax similar to how it would have parsed if encountered by the module's own keyword parser plugin, except that the second set of hooks given here also take effect.

If a regular "sub" is encountered, then this is parsed using the hooks in a similar way to xs_parse_sublike().

If a different registered "sub"-like keyword is encountered, then parsing is performed using both sets of hooks - the ones given to this function as well as the ones registered with the keyword. This allows their effects to combined. The hooks given by the hooks argument are considered to be on the "outside" from those of the registered keyword "inside". The outside ones run first for all stages, except "pre_blockend" which runs them inside-out.

hookdata is an opaque pointer which is passed through to each of the hook stage functions when they are invoked.

Note that this function is now vaguely discouraged, in favour of using a prefixing keyword instead, by using the "XS_PARSE_SUBLIKE_FLAG_PREFIX" flag.

  xps_signature_add_param
   void xps_signature_add_param(struct XSParseSublikeContext *ctx,
      struct XPSSignatureParamDetails *details);

Since version 0.31; experimental.

This experimental function may only be called during the "start_signature" or "finish_signature" hook stages. It is used to insert extra signature parameters, either at the beginning (when called by the start hook), or at the end (when called by the finish hook). It takes details of the parameter to add from an addressed structure, which has the following fields.

   struct XPSSignatureParamDetails {
      U32 ver;

      char sigil;
      PADOFFSET padix;
   };

The caller must set the ver field equal to "XSPARSESUBLIKE_ABI_VERSION".

The sigil field gives the leading sigil of the parameter; "$" for mandatory scalars, "@" or "%" for a final slurpy. The padix field gives the pad offset for a pad variable to store the value into. The caller is (currently) responsible for creating that pad variable.

At the present version, this API cannot create optional, or named parameters. These abilities may be added in a later version which expands on the structure's definition to add new fields to support this.

  xps_signature_query_*
   IV xps_signature_query_params(struct XSParseSublikeContext *ctx);

   IV xps_signature_query_optparams(struct XSParseSublikeContext *ctx);

   char xps_signature_query_slurpy(struct XSParseSublikeContext *ctx);

Since version 0.37; experimental.

These experimental functions may only be called during the "start_signature" or "finish_signature" hook stages. They are used to query details about the accumulated set of signature parameters. The "_params" query returns the total number of defined parameters - counting all mandatory, optional, and a final slurpy if present. "_optparams" returns a count of only the optional parameters. "_slurpy" returns the sigil character of a final slurpy parameter ("@" or "%"), or zero if no slurpy parameter is present.

PARSE CONTEXT
The various hook stages all share state about the ongoing parse process using various fields of the "XSParseSublikeContext" structure.

   struct XSParseSublikeContext {
      SV *name;
      OP *attrs;
      OP *body;
      CV *cv;
      U32 actions;
      HV *moddata;
   }

The "actions" field will contain a bitmask of action flags that control the various steps that "XS::Parse::Sublike" might take inbetween invoking hook stages. The initial value of this field is set after the name-parsing stage, depending on whether or not a name is found. Stage hook functions may modify the field to adjust the subsequent behaviour.

At the current ABI version, a module will have to set the "XS_PARSE_SUBLIKE_COMPAT_FLAG_DYNAMIC_ACTIONS" bit of the "flags" field in order to make use of the actions field. A future ABI version may remove this restriction.

XS_PARSE_SUBLIKE_ACTION_CVf_ANON
    If set, the start_subparse() call will be set up for an anonymous function protosub; if not it will be set for a named function. This is set by default if a name was not found.

XS_PARSE_SUBLIKE_ACTION_SET_CVNAME
    If set, the newly-constructed CV will have the given name set on it. This is set by default if a name was found.

    On Perl versions 5.22 and above, this flag can be set even if "XS_PARSE_SUBLIKE_ACTION_INSTALL_SYMBOL" is not. In this case, the CV will not be reachable via the symbol table, even though it knows its own name and pretends that it is. On earlier versions of perl this flag will be ignored in that case.

XS_PARSE_SUBLIKE_ACTION_INSTALL_SYMBOL
    If set, the newly-constructed CV will be installed into the symbol table at its given name. Note that it is not possible to enable this flag without also enabling "XS_PARSE_SUBLIKE_ACTION_SET_CVNAME". This is set by default if a name was found.

XS_PARSE_SUBLIKE_ACTION_INSTALL_LEXICAL
    If set, the newly-constructed CV will be installed into the currently compiling lexical pad as its given name. This is only available on Perl version 5.18 or above, and conflicts with the alternative of "XS_PARSE_SUBLIKE_ACTION_INSTALL_SYMBOL". This is set by default if a name was found and the "my" keyword appeared before the construction.

XS_PARSE_SUBLIKE_ACTION_REFGEN_ANONCODE
    If set, the syntax will yield the "OP_REFGEN" / "OP_ANONCODE" optree fragment typical of anonymous code expressions; if not it will be "OP_NULL". This is set by default if a name was not found.

XS_PARSE_SUBLIKE_ACTION_RET_EXPR
    If set, the syntax will parse like an expression; if not it will parse like a statement. This is set by default if a name was not found.

The moddata field will point towards an HV that modules can used to store extra data between stages. As a naming convention a module should prefix its keys with its own module name and a slash character, "Some::Module/field". The field will point to a newly-created HV for every parse invocation, and will be released when each parse is complete.

PARSE HOOKS
The "XSParseSublikeHooks" structure provides the following hook stages, which are invoked in the given order.

The structure has a flags field, which controls various optional parts of operation. The following flags are defined.

XS_PARSE_SUBLIKE_FLAG_BODY_OPTIONAL
    If not set, the require_parts field will imply the "XS_PARSE_SUBLIKE_PART_BODY" flag, making the body part required. By setting this flag this will no longer happen. If all hooks agree, then the body will become optional.

XS_PARSE_SUBLIKE_FLAG_PREFIX
    If set, the keyword is considered to be a prefix that can be placed in front of "sub" or another sub-like keyword, to add its set of hooks in addition to those of the following keyword. These prefices may be further stacked.

XS_PARSE_SUBLIKE_FLAG_ALLOW_PKGNAME
    Since version 0.31.

    If not set, then fully-qualified identifiers that include a package name are not allowed when declaring a named function. If all hooks agree by all setting the flag, then the name may be fully-qualified to add the newly-declared function into a different package.

XS_PARSE_SUBLIKE_FLAG_SIGNATURE_NAMED_PARAMS
    If set, use the extended signature parser of this module when parsing a signature and additionally permit the use of named parameter syntax, as documented in Sublike::Extended.

XS_PARSE_SUBLIKE_FLAG_SIGNATURE_PARAM_ATTRIBUTES
    If set, use the extended signature parser of this module when parsing a signature and additionally permit the use of attribute declarations on parameter variables, as documented in Sublike::Extended.

In addition there are two "U8" fields named require_parts and skip_parts which control the behaviour of various parts of the syntax which are usually optional. Any parts with bits set in require_parts become non-optional, and an error if they are missing. Any parts with bits set in skip_parts will skip the relevant part of the parsing process.

When multiple sets of hooks are combined by the "xs_parse_sublike_any" function, or as part of parsing prefixing keywords, these bitmasks are accumulated together with inclusive or. Any part required by any set of hooks will still be required; any step skipped by either will be skipped entirely.

If the same bit is set in both fields then the relevant parsing step will not be performed but it will still be an error for that section to be missing. This is likely not useful.

Note that for skipped parts, only the actual parsing steps are skipped. A hook function can still set the relevant fields in the context structure anyway to force a particular value for those parts.

XS_PARSE_SUBLIKE_PART_NAME
    The name of the function.

XS_PARSE_SUBLIKE_PART_ATTRS
    The attributes of the function.

    This part can be skipped, but the bit is ignored when in require_parts. It is always permitted to not provide any additional attributes to a function definition.

XS_PARSE_SUBLIKE_PART_SIGNATURE
    The parameter signature of the function.

    This part can be skipped, but it is always permitted not to provide a signature for a function definition even if the bit it set in require_parts. This is because such syntax only applies when "use feature 'signatures'" is in effect, and only on supporting perl versions.

    However, setting the bit in require_parts instead has the effect of enabling "use feature 'signatures'" (at least on supporting perl versions), thus permitting the syntax to use a signature even if the signatures feature was not previously enabled.

XS_PARSE_SUBLIKE_PART_BODY
    The actual body of the function, expressed as a brace-delimited block.

    This part cannot be skipped, but it can be made optional by omitting it from the require_parts field. Instead of the block, it is permitted to place a single semicolon (";") to act as a statement terminator; thus giving the same syntax as a subroutine forward declaration.

    In this case, the "body" and "cv" fields of the context structure will remain "NULL".

    This flag is currently implied on the require_parts field if the hook does not supply the "XS_PARSE_SUBLIKE_FLAG_BODY_OPTIONAL" flag; meaning that most use-cases will make it a required part.

  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_sublike".

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.

  Parse Name
At this point, the optional name is parsed and filled into the "name" field of the context.

  The "pre_subparse" Stage
   void (*pre_subparse)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata)

Invoked just before start_subparse() is called.

  Parse Attrs
At this point the optional sub attributes are parsed and filled into the "attrs" field of the context, then block_start() is called.

  The "filter_attr" Stage
   bool (*filter_attr)(pTHX_ struct XSParseSublikeContext *ctx,
      SV *attr, SV *val, void *hookdata);

If this optional stage is defined, then each individual attribute is passed through this optional filter function immediately as each is parsed. attr will be a string SV containing the name of the attribute, and val will either be "NULL", or a string SV containing the contents of the parens after its name (without the parens themselves).

If the filter returns "true", it indicates that it has in some way handled the attribute and it should not be added to the list given to newATTRSUB(). If the filter returns "false" it will be handled in the usual way; equivalent to the case where the filter function did not exist.

  The "post_blockstart" Stage
   void (*post_blockstart)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata)

Invoked after the block_start() function has been called. This hook stage may wish to perform any alterations of "PL_compcv" or related, inspect or alter the lexical pad, provide hints hash values, or any other tasks before the signature and code body are parsed.

  Parse Signature
If the perl version supports subroutine signatures, and the feature is enabled at this point, then an optional signatured is expected.

Since version 0.31: if the open parenthesis of a signature declaration is found, then the "start_signature" stage is invoked. Once the signature has been parsed, the "finish_signature" stage is invoked.

   void (*start_signature)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata);

   void (*finish_signature)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata);

Code in either of these hook stages is permitted to call "xps_signature_add_param", or any of the "xps_signature_query_*()" functions.

  Parse Body
At this point, the main body of the function is parsed and the optree is stored in the "body" field of the context. If a subroutine signature was found, the body will be prefixed with the signature ops as well.

  The "pre_blockend" Stage
   void (*pre_blockend)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata)

Invoked just before the block_end() function is invoked. The hook stage may wish to inspect or alter the optree stored in the "body" context field.

  The "post_newcv" Stage
   void (*post_newcv)(pTHX_ struct XSParseSublikeContext *ctx, void *hookdata)

Invoked just after newATTRSUB() has been invoked on the optree. The hook stage may wish to inspect or alter the CV stored in the "cv" context field.

AUTHOR
Paul Evans <leonerd@leonerd.org.uk>