noble (3) XS::Parse::Infix.3pm.gz
NAME
"XS::Parse::Infix" - XS functions to assist in parsing infix operators
DESCRIPTION
This module provides some XS functions to assist in writing syntax modules that provide new infix
operators as perl syntax, primarily for authors of syntax plugins. It is unlikely to be of much use to
anyone else; and highly unlikely to be of any use when writing perl code using these. Unless you are
writing a syntax 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.
In addition, the places this functionality can be used are relatively small. Support for custom infix
operators as added in Perl development release "v5.37.7", and is therefore present in Perl "v5.38.0".
In addition, the various "XPK_INFIX_*" token types of XS::Parse::Keyword support querying on this module,
so some syntax provided by other modules may be able to make use of these new infix operators.
CONSTANTS
HAVE_PL_INFIX_PLUGIN
if( XS::Parse::Infix::HAVE_PL_INFIX_PLUGIN ) { ... }
This constant is true if built on a perl that supports the "PL_infix_plugin" extension mechanism, meaning
that custom infix operators registered with this module will actually be recognised by the perl parser.
No actual production releases of perl yet support this feature, but see above for details of development
versions which do.
XS FUNCTIONS
boot_xs_parse_infix
void boot_xs_parse_infix(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_infix(0.14);
parse_infix
bool parse_infix(enum XSParseInfixSelection select, struct XSParseInfixInfo **infop);
Since version 0.27.
This function attempts to parse syntax for an infix operator from the current parser position. If it is
successful, it fills in the variable pointed to by infop with a pointer to the actual information
structure and returns "true". If no suitable operator is found, returns "false".
xs_parse_infix_new_op
OP *xs_parse_infix_new_op(const struct XSParseInfixInfo *info, U32 flags,
OP *lhs, OP *rhs);
This function constructs a new optree fragment to represent invoking the infix operator with the given
operands. It should be used much the same as core perl's "newBINOP" function.
The "info" structure pointer would be obtained from the "infix" field of the result of invoking the
various "XPK_INFIX_*" token types from "XS::Parse::Keyword", or by calling "parse_infix" directly.
register_xs_parse_infix
void register_xs_parse_infix(const char *opname,
const struct XSParseInfixHooks *hooks, void *hookdata);
This function installs a set of parsing hooks to be associated with the given operator name. This new
operator will then be available via XS::Parse::Keyword by the various "XPK_INFIX_*" token types,
"parse_infix", or to core perl's "PL_infix_plugin" if available.
These tokens will all yield an info structure, with the following fields:
struct XSParseInfixInfo {
const char *opname;
OPCODE opcode; /* for built-in operators, or OP_CUSTOM for
custom-registered ones */
struct XSParseInfixHooks *hooks;
void *hookdata;
enum XSParseInfixClassification cls; /* since version 0.28 */
};
If the operator name contains any non-ASCII characters they are presumed to be in UTF-8 encoding. This
will matter for deparse purposes.
PARSE HOOKS
The "XSParseInfixHooks" structure provides the following fields which are used at various stages of
parsing.
struct XSParseInfixHooks {
U16 flags; /* currently ignored */
U8 lhs_flags;
U8 rhs_flags;
enum XSParseInfixClassification cls;
const char *wrapper_func_name;
const char *permit_hintkey;
bool (*permit)(pTHX_ void *hookdata);
OP *(*new_op)(pTHX_ U32 flags, OP *lhs, OP *rhs, SV **parsedata, void *hookdata);
OP *(*ppaddr)(pTHX);
/* optional */
void (*parse)(pTHX_ U32 flags, SV **parsedata, void *hookdata);
};
Flags
The "flags" field is currently ignored. It is defined simply to reserve the space in case used in a later
version. It should be set to zero.
The "lhs_flags" and "rhs_flags" fields give details on how to handle the left- and right-hand side
operands, respectively.
It should be set to one of the following constants, or left as zero:
XPI_OPERAND_TERM_LIST
The operand will be foced into list context, preserving the "OP_PUSHMARK" at the beginning. This
means that the ppfunc for this infix operator will have to "POPMARK" to find that.
XPI_OPERAND_LIST
The same as above.
Older versions used to provide constants named "XPI_OPERAND_ARITH" and "XPI_OPERAND_TERM" but they
related to an older version of the core perl branch. These names are now aliases for zero, and can be
removed from new code.
In addition the following extra bitflags are defined:
XPI_OPERAND_ONLY_LOOK
If set, the operator function promises that it will not mutate any of its passed values, nor allow
leaking of direct alias pointers to them via return value or other locations.
This flag is optional; omitting it when applicable will not change any observed behaviour. Setting it
may enable certain optimisations to be performed.
Currently, this flag simply enables an optimisation in the call-checker for infix operator wrapper
functions that take list-shaped operands. This optimisation discards an "OP_ANONLIST" operation which
would create a temporary anonymous array reference for its operand values, allowing a slight saving
of memory use and CPU time. This optimisation is only safe to perform if the operator does not mutate
or retain aliases of any of the arguments, as otherwise the caller might see unexpected modifications
or value references to the values passed.
The Selection Stage
The "cls" field gives a "classification" of the operator, suggesting what sort of operation it provides.
This is used as a filter by the various "XS::Parse::Keyword" selection macros.
The classification should be one of the "XPI_CLS_*" constants found and described further in the main
XSParseInfix.h file.
The "permit" Stage
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 "parse" Stage
If the optional "parse" hook function is present, it is called immediately after the parser has
recognised the presence of the named operator itself but before it attempts to consume the right-hand
side term. This hook function can attempt further parsing, in order to implement more complex syntax such
as hyper-operators.
When invoked, it is passed a pointer to an "SV *"-typed storage variable. It is free to use this variable
it desires to store a result, which will then later be made available to the "new_op" function.
The Op Generation Stage
If the infix operator is going to be used, then one of the "new_op" or the "ppaddr" fields explain how to
create a new optree fragment.
If "new_op" is defined then it will be used, and is expected to return an optree fragment that consumes
the LHS and RHS arguments to implement the semantics of the operator. If the optional "parse" stage had
been present earlier, the "SV **" pointer passed here will point to the same storage that "parse" had
previously had access to, so it can retrieve the results.
If "new_op" is not present, then the "ppaddr" will be used instead to construct a new BINOP of the
"OP_CUSTOM" type. If an earlier "parse" stage had stored additional results into the "SV *" variable
these will be lost here.
The Wrapper Function
Additionally, if the "wrapper_func_name" field is set to a string, this gives the (fully-qualified) name
for a function to be generated as part of registering the operator. This newly-generated function will
act as a wrapper for the operator.
For operators whose RHS is a scalar, the wrapper function is assumed to take two simple scalar arguments.
The result of invoking the function on those arguments will be determined by using the operator code.
$result = $lhs OP $rhs;
$result = WRAPPERFUNC( $lhs, $rhs );
For operators whose RHS is a list, the wrapper function takes at least one argument, possibly more. The
first argument is the scalar on the LHS, and the remaining arguments, however many there are, form the
RHS:
$result = $lhs OP @rhs;
$result = WRAPPERFUNC( $lhs, @rhs );
For operators whose LHS and RHS is a list, the wrapper function takes two arguments which must be array
references containing the lists.
$result = @lhs OP @rhs;
$result = WRAPPERFUNC( \@lhs, \@rhs );
This creates a convenience for accessing the operator from perls that do not support "PL_infix_plugin".
In the case of scalar infix operators, the wrapper function also includes a call-checker which attempts
to inline the operator directly into the callsite. Thus, in simple cases where the function is called
directly on exactly two scalar arguments (such as in the following), no "ENTERSUB" overhead will be
incurred and the generated optree will be identical to that which would have been generated by using
infix operator syntax directly:
WRAPPERFUNC( $lhs, $rhs );
WRAPPERFUNC( $lhs, CONSTANT );
WRAPPERFUNC( $args[0], $args[1] );
WRAPPERFUNC( $lhs, scalar otherfunc() );
The checker is very pessimistic and will only rewrite callsites where it determines this can be done
safely. It will not rewrite any of the following forms:
WRAPPERFUNC( $onearg ); # not enough args
WRAPPERFUNC( $x, $y, $z ); # too many args
WRAPPERFUNC( @args[0,1] ); # not a scalar
WRAPPERFUNC( $lhs, otherfunc() ); # not a scalar
The wrapper function for infix operators which take lists on both sides also has a call-checker which
will attempt to inline the operator in similar circumstances. In addition to the optimisations described
above for scalar operators, this checker will also inline an array-reference operator and omit the
resulting dereference behaviour. Thus, the two following lines emit the same optree, without an
"OP_SREFGEN" or "OP_RV2AV":
@lhs OP @rhs;
WRAPPERFUNC( \@lhs, \@rhs );
Note that technically, this optimisation isn't strictly transparent in the odd cornercase that one of the
referenced arrays is also the backing store for a blessed object reference, and that object class has a
"@{}" overload.
my @arr;
package SomeClass {
use overload '@{}' => sub { return ["values", "go", "here"]; };
}
bless \@arr, "SomeClass";
# this will not actually invoke the overload operator
WRAPPERFUNC( \@arr, [4, 5, 6] );
As this cornercase relates to taking duplicate references to the same blessed object's backing store
variable, it should not matter to any real code; regular objects that are passed by reference into the
wrapper function will run their overload methods as normal.
The callchecker for list operands can optionally also discard an op of the "OP_ANONLIST" type, which is
used by anonymous array-ref construction:
($u, $v, $w) OP ($x, $y, $z);
WRAPPERFUNC( [$u, $v, $w], [$x, $y, $z] );
This optimisation is only performed if the operator declared it safe to do so, via the
"XPI_OPERAND_ONLY_LOOK" flag.
If a function of the given name already exists at registration time it will be left undisturbed and no
new wrapper will be created. This permits the same infix operator to have multiple spellings of its name;
for example to allow both a real Unicode and a fallback ASCII transliteration of the same operator. The
first registration will create the wrapper function; the subsequent one will skip it because it would
otherwise be identical.
Note that when generating an optree for a wrapper function call, the "new_op" hook function will be
invoked with a "NULL" pointer for the "SV *"-typed parse data storage, as there won't be an opporunity
for the "parse" hook to run in this case.
DEPARSE
This module operates with B::Deparse in order to automatically provide deparse support for infix
operators. Every infix operator that is implemented as a custom op (and thus has the "ppaddr" hook field
set) will have deparse logic added. This will allow it to deparse to either the named wrapper function,
or to the infix operator syntax if on a "PL_infix_plugin"-enabled perl and the appropriate lexical hint
is enabled at the callsite.
In order for this to work, it is important that your custom operator is not registered as a custom op
using the Perl_register_custom_op() function. This registration will be performed by "XS::Parse::Infix"
itself at the time the infix operator is registered.
TODO
• Have the entersub checker for list/list operators unwrap arrayref or anon-array argument forms
("WRAPPERFUNC( \@lhs, \@rhs )" or "WRAPPERFUNC( [LHS], [RHS] )").
• Further thoughts about how infix operators with "parse" hooks will work with automatic deparse, and
also how to integrate them with XS::Parse::Keyword's grammar piece.
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>