XS::Parse::Infix(3pm) User Contributed Perl DocumentationXS::Parse::Infix(3pm)
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. No current release of perl actually supports custom infix
operators, though I have a branch where I am currently experimenting
with such support:
<https://github.com/leonerd/perl5/tree/infix-plugin>
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 or development releases of perl yet support this
feature, but see above for details of a branch which does.
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);
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".
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, or to core
perl's "PL_infix_plugin" if availble.
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;
};
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, void *hookdata);
OP *(*ppaddr)(pTHX);
};
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 "rhs_flags" field gives details on how to parse and handle the
right-hand side of the operator syntax. It should be set to one of the
following constants:
XPI_OPERAND_TERM (0)
Default. The operand is a term expression.
XPI_OPERAND_TERM_LIST
The operand is a term expression. It 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 operand is a list expression. It will be forced into list
context, the same as above.
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 "lhs_flags" field gives details on how to handle the left-hand side
of the operator syntax. It takes similar values to "rhs_flags", except
that it does not accept the "XPI_OPERAND_LIST" value. Parsing always
happens on just a term expression, though it may be placed into list
context (which therefore still permits things like parenthesized lists,
or array variables).
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 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 this is not present, then the
"ppaddr" will be used instead to construct a new BINOP of the
"OP_CUSTOM" type.
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.
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] )").
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.34.0 2022-02-06 XS::Parse::Infix(3pm)
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