Implement named parameters in signatures (PPC0024) #23871

leonerd · 2025-10-22T12:50:11Z

This adds a major new ability to subroutine signatures, allowing callers to pass parameters by name/value pairs rather than by position.

sub f ($x, $y, :$alpha, :$beta = undef) { ... }

f( 123, 456, alpha => 789 );

Originally specified in

https://github.com/Perl/PPCs/blob/main/ppcs/ppc0024-signature-named-parameters.md

This feature is currently considered experimental.

This set of changes requires a perldelta entry, and it is included.

wolfsage · 2025-10-22T20:55:03Z

pp.c

+            else {
+                // TODO: Consider collecting up all the names of unrecognised
+                // in one string
+                croak("Unrecognized named parameter '%s' to subroutine '%" SVf "'",


If you change these croaks to Perl_croak_caller() the errors will include the line number of the caller rather than where the subroutine is defined which would be way more helpful (and in line with what signatures do without named parameters)

namepv may or may not be utf8 encoded, and this message doesn't account for that.

croak_caller("Unrecognized named parameter '" UTF8f "' to subroutine '%" SVf "'", UTF8fARG(SvUTF8(name), namepv), S_find_runcv_name());

If the above changes to fetch the UTF8 name this should pass true instead of SvUTF8(name).

wolfsage · 2025-10-22T21:19:58Z

It seems that a slurp '@' doesn't allow you to pass extra parameters:

sub foo (:$x, @) {}
sub bar (:$y, @extra) {}

bar(y => 1, x => "a");
# This one fails with Unrecognized named parameter 'a' to subroutine 'main::foo'
foo(x => 1, "a");

wolfsage · 2025-10-22T21:23:43Z

Extra parameters aren't detected if they have no key value pair. Consider:

sub foo (:$x) {}
foo(x => 1, bar => 1); # Unrecognized named parameter 'bar' to subroutine 'main::foo'

vs

sub foo (:$x) {}
foo(x => 1, 1); # Odd name/value argument for subroutine 'main::foo'

... compared with normal sub sigs which gives "Too many arguments for subroutine 'main::foo'"

wolfsage · 2025-10-22T21:28:25Z

This recurses into madness consuming memory:

sub foo (:$x, @rest) {}
foo(x => 1, "oh");

Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.
Use of uninitialized value in signature processing at sigs line 7.

wolfsage · 2025-10-22T21:50:21Z

pp.c

+            if(!named->is_required || !SvPADSTALE(PAD_SVl(named->padix)))
+                continue;
+
+            // TODO: Consider collecting up all the names of missing


Yes please (and unrecognized ones above...)

I'll fix those TODOs in a follow-up commit once the initial bit is merged. That's more of a "nice-to-have" feature.

wolfsage · 2025-10-22T21:51:04Z

op.c

+
+        /* For now, build the named param array in iteration order. We'll
+         * sort it at the end.
+         */


Is it actually sorted? If I do sub foo (:$z, :$x) {} foo(); the error I get is for 'z' missing, but I'd expect 'x'

Ah it's currently not sorted at all. Eventually it'll be sorted by order of the hash value - so not something that's useful for end users.

wolfsage · 2025-10-22T21:58:27Z

This recurses into madness consuming memory:
sub foo (:$x, @rest) {}
foo(x => 1, "oh");

From reading the docs and the code, it seems like the intent is.. if you have named parameters then anything slurpy at the end should be key / values pairs even if it's @foo.

I'm not sure I agree with this. I think if you want k/v pairs still, use %foo, otherwise anything extra can be ... whatever. This is consistent with normal sub signatures and least surprising.

wolfsage · 2025-10-22T21:59:00Z

With all of that said, I'm very excited for this! Thanks again!

op.c

wolfsage · 2025-10-22T22:28:46Z

sub foo(:&cat) {} # A signature parameter must start with '$', '@' or '%'

If we detect it's a named parameter can we give a more accurate error message? The above suggests :@cat is legal but of course:

Only a scalar parameter may be named

pp.c

mauke · 2025-10-23T01:16:37Z

Something odd is happening with eval:

$ ./perl -Ilib -wE 'sub (:$x, $y) {}'
Named parameters in signatures are experimental at -e line 1.
Positional parameter follows named parameter at -e line 1, near "$y) "
Execution of -e aborted due to compilation errors.
$ ./perl -Ilib -wE 'my $f = eval(q{ sub (:$x, $y) {} }); say $f; $f->()'
CODE(0x5712557d23f8)
Too few arguments for subroutine 'main::__ANON__' (got 0; expected 2) at -e line 1.

If (:$x, $y) is a compilation error, why doesn't the eval return undef?

mauke · 2025-10-23T01:21:48Z

$ ./perl -Ilib -E 'sub f(:$x, :$x) {} say 42'
Named parameters in signatures are experimental at -e line 1.
42

I think declaring the same named parameter twice in one signature should be a compiler error.

mauke · 2025-10-23T01:28:22Z

$ ./perl -Ilib -wE 'sub f($x = 1, :$y) {} say 42'
Named parameters in signatures are experimental at -e line 1.
42

I think this should be a compiler error as well. It doesn't make sense to have optional positional parameters if there are required named parameters in the same signature because you can never call f without explicitly giving a value for $x anyway.

davorg · 2025-10-23T10:13:14Z

pod/perlsub.pod

+    foo(%params);
+
+As with positional parameters, named parameters may be declared with a
+defaulting expression to provide a value to assign into the varible if the


leonerd · 2025-10-23T16:27:46Z

@wolfsage

It seems that a slurp '@' doesn't allow you to pass extra parameters:

Fixed.

This recurses into madness consuming memory:

Fixed. This was a while loop that decremented 2 from argc and waited until it was zero. Of course if it started at an odd number that never happened; it wrapped around to overflow and went off the rails there.

... compared with normal sub sigs which gives "Too many arguments for subroutine 'main::foo'"

Ahyes, that's because the argc-parity check comes quite early on. Here, it realises the parity of argc is wrong, so complains about that before it does anything else.

I'm not sure I agree with this. I think if you want k/v pairs still, use %foo, otherwise anything extra can be ... whatever. This is consistent with normal sub signatures and least surprising.

This is now fixed and a test added. Specifically, with a slurpy array and a single spare value, it checks only that value is pushed, and not an additional undef.

If we detect it's a named parameter can we give a more accurate error message? The above suggests :@cat is legal but of course:

Fixed, by adding a new message.

leonerd · 2025-10-23T16:34:22Z

@mauke

I think declaring the same named parameter twice in one signature should be a compiler error.

Fixed.

I think this should be a compiler error as well. It doesn't make sense to have optional positional parameters if there are required named parameters in the same signature because you can never call f without explicitly giving a value for $x anyway.

Fixed.

Something odd is happening with eval:

Hrmmm. Something tricky, probably because the yyerror() function doesn't actually have croak-like behaviour but just sets an error flag and carries on anyway. I expect something somewhere isn't checking correctly in that case. I'll have a look shortly.

leonerd · 2025-10-23T16:37:12Z

Reviewers: Thanks all for the many and varied comments.

Besides one awkward bug to do with eval(), I think I've addressed them all. For now, I've left the fixes in various "fixup" commits, but I'll squash those down in the main one before merge.

op.c

op.h

leonerd · 2025-10-24T12:59:53Z

Something odd is happening with eval:

$ ./perl -Ilib -wE 'sub (:$x, $y) {}'
Named parameters in signatures are experimental at -e line 1.
Positional parameter follows named parameter at -e line 1, near "$y) "
Execution of -e aborted due to compilation errors.
$ ./perl -Ilib -wE 'my $f = eval(q{ sub (:$x, $y) {} }); say $f; $f->()'
CODE(0x5712557d23f8)
Too few arguments for subroutine 'main::__ANON__' (got 0; expected 2) at -e line 1.

If (:$x, $y) is a compilation error, why doesn't the eval return undef?

Actually, this is way weirder. I don't think it is related to the eval(), I think it's about context of the anonymous sub expression.

Having added some debug prints, I can get this:

$ ./perl -E 'my $sub = sub (:$x, :$y) {}; say $sub'
subsignature append positional padix=1<$x>
subsignature append named padix=2<$y> name=<y>
Named parameters in signatures are experimental at -e line 1.
CODE(0x5583e39e1358)

leo@vel:~/src/bleadperl/perl [git]
$ ./perl -E 'my $sub; $sub = sub (:$x, :$y) {}; say $sub'
subsignature append positional padix=1<$x>
subsignature append named padix=2<$y> name=<y>
Named parameters in signatures are experimental at -e line 1.
CODE(0x55d4124f35a8)

leo@vel:~/src/bleadperl/perl [git]
$ ./perl -E 'my $sub; sub (:$x, :$y) {}; say $sub'
subsignature append named padix=1<$x> name=<x>
Named parameters in signatures are experimental at -e line 1.
subsignature append named padix=2<$y> name=<y>

It seems that when the anonymous sub is on RHS of an assignment expression, the first param is treated as positional, not named. Most odd indeed.

leonerd · 2025-10-24T16:40:36Z

Last bug now fixed by e287ee0 - turned out to be an action block in perly.y leaving the llval uninitialised, which fortunately most of the time happened to be true, but in some cases was false and upset the logic.

I believe that's all the issues fixed now

jkeenan · 2025-10-24T22:25:03Z

Last bug now fixed by e287ee0 - turned out to be an action block in perly.y leaving the llval uninitialised, which fortunately most of the time happened to be true, but in some cases was false and upset the logic.

I believe that's all the issues fixed now

I tested the pull request with Perl5::TestEachCommit. All commits PASSed on their own. LGTM.

rwp0

Made a review on docs' (perlsub.pod) spelling etc

rwp0 · 2025-10-25T02:41:22Z

pod/perlsub.pod

+passed by the caller when the subroutine is invoked.  Whereas positional
+parameters take their values from the corresponding position in the list
+passed by the caller, named parameters are matched by name-value pairs
+from the caller, where the name matches the variable name of the parameter


I think , (the comma) is not needed here before "where".

It probably reads better as a separate sentence even.

rwp0 · 2025-10-25T02:58:35Z

pod/perlsub.pod

+Named parameters in signatures are currently considered B<experimental>, and
+their behaviour may change in future releases of Perl.  Uses of named
+parameters in a signature will currently emit a warning in the
+C<experimental::signature_named_parameters> category.


This could instead be a link to that category ie. within the pod of the experimental pragma (so that the users could conveniently read more if needed).

experimental isn't (exactly) a core module, but also its documentation doesn't have separate sections per warning category. Nor does warnings.

rwp0 · 2025-10-25T03:01:12Z

pod/perlsub.pod

+
+To invoke this function, the caller must pass pairs of names and values to
+provide values for the parameters.  Each name must match the name of one of
+the variables, omimtting its leading C<$> sigil.  The order of these values


"omitting", without the second M, also as previously I find the comma to be unnecessary and confusing here too.

rwp0 · 2025-10-25T14:40:38Z

pod/perlsub.pod

+To invoke this function, the caller must pass pairs of names and values to
+provide values for the parameters.  Each name must match the name of one of
+the variables, omimtting its leading C<$> sigil.  The order of these values
+is not important; both of the following lines will cause the same behaviour.


Expected : (colon) rather than . (comma) in the end to introduce the following lines (code samples).

rwp0 · 2025-10-25T14:59:55Z

pod/perlsub.pod

+=item *
+
+zero or more mandatory positional scalar parameters
+
+=item *
+
+zero or more optional positional scalar parameters
+


Maybe merge into a single list item as with the following named scalar parameters using OR (ie. mandatory or optional) for consistency in the Summary.

No; the entire point here is that there's zero-or-more mandatory, followed by zero-or-more optional, in two distinct groups. They aren't all mixed together.

karenetheridge

Awesome!

Just a few minor tweaks to some of the doc wording.

pod/perldiag.pod

pod/perlsub.pod

tonycoz · 2025-10-26T22:19:55Z

pp.c

+
+            /* namepv / namelen are always UTF-8 */
+            STRLEN namelen;
+            const char *namepv = SvPVutf8(name, namelen);


If the callers SV is modifiable and isn't special (it won't overwrite references or GVs for example) this will upgrade it in place, modifying the caller's SV.

If the SV isn't utf8 you could use bytes_to_utf8_free_me() or bytes_to_utf8_temp_pv() to get the UTF8 version cheaply:

const char *namepv = SvPV(name, namelen); if (!SvUTF8(name)) namepv = bytes_to_utf8_temp_pv(namepv, &namelen);

which saves the cost of making a new SV, and bytes_to_utf8_temp_pv() is smart about invariants (it will return namepv if the string contains only invariants).

The _temp_pv() variant uses SAVEFREEPV() to ensure the namepv is release (if allocated), alternatively you can use the _free_me() variant where you need to Safefree() the allocation, but the croak() makes that difficult to use here.

Ahyes, now changed and added a test to check that non-UTF8 SVs from the caller are not upgraded like that any more.

leonerd · 2025-10-27T12:42:08Z

Awesome!

Just a few minor tweaks to some of the doc wording.

Thanks, now all applied.

leonerd · 2025-10-28T12:23:59Z

If reviewers are happy their comments have been addressed, I shall squash all the fixup commits back down to a single one

leonerd · 2025-10-29T13:16:56Z

Intending to merge on Friday (31st oct) if no further comments

This adds a major new ability to subroutine signatures, allowing callers to pass parameters by name/value pairs rather than by position. sub f ($x, $y, :$alpha, :$beta = undef) { ... } f( 123, 456, alpha => 789 ); Originally specified in https://github.com/Perl/PPCs/blob/main/ppcs/ppc0024-signature-named-parameters.md This feature is currently considered experimental.

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Implement named parameters in signatures (PPC0024) #23871

Implement named parameters in signatures (PPC0024) #23871

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