Dear Nuns and Monks,

after some digging into subroutine signatures I stumbled upon a little problem that seems to be non-existing without signatures: How to distinguish between a missing and an undefined argument? Without signatures it goes straight forward: shift arguments as long as there are any left over. There is no problem distinguishing between non-existing and undefined. See no_sig in the example below.

With signatures, a naive approach would be to check the size of @_, too. This appears to be some kind of illogical to me, see sig_std in the example.

Going one step further, I came to a lexical variable defined inside the signature that is set only in case of a missing argument. This apparently works but I'm not sure if this is indeed valid. The argument and the new variable are named alike to establish a connection between them. See sig_lexical in the example.

Is there a common idiom how to handle such case with signatures? I'm not satisfied with my approaches in the example.

Any other ideas? 

#!/usr/bin/perl

use v5.12;
use Test2::V0;
use experimental 'signatures';

sub show ($arg) {
    defined $arg ? $arg : 'undef';
}

# No signature, no check.
sub no_sig {
    my $what = shift;
    state $data;
    # Straight forward: shift off first arg if there is any.
    if (@_) {
        my $arg = shift;
        say "$what: ", show $arg;
        $data = $arg;
    } else {
        say "$what: ", show $data;
        return $data;
    }
}

# Standard signature with optional second arg.
sub sig_std ($what, $arg=undef) {
    state $data;
    # Kind of illogical: there is no obvious connection between @_, it
+s
    # size and $arg.
    if (@_ >= 2) {
        say "$what: ", show $arg;
        $data = $arg;
    } else {
        say "$what: ", show $data;
        return $data;
    }
}

# Signature with side effect on missing arg.
sub sig_lexical ($what, $arg=(my $no_arg=1, undef)) {
    state $data;
    # $arg and $no_arg are (loosely) connected by their names.  Howeve
+r,
    # there seems to be no specification about lexical variables
    # declared within a signature.
    if (!$no_arg) {
        say "$what: ", show $arg;
        $data = $arg;
    } else {
        say "$what: ", show $data;
        return $data;
    }
}

pass 'init';
no strict 'refs';
for my $sub (qw(no_sig sig_std sig_lexical)) {

    # Set to 1 and retrieve:
    &$sub("$sub setter", 1);
    is &$sub("$sub getter"), 1, "$sub get 1";

    # Set to undef and retrieve:
    &$sub("$sub setter", undef);
    is &$sub("$sub getter"), U(), "$sub get undef";

    SKIP:
    {
        skip "arg check without signature" if $sub =~ /^no/;

        # Too many arguments:
        like dies {&$sub("$sub invalid args", 42, 1)},
            qr/Too many arguments/,
            "$sub with three args";
    }
}

done_testing;

__DATA__
# Seeded srand with seed '20201227' from local date.
ok 1 - init
no_sig setter: 1
no_sig getter: 1
ok 2 - no_sig get 1
no_sig setter: undef
no_sig getter: undef
ok 3 - no_sig get undef
ok 4 - skipped test # skip arg check without signature
sig_std setter: 1
sig_std getter: 1
ok 5 - sig_std get 1
sig_std setter: undef
sig_std getter: undef
ok 6 - sig_std get undef
ok 7 - sig_std with three args
sig_lexical setter: 1
sig_lexical getter: 1
ok 8 - sig_lexical get 1
sig_lexical setter: undef
sig_lexical getter: undef
ok 9 - sig_lexical get undef
ok 10 - sig_lexical with three args
1..10

[download]

Greetings,
-jo

$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$

In reply to Distinguish between missing and undefined arguments with subroutine signatures by jo37

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