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Postings from February 2022
I've just encountered another fine use-case for wanting to know the
min/max arity count of a sub before calling it.
I'm writing an nroff parser... I'll spare you the full details but in
brief I have a plaintext input that consists of a short two-letter
"directive" code, followed by some positional arguments:
.TH NAME 3 "date here"
.SH HEADING
.SS Subheading
etc...
My code to parse/dispatch these basically takes the two-letter
directive code and the args in an array, and does:
my $method = $self->can( "handle_directive_$d" ) or
$self->die( "Unrecognised directive .$d" );
return $self->$method( @args );
I can then go off and implement
sub handle_directive_TH {
my $self = shift;
my ( $title, $section, $date ) = @_;
...
}
# and so on for handle_directive_SH, handle_directive_SS and many
# many others
Without sub signatures I get no min/max arity checking of these things.
I could hack that in manually via some lookup table or whatever, but
it'd be great if I can just use the sub signatures themselves:
# date is optional
sub handle_directive_TH ( $self, $title, $section, $date = undef )
{
...
Problem is if I do that, any arity failures appear to come from
*perl* land in the dispatch logic in the toplevel. My process dies with
Too few arguments for subroutine 'main::handle_directive_TH' at
Some/Perl/File.pm line 1234.
That's not very friendly to my end-users. I want them to see a failure
as generated by my parser logic; something like:
Too few arguments for .TH directive on line 1 at:
.TH TITLE
^
If perl provided me a way to query the min/max arity of a coderef
without invoking it, well this is easy enough to write:
my $method = $self->can( "handle_directive_$d" ) or
$self->die( "Unrecognised directive .$d" );
my ( $minargs, $maxargs ) = builtin::sub_arity $method;
$self->die( "Too few arguments for .$d directive" ) if 1+@args < $minargs;
$self->die( "Too many arguments for .$d directive" ) if 1+@args > $minargs;
return $self->$method( @args );
Perl currently does not provide this. So I can't.
--
Paul "LeoNerd" Evans
leonerd@leonerd.org.uk | https://metacpan.org/author/PEVANS
http://www.leonerd.org.uk/ | https://www.tindie.com/stores/leonerd/
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