The Perl 5 prototypes mechanism seems to be somewhat deprecated, at least within the commnity right now. We had something of a discussion in the CB this week and Far more than you ever wanted to know about prototypes has a lengthy, and substantial discussion about prototypes.

TheDamian in his recent tome, Perl Best Pratices explains on pp194-196 just what can go wrong with code using prototypes and why. When I first started using Perl I used prototypes, for, oh, about the first week! I had come from 'C' and expected that this was how things should be done. My problem was the one pointed out on p195. If a prototype is given thus (stolen shamelessly from shmem's reply

#!/usr/bin/perl

sub getInfo ($\%) {
    my $scalar = shift;
    my $hash = shift;
    print "scalar = '$scalar'\n";
    print "hash = $hash\n";
    print "$_ => $hash->{$_}\n" for keys %$hash;
}   

my %hash = (foo => 'bar', baz => 'quux');
my $scalar = 'string';

getInfo($scalar,%hash);
__END__

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Will give the desired result. BUT there is a problem of magic being perpetrated here, the call to the sub,
getInfo($scalar,%hash);

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implies that the hash is going to be "squashed" into the list context and unless you are aware that a prototype was being used you would expect to see
my ( $thescalar, %thehash ) = @_;

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in the sub. The call to the prototyped function doesn't pass the hash at all! It passes a reference, but that fact is obscured. So by looking at the subroutine call we dont really know what the subroutine is doing.

If we want to pass a hash reference, why not do it explicitly: 

sub getInfo {
  my ( $thescalar, $thehashref ) = @_;
  ....
}

getInfo( $scalar, \%hash );

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at least then everybody will know that the subroutine is expecting a hash and not a hash reference by some magic. Then if you want the hash itself to be copied in: 
sub getInfo {
  my ( $thescalar, %thehash ) = @_;
  ....
}

getInfo( $scalar, %hash );

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and then the subroutine definition and its invocation actually look alike! But don't believe me, search the Monastery on prototypes and read TheDamian.

jdtoronto

In reply to Re: Parsing a hash into a sub by jdtoronto
in thread Parsing a hash into a sub by RaginElmo

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