Building notes, projects, and occasional rants

Bitten by prototypes

I just spent the best part of an hour around a problem caused by the behavior of Perl prototypes.

I used the following test case to figure it out:

use Test::More tests => 1;
use Encode qw( encode decode );

sub u8l1 {
  return encode('iso-8859-1', @_);

my $ola_u8 = decode('utf8', 'Olá');
my $ola_l1 = encode('iso-8859-1', $ola_u8);
is(u8l1($ola_u8), $ola_l1);

The output of prove x.t is this:

t/x.t .. 1/1 
#   Failed test at t/x.t line 12.
#          got: '1'
#     expected: 'Ol?'
# Looks like you failed 1 test of 1.
t/x.t .. Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/1 subtests

The got: '1' had me for quite some time. Until I changed the u8l1() helper to this:

sub u8l1 {
  return encode('iso-8859-1', $_[0]);

And it just works.

The problem is the definition of the Encode::encode() function. It has a prototype like this:

sub encode($$;$)

So our @_ is interpreted in scalar context, and so evaluates to the number of parameters, 1.

I don't like it at all because it changes the standard Perl behavior of expanding lists. Its action at the distance. The fact that you cannot pass a single element list is also not mentioned in the documentation.

The only really useful use of Perl prototypes is using a & as the initial char, that allows you to write a function that looks like some built-ins like sort or map, that take a anonymous sub as the first parameter.