Different behavior of $foo ~~ 0 in 5.10.0.

[schwern]

t/zero_defaults.t is failing on 5.10.0. The problem is the copy_cat() method. It does not consider an empty string to be a default. The issue is with how $foo ~~ 0 behaves.

copy_cat expands out into this:

sub copy_cat {
    my $self = shift;
    Method::Signatures->required_arg('$this') unless (@_ > 0);
    my $this = $_[0];
    my $that = !(@_ > 1) ? ( $this ) : do {
        no warnings;
        my $arg = $_[1];
        $arg ~~ 0 ? $this : $arg
    };
    Method::Signatures->too_many_args_error(2) if @_ > 2; 

    return $that;
}

On 5.10.0, "" ~~ 0 is false. From 5.10.1 on it's true.

Smart match before 5.10.1 is unstable. Simplest thing to do might be to boost the minimum requirement for that feature to 5.10.1.

[thoughtstream]

Arguably it's post-5.10.1 that's broken... for MS purposes. :-)

Actually, though, it's the test that's broken.
Cut-and-paste error. Mea culpa!

The line:

    is( Stuff->copy_cat("wibble", ""), "wibble" );

should be:

    is( Stuff->copy_cat("wibble", 0), "wibble" );

[thoughtstream]

BTW, the new new smartmatch semantics currently proposed
for 5.18 are going to mess this mechanism up again and will
necessitate a rewrite to support backwards compatibility. I'll volunteer
to provide that rewrite when the time comes, as I'll have to do the
same for various other modules that rely on 5.10.1 smartmatching
semantics.

[schwern]

I think the behavior of when needs to be clarified. The examples in the
docs make it pretty clear that it's an exact match.

# Use default if no argument passed OR argument is undef
method get_results($how_many = 1 when undef) {...}

# Use default if no argument passed OR argument is empty string
method get_results($how_many = 1 when "") {...}

# Use default if no argument passed OR argument is zero
method get_results($how_many = 1 when 0) {...}

If that's the case, if when is going to try and differentiate undef from 0
from empty string, the test needs all of them.

is( Stuff->copy_cat("wibble"),        "wibble" );
is( Stuff->copy_cat("wibble", 0),     "wibble" );
is( Stuff->copy_cat("wibble", ""),    "" );       # fails in 5.10.1 and up
is( Stuff->copy_cat("wibble", undef), undef );

I would suggest not using smart match. It's unstable across versions and
doing it wrong for a large spread of them (5.10.1 to now). I'm guessing its
there to avoid having to play the "is this a number or a string" game which is
an equally obnoxious hassle.

Alternatively we special case empty string.

Alternatively we define "when" as "whatever smart match does".

If we're going to use smart match, I like the last one. We don't have to work
around smart match changes. It also means things like "when []" or "when {}"
are legit without us having to explain anything.

[thoughtstream]

I think we ought to go with "whatever smartmatch does".

If they need to be more specific, there's always:

 method get_results($how_many = 1 when qr/^0$/) {...}     

or:

use Smart::Match 'numwise';
method get_results($how_many = 1 when numwise 0) {...} 

They're going to have to get used to being more specific anyway,
once 5.18 is out.

Damian

[barefootcoder]

Okay, this was causing 12 of the 17 failures on CPAN Testers. Fixed as per @thoughtstream suggested. I'm closing this one down.