Re: Coerce or transform with Types::Param

[tobyink]

Something like this?

use Types::Standard qw( Str Maybe );
use Types::Common::String qw( StrLength );

my $Caption      = StrLength[ 1, 2780 ];
my $EmptyStr     = Str->where( q{ $_ eq '' } );
my $MaybeCaption = Maybe->of( $Caption )->plus_coercions( $EmptyStr =>
+ q{ undef } );


use Test::More;

ok( $MaybeCaption->check( 'My pic' ), 'Good caption passes type constr
+aint' );
ok( $MaybeCaption->check( undef ), 'Undef passes type constraint' );
ok( ! $MaybeCaption->check( [] ), 'Arrayref fails type constraint' );
is( $MaybeCaption->coerce( '' ), undef, 'Empty string coerced to undef
+' );

Basically, add the coercion to the Maybe[Caption], not to Caption.

But the important thing is to make sure that the empty string isn't considered a valid Caption. Because if a value is valid, it is never coerced.