I was hoping you'd reply Toby, thank you.

I made a couple of changes to the code so it compiles, and added a test to see that one can pass in the empty string and it will pass validation by being coerced to undef. But the syntax to do this must be wrong:

ok( $Caption->check( $Caption->coerce('') ), 'Empty string passes' );
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Here's the revised test script:

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

my $NonEmptyCaption = StrLength[ 1, 10 ];

my $EmptyStr = Str->where( q{ $_ eq '' } );

my $Caption = (Maybe[$NonEmptyCaption])->plus_coercions(
    $EmptyStr => 'undef',
);


use Test::More;

ok( $Caption->check('My pic'),               'Good caption passes' );
ok( ! $Caption->check('This is too long'),   'Bad caption fails' );
ok( $Caption->check(undef),                  'Undef passes' );
ok( ! $Caption->check([]),                   'Arrayref fails' );
is( $Caption->coerce(''), undef,             'Empty string coerced to 
+undef' );
ok( $Caption->check( $Caption->coerce('') ), 'Empty string passes' );


done_testing;
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perl 11130005-3.pl

ok 1 - Good caption passes
ok 2 - Bad caption fails
ok 3 - Undef passes
ok 4 - Arrayref fails
ok 5 - Empty string coerced to undef
ok 6 - Empty string passes
1..6
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Wonder if there is a more idiomatic way to do the check when a coercion is used. Thanks!

... I feel sure that there's a way to avoid calling coerce manually.

That was why I used Types::Params because the POD says: ...it will check the arguments passed to it conform to the spec (coercing them if appropriate)

Otherwise I've had to relie on has_coercions before attempting to call coerce

If there is another way though, I would love to know about it.

Dermot

Your code is far less verbose than mine too.

There are a couple of concepts that are new to me there, I've not come across of before and I haven't see a Type modified with where in that way either. I only considered where in the constructor so that is really useful.

my $MaybeCaption = Maybe
    ->of( $LongCaption )
    ->plus_coercions( $EmptyStr => q{ undef } );
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of is a shorthand for parameterizing a type. If you do something like this, it will fail:

if ( ArrayRef[Int]->check( \@numbers ) ) {
  ...;
}
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And you need to add some parentheses which look pretty ugly:

if ( ( ArrayRef[Int] )->check( \@numbers ) ) {
  ...;
}
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The of method look a little nicer:

if ( ArrayRef->of( Int )->check( \@numbers ) ) {
  ...;
}
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And where is a shorthand for creating a child type with an additional restriction.

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