It took me a little while to figure this one out so I thought I post it here, partly to make it easier for myself to find it next time ;-)

I write software for biological applications and I use Moose. My modules often take biological sequence data as input, so I create a subtype for strings that fulfill a regex for valid DNA sequences (in this case I only allow the letters A,G,T, C and N). This is in a package MyApp::MooseX::MyTypes:

subtype 'DNAString'
  => as Str
  => where { $_ && $_=~/^[AGCTN]+$/i}
  => message {"Not a DNA string: only AGCT or N allowed and cannot be 
+empty"};
[download]

Now my modules can define attributes like this:

use MyApp::MooseX::MyTypes:
has sequence => (
  isa      => 'DNAString',
);
[download]

So far so good, but sequences in biological applications in Perl often exist in the form of Bio::Seq objects and I thought it would be nice if my DNAString type could automatically get the sequence string from such objects. However, you can only coerce from Moose built-in types, so I wanted to say: coerce from an incoming object into a sequence string, if that object has a 'seq' method (which returns the sequence as a string in BioPerl).
Turns out that this can be done like this (in MyApp::MooseX::MyTypes again):

coerce 'DNAString'
  => from duck_type( ['seq'] )
  => via { $_->seq };
[download]

And then you need to tell the constructor that you want to use coercions for this attribute by adding the 'coerce' switch:

use MyApp::MooseX::MyTypes:
has sequence => (
  isa      => 'DNAString',
  coerce   => 1, 
);
[download]

Now all my modules that used to only accept sequence strings, automatically except Bio::Seq objects as well.