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perlmoose

Moose "builder" vs "default"

I understand that using builder enables subclasses to override attribute defaults easily and roles can require them. This can also be accomplished using default like so:

has 'foo' =>
    is       => 'rw',
    isa      => 'Str',
    default  => sub { $_[0]->_build_foo };

I'm wondering if there are further advantages to using builder I'm not aware of? I've come up with some myself:

  • builder is declarative so you can introspect that foo is built by _build_foo
  • builder eliminates a subroutine wrapper making it a bit faster
  • builder allows the use of the helpful lazy_build.

UPDATE To clarify, this isn't about default vs builder in general but default => sub { $_[0]->_build_foo } vs builder => '_build_foo'.

Solution

  • I think you've already answered your own question. Using builder allows late-binding, which plays nicely with roles and classes that are intended to be subclassed. It's also valuable if the builder is pretty long — I never put a default more than a line long into an attribute definition. There's no real functional difference; default can easily emulate builder, but the result isn't very pretty.