It is relatively easy to adapt a closure to a one-method interface. And such conversion should be very useful for implementing functors from within groovy. syntax can be: {a->a.name=="a"} as Predicate The attached class is a dynamic proxy that implements this adapter functionality. It is currently used in a category class to enable a syntax like "Predicate.impl {...}". Just hook it up to the "as" keyword should make it work.