[Chattered]

Functors are modules that depend on another module. This works like a function whose argument is a module with type given by a signature.

There isn't anything exactly analogous in Java. Functor application can introduce new type definitions at compile time and enforce static type safety. You need this so that you can statically require that you can only take unions of sorted collections whose types came from the same ordering module applied to the same ordered set module.

[ergl]

Yes, I know my example isn't perfect, but I've found is a good way of explaining the general concept to people not familiar with ML's module system. Thanks for your explanation though!