Re: [Haskell-cafe] Double-dispatch

On 3/5/12 4:24 PM, Clark Gaebel wrote:

Well look at that.

Thanks!

On Mon, Mar 5, 2012 at 4:07 PM, Felipe Almeida Lessa wrote:

...

{-# LANGUAGE MultiParamTypeClasses #-}

class Intersectable a b where intersectsWith :: a -> b -> Bool

Assuming that intersectsWith is something like unification, equality, or similar operations: Do note that this can lead to needing quadratically many instances, about half of which will be redundant if intersectsWith is supposed to be symmetric. Often times we know that the vast majority of these quadratically many instances should be vacuous (i.e., always return False), and it'd be nice to avoid writing them out. This can be achieved via -XOverlappingInstances where you give a default instance: instance Intersectable a b where intersectsWith _ _ = False and then override it for more specific choices of a and b. Beware that if you want to have other polymorphic instances you may be forced to use -XIncoherentInstances, or else resolve the incoherence by filling out the lattice of instances. The other notable complication is if you want your collection of types to have more than just a set structure (e.g., if you want some kind of subtyping hierarchy). It's doable, but things get complicated quickly. Other than those caveats, have at it! The ability to do this sort of thing is part of what makes Haskell great. Few languages have multiple-dispatch this powerful. -- Live well, ~wren