2008/12/11 Thomas DuBuisson
I see Lennart answered your question. For more fun you could also do this with TypeFamilies, which are the new hot thing in Haskell type level logic. Since you are just getting into MPTC, FunDeps etc I figured you'd be interested.
------ START CODE ------ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances, FlexibleInstances, TypeFamilies #-}
data Foo = Foo Bar deriving(Show) data Bar = Bar String deriving(Show)
-- A family of types will evaluate from one type to another. -- Here, I chose the word 'Eval', which you could make more meaningful. -- It is basically a function over types. type family Eval b
-- This is three definitions for the type function 'Eval' type instance Eval Foo = Integer type instance Eval Bar = String type instance Eval [x] = [Eval x]
-- And instead of a functional dependency -- you have a type level function (Eval) that operates on the type 'a'. class ZOT a where zot :: a -> Eval a
instance ZOT Foo where zot x = 17
instance ZOT Bar where zot x = "Eighteen"
-- And don't forget that x must be an instance of ZOT to apply zot. instance (ZOT x) => ZOT [x] where zot xs = map zot xs
main = do print $ zot $ Foo $ Bar "Blah" print $ zot $ Bar "Blah" print $ zot $ [Bar "Blah", Bar "Blah"] -- No map here please ----
I don't mean to hijack the original question, but I have a question about this code. Is this the same as saying class ZOT a where type Eval a zot :: a -> Eval a and then appropriate instance declarations? Is there any reason to have the type function inside or outside of the class? Thanks, Alex