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
----
2008/12/11 Paul Keir
Hi all,
I've been trying to refactor my tree conversion code to make better use of type classes; and I've discovered multi-parameter type classes and functional dependencies. I have a class with a function a2b, and I'd like "map" to be used when it's a list of type a.
I've created a simple failing example:
data Foo = Foo Bar deriving(Show) data Bar = Bar String deriving(Show)
class ZOT a b | a -> b where zot :: a -> b
instance ZOT Foo Integer where zot x = 17
instance ZOT Bar String where zot x = "Eighteen"
instance ZOT [x] [y] where -- This bit zot xs = map zot xs -- fails
main = do print $ zot $ Foo $ Bar "Blah" print $ zot $ Bar "Blah" print $ zot $ [Bar "Blah", Bar "Blah"] -- No map here please
I know this would work if the third instance of zot explicitly took [Bar] and [String]. Can I not instead generalise for all the ADTs in my tree in the way I've outlined? Must I instantiate for the type of each list pair?
Cheers, Paul
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