On Tue, Apr 27, 2010 at 10:20 AM, John Creighton
I was wondering if it is possible to sort types in hakell and if so what language extension I should use. Not sure if this is possible but here is my attempt:
(I'm aware I don't need so many pragmas
{-# LANGUAGE GADTs #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeSynonymInstances #-} data Z=Z deriving (Show) data S i=S i deriving (Show) data family N a type family Add n m type instance Add Z m = m type instance Add m Z = m type instance Add (S n) (S m) = S (S (Add n m)) --14 type family LT a b data Cat=Cat data Dog=Dog data Fish=Fish type family Sort a --19 data And a b=And a b
type instance LT Dog Z = Cat type instance LT Fish Z = Dog type instance LT a (S i) = LT (LT a Z) i type instance Sort (And a (LT a i))=And (LT a i) a
I get the following error:
Illegal type synonym family application in instance: And a (LT a i) In the type synonym instance declaration for 'Sort' Failed, modules loaded: none,
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It's certainly possible. There's a sample on the haskell wiki using functional dependencies: http://www.haskell.org/haskellwiki/Type_arithmetic#An_Advanced_Example_:_Typ... This could be translated to type families. Your instance: type instance Sort (And a (LT a i))=And (LT a i) a is illegal because you are using a type function (LT) in the instance head.