Hello Greg, The idea however is that MonoType is going to be used in a recursive way. For instance: newtype FMT = FMT MonoType FMT instance FMT where... And this definition will have to reside on recursive definitions. In the style of how HasVars was instantiated: instance HasVars a => HasVars (MonoType a) where freeVars (TyVar x) = [x] freeVars (TyConst _ xs) = nub . concatMap freeVars $ xs occurs x (TyVar y) = x == y occurs x (TyConst _ xs) = or . map (occurs x) $ xs So for Type instance Type a => Type (MonoType a) where ... That's where it becomes rather troublesome. Greg Buchholz wrote:
Christophe Poucet wrote:
What I would like to do is combine HasVars and Type (mostly because in my framework the two concepts shouldn't be divided from a design perspective) into one type class to clean it up a bit. However I fail to see how I would implement toType and fromType for the given instance. Is this feasible without resorting to ugly hacks?
{-# OPTIONS -fglasgow-exts #-}
-- Multiparameter type classes?
import List
type Var = String type Const = String
data MonoType mt = TyVar Var | TyConst Const [mt] deriving (Eq, Show)
data PolyType mt = TyPoly [Var] mt deriving (Show)
class Type a b where toType :: b -> a b fromType :: a b -> b freeVars :: a b -> [Var] occurs :: Var -> a b -> Bool
instance Type MonoType Int -- yada, yada, yada...
instance Type MonoType (MonoType Int) where freeVars (TyVar x) = [x] freeVars (TyConst _ xs) = nub . concatMap freeVars $ xs occurs x (TyVar y) = x == y occurs x (TyConst _ xs) = or . map (occurs x) $ xs
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