I'm not certain but I think this will still fail for exactly the piece that
you ignored, which is the crux of the problem.
On 6/8/06, Greg Buchholz
Christophe Poucet wrote:
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...
Er, I'll ignore this part.
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.
Yeah, after a certain point of complexity with type classes, it starts to look like C++ templates. How about something like...
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} 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
data Nil = Nil
instance Type MonoType Nil where freeVars (TyVar x) = [x] freeVars (TyConst _ xs) = ["???"]
instance (Type a b) => Type MonoType (a b) 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
main = print $ freeVars $ TyConst "foo" [TyConst "bar" [Nil], TyConst "baz" [Nil], TyVar "quux" ]
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