[Haskell-cafe] Type class hell

[Christophe Poucet]

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 <haskell at sleepingsquirrel.org> wrote:
>
> 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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