[Haskell-cafe] Impossible class instance?

[Emil Axelsson]

Ahh, never mind... I just realized there's no way to relate the `info` 
in the instance to the `info` in the class definition.

Alright, I'll keep trying to make this work. Sorry for the noise!

/ Emil


2011-05-16 12:19, Emil Axelsson skrev:
> Hello!
>
> At the end of this message is a program with a simple expression type,
> and a class `ToExpr` that generalizes expressions to arbitrary Haskell
> types. Every node in `Expr` is annotated with some abstract information.
> The program raises the following type error:
>
> test.hs:13:5:
>       Couldn't match type `(,) a' with `(,) (a, a)'
>       Inaccessible code in the instance declaration
>       In the instance declaration for `ToExpr (a, b)'
>
> It seems that the mere existence of the constraint
>
>     info (a,b) ~ (info a, info b)
>
> causes this error. I was hoping that this constraint would make it
> possible to construct the value (ia,ib) in the class instance, which is
> otherwise not allowed.
>
> Note: I don't want to make `info` an associated type. The idea is to
> make this work with any type function `info` that fulfills the above
> constraint.
>
> Is there any way to make this work?
>
> / Emil
>
>
> --------------------
>
> {-# LANGUAGE UndecidableInstances #-}
>
> data Expr info a
>     where
>       Int  :: info a     ->  Int ->  Expr info a
>       Pair :: info (a,b) ->  Expr info a ->  Expr info b ->  Expr info (a,b)
>
> getInfo :: Expr info a ->  info a
> getInfo (Int info _)    = info
> getInfo (Pair info _ _) = info
>
> class ToExpr a
>     where
>       type Internal a
>       toExpr :: a ->  Expr info (Internal a)
>
> instance
>       ( ToExpr a
>       , ToExpr b
>       , info (a,b) ~ (info a, info b)
>       ) =>
>         ToExpr (a,b)
>     where
>       type Internal (a,b) = (Internal a, Internal b)
>       toExpr (a,b) = Pair (ia,ib) (toExpr a) (toExpr b)
>         where
>           ia = getInfo a
>           ib = getInfo b
>
>
>
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