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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