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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