Hello folks
A section on GADTs in the GHC user's guide gives a simple example:
data Term a where
Lit :: Int -> Term Int
Succ :: Term Int -> Term Int
IsZero :: Term Int -> Term Bool
eval :: Term a -> a
eval (Lit i) = i
eval (Succ t) = 1 + eval t
eval (IsZero t) = eval t == 0
let's add
Sum :: Term Int -> Term Int -> Term Int
eval (Sum l r) = eval l + eval r
it's ok
But when I do this
Sum :: (Num a) => Term a -> Term a -> Term a
eval (Sum l r) = eval l + eval r --- same
it doesn't typecheck with the error "no isntance Num a"
writing
eval (Sum (l::Num b => Term b) (r::Num b => Term b))
doesn't help.
I wonder, if there's a way to do what I'm trying to do. After all, I'm
allowed to declare Term that way. Or is this a restriction of type
refinement?
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