You need to enable ScopedTypeVariables, and add a forall to introduce the
type variable at the top level. The local variable will then be the *same*
'a' instead of a fresh one:
{-# LANGUAGE ScopedTypeVariables #-}
data D a = D1 a | D2 a (a -> a)
f :: forall a. Eq a => D a -> a
f (D1 x) = x
f (D2 x g) = let y :: Eq a => a
y = g x
in if x == y then x else g y
main = putStr $ shows (f (D2 (1 :: Int) succ)) "\n"
On Wed, Nov 14, 2012 at 1:03 PM, Serge D. Mechveliani
Please, how to correctly set an explicit type for a local value in the body of a polymorphic function?
Example (tested under ghc-7.6.1):
data D a = D1 a | D2 a (a -> a)
f :: Eq a => D a -> a f (D1 x) = x f (D2 x g) = let -- y :: Eq a => a y = g x in if x == y then x else g y
main = putStr $ shows (f (D2 (1 :: Int) succ)) "\n"
This is compiled by ghc --make Main
Now I need, for a certain reason, to explicitly set the type for y in `let', with the meaning: "this very `a' which is in the signature for f" (and I think that this type Haskell assignes to y in "y = g x").
I need to declare this type in a separate line: y :: <what ever it is>.
Both `y :: a' and `y :: Eq a => a' are not compiled.
Please, copy the answer to mechvel@botik.ru
Thanks,
------ Sergei
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