
{-# LANGUAGE ScopedTypeVariables #-}
Отправлено с iPhone
14.11.2012, в 16:03, "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
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe