7 Feb
2006
7 Feb
'06
3:21 p.m.
Bulat Ziganshin wrote:
{-# OPTIONS_GHC -fglasgow-exts #-} main = do return "xx" >>= ((\x -> print x) :: Show a => a -> IO ()) main2 = do return "xx" >>= (\(x:: (forall a . (Show a) => a)) -> print x) main3 = do (x :: forall a . Show a => a) <- return "xx" print x
in this module, only "main" compiles ok
The other two need "exists" rather than "forall", which isn't supported by GHC. As written, they say that x can produce a value of any type that's an instance of Show, but the value you're binding to x has type String, which can only produce a value via Show String. -- Ben