Am Samstag 03 April 2010 15:40:03 schrieb Vladimir Reshetnikov:

Hi list,

GHC 6.10.1:

Prelude> :t let f x y = return x == return y in f let f x y = return x == return y in f :: (Eq (m a), Monad m) => a -> a -> Bool

Hugs (Sep 2006):

Hugs> :t let f x y = return x == return y in f ERROR - Ambiguous type signature in inferred type *** ambiguous type : (Eq (a b), Monad a) => b -> b -> Bool *** assigned to : f

Who is right?

I think hugs is righter. GHC lets you define f, but you can't use it; if you try, you'll get No instance for (Eq (m Bool)) arising from a use of `f' at TypeTest.hs:5:6-17 Possible fix: add an instance declaration for (Eq (m Bool)) In the expression: f True False In the definition of `res': res = f True False and if you provide e.g. instance (Monad m) => Eq (m Bool) where _ == _ = False , you'll get Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `f' at <interactive>:1:0-11 Probable fix: add a type signature that fixes these type variable(s) The only difference is that GHC fails more lazily for such ambiguous types. than hugs. I think, according to http://haskell.org/onlinereport/decls.html#sect4.3.4 , GHC also should fail on the definition and not only on use.

-- Thanks Vladimir