On Sun, Dec 21, 2008 at 10:28 AM, Maurício
Hi,
Why isn't the last line of this code allowed?
f :: (TestClass a) => a -> Integer f = const 1 a = (f,f) g = fst a
The only thing I can think about is monomorphism restriction, but it's allowed (or even the third line would not be accepted). Is there something I could read to understand that?
Thanks, Maurício
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The monomorphism restriction refuses to accept the definition of a. However, even if we turn the monomorphism restriction off there is still a problem.
{-# LANGUAGE NoMonomorphismRestriction #-}
f :: (TestClass a) => a -> Integer f = const 1 a = (f,f) g = fst a
In this case, the definition of a is accepted, but not the definition of g. The reason is that a has type a :: (TestClass a, TestClass b) => (a,b) and then when we take 'fst' of this value (as in g) we get g :: (TestClass a, TestClass b) => a which is an ambiguous type, since there is no way to tell the compiler what 'b' is when running g. Cheers, Reiner