just try to compile it with ghc -fallow-overlapping-instances -Wall --make Main.hs or inseart sth. like this at the first line: {-# OPTIONS -fglasgow-exts -fffi -fallow-undecidable-instances -fallow-overlapping-instances #-} - marc Am Montag, 29. August 2005 05:25 schrieb Juan Carlos Arevalo Baeza:
David Menendez wrote:
Juan Carlos Arevalo Baeza writes:
The way Haskell type classes work, the overlap is determined without looking at the context, so "Show a" will overlap with every possible instance for Show, including Show Int, which is predefined.
Ah. :-P Bummer.
I'm not sure what the official justification for that is, but reason is to avoid situations like this:
class A t where a :: t class B t where b :: t class C t where c :: t
instance A t => C t where c = a instance B t => C t where c = b
instance A Char where a = 'a' instance B Char where b = 'b'
What should c :: Char evaluate to?
Right. Ambiguity. But as long as there's no ambiguity, there's no reason to be this restrictive. Oh, well.
JCAB
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