G'day all. On Tue, May 13, 2003 at 08:58:35AM +0100, Simon Peyton-Jones wrote:
Think of it like this. Should this be acceptable?
f :: a -> a -> a f x y = x && y
No, because (&&) is not defined on anything but Bool.
Your signature is bar :: (Foo Char t) => t and we know (from the instance decl) that t must be Bool.
Even if we don't have the instance declaration, we know that t must be some unique type because of the fundep. (Of course that type may be polymorphic, which probably complicates things.)
It is undoubtedly odd that adding an instance declaration makes a legal program illegal, and that might be a reason for relaxing the rule... but it's not entirely easy to implement such a relaxation.
Understood, and I'm not so worried about it now that we've found a few ways to get around my motivating problem. BTW, even worse than adding an instance declaration making a legal program illegal is moving an instance declaration across a module boundary. GHC appears perfectly happy with this: << module Foo where class Foo a b | a -> b where foo :: a -> b bar :: (Foo Char t) => t bar = foo 'a'
<< module Bar where import Foo instance Foo Char Bool where foo a = a == 'a'
<< % ghci -fglasgow-exts Bar.hs [deletia] Ok, modules loaded: Bar, Foo. *Bar> :t bar bar :: Bool *Bar> bar True
Cheers, Andrew Bromage