Functional dependencies question
Andrew J Bromage
Tue, 13 May 2003 18:16:08 +1000
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
instance Foo Char Bool where
foo a = a == 'a'
% ghci -fglasgow-exts Bar.hs
Ok, modules loaded: Bar, Foo.
*Bar> :t bar
bar :: Bool