On Mon, Feb 28, 2005 at 11:25:26AM +0000, Keean Schupke wrote:
Further I think functional dependancies (without overlapping instances), and undecidable instances are perfectly well understood. (incoherent instances would join overlapping instances and not be included).
Where can I read this understanding? The GHC docs on FDs are terse, referring to the original paper, but that paper is somewhat informal, and describes a weaker system than what is implemented. For example, (taken from s7 of the paper), given class U a b | a -> b where u :: a -> b class U a b => V a b where v :: a -> b the paper says that the principal type of \x -> (u x, v x) is (U a b, V a c) => a -> (b,c) According to GHC and Hugs, it is V a b => a -> (b,b) I think that's sensible, but where are the rules that give it? I suspect that writing this addendum may take a while.