On Wed, Feb 15, 2006 at 08:29:48PM -0800, oleg@pobox.com wrote:
This message shows the (extensible polymorphic) record selection code that requires neither overlapping instances and not even undecidable instances. Therefore, it works both in GHC and Hugs. This constructively shows that it is possible to write less-trivial code using functional dependencies and multi-parameter type classes as had been specified by Mark P. Jones -- that is, abiding by his syntactic decidability constraints. Therefore, if Haskell' standardized the current practice -- which is implemented _both_ by GHC and Hugs (with no overlapping instances, no undecidable instances), the result can still be useful.
I'm afraid it's worse than that: current practice lacks a foundation, and attempts to provide one have exposed flaws (see the FunctionalDependencies page). FDs as specified by Mark are significantly weaker than what is implemented by GHC and Hugs. Even so, the original specification needs an additional restriction to prevent some non-terminating instances. In line with recent studies of FDs by Sulzmann et al, the CVS version of GHC has much tighter rules (closer to the original specification). It also has a more liberal termination condition for instances (see the end of FlexibleInstances), but this is only partial compensation. For example,
class TypeEq a b c | a b -> c class EqL1 a c | a -> c class EqL2 a c | a -> c instance EqL1 b c => TypeEq L1 b c instance EqL2 b c => TypeEq L2 b c
These instances are not permitted by the original restrictions: the third (range) argument contains a variable (c) that does not occur in the first two (domain) arguments. A safe relaxation has been proposed (see end of FunctionalDependencies) and Simon favours adding it to GHC. It is quite complex, though.
class Select dummy label val rec | label rec -> val where lookp :: dummy -> rec -> label -> val
instance (TypeEq label l tr, ProjLabel r l, Select' tr label val r) => Select () label val r where lookp () r label = lookup' (typeEq label (proj'label r)) r label
This instance (and the similar one for Select') were permitted by the original rules, under a relaxation that has since been shown to produce non-termination in some cases (CHR paper, ex. 15). No replacement has been proposed, so the relaxation has been removed from the CVS version of GHC, which thus rejects this.