On Tue Apr 18 10:31:30 UTC 2017, Simon Peyton Jones wrote:
Moreover, as discussed in the user manual section, GHC doesnt complain about overlapping instances at the instance decl, but rather where the instances are used.
Thank you Simon, yes I knew that, so I'd written a usage (just didn't bother putting it in the message ): foo :: (TypeEq a a' b) => a -> a' -> String foo _ _ = "blah" x = foo 'c' "String"
Thats why there is no overlap complaint here
I didn't get a complaint about `x`, contrary to what I expected. On trying again just now: y = foo 'c' 'd' GHC _does_ complain of overlap. I apologise for the distraction. AntC
On 18 April 2017 01:50, Iavor Diatchki wrote
these two instances really should be rejected as they violate the FD of the class: we can derive `TypeEq a a True` using the first instance and `TypeEq a a False` using the second one. Unfortunately, the check that we are using to validate FDs when `UndecidableInstances` is on, is not quite correct (relevant tickets are #9210 and #10675 where there are similar examples).
On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden wrote:
--ghc 7.10 or 8.0.1
{-# LANGUAGE DataKinds, KindSignatures, GADTs, MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, NoOverlappingInstances #-}
class TypeEq a a' (b :: Bool) | a a' -> b
instance (b ~ True) => TypeEq a a b instance (b ~ False) => TypeEq a a' b
Those two instance heads are nearly identical, surely they overlap? And for a type-level type equality test, they must be unifiable. But GHC doesn't complain.
If I take off the FunDep, then GHC complains.
AFAICT none of those extensions imply Overlaps, but to be sure I've put NoOverlapping.