Moreover, as discussed in the user manual section, GHC doesn’t complain about overlapping instances at the instance decl, but rather where the instances are used. That’s why there is no overlap complaint here

Simon

From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Iavor Diatchki
Sent: 18 April 2017 01:50
To: anthony_clayden@clear.net.nz
Cc: GHC Users Mailing List <glasgow-haskell-users@haskell.org>
Subject: Re: Why isn't this Overlapping?

Hello,

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).

-Iavor

On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden <anthony_clayden@clear.net.nz> 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.


AntC
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