Why isn't this Overlapping?

Simon Peyton Jones simonpj at microsoft.com
Tue Apr 18 10:31:30 UTC 2017

Moreover, as discussed in the user manual section<http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#overlapping-instances>, 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


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


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 <anthony_clayden at clear.net.nz<mailto:anthony_clayden at clear.net.nz>> wrote:
--ghc 7.10 or 8.0.1

{-# LANGUAGE DataKinds, KindSignatures, GADTs,
FunctionalDependencies, FlexibleInstances,
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
And for a type-level type equality test, they must be
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.

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