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
Simon From: Glasgow-haskell-users [mailto:glasgow-haskell-users-boun...@haskell.org] On Behalf Of Iavor Diatchki Sent: 18 April 2017 01:50 To: anthony_clay...@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_clay...@clear.net.nz<mailto:anthony_clay...@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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org<mailto:Glasgow-haskell-users@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell-users&data=02%7C01%7Csimonpj%40microsoft.com%7Cc6be4bcfd23946f5f4fc08d485f4f93a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636280734593515972&sdata=Us2Ovast3dKEpNkfEDbejyBHGmGKh2ElAi6%2FqQY8iFE%3D&reserved=0>
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