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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Moreover, as discussed in
<a href="http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#overlapping-instances">
the user manual section</a>, GHC doesn’t complain about overlapping instances at the
<b>instance decl</b>, but rather where the instances are <b>used</b>. That’s why there is no overlap complaint here<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>Iavor Diatchki<br>
<b>Sent:</b> 18 April 2017 01:50<br>
<b>To:</b> anthony_clayden@clear.net.nz<br>
<b>Cc:</b> GHC Users Mailing List <glasgow-haskell-users@haskell.org><br>
<b>Subject:</b> Re: Why isn't this Overlapping?<o:p></o:p></span></p>
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Hello,<o:p></o:p></p>
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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).<o:p></o:p></p>
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-Iavor<o:p></o:p></p>
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On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden <<a href="mailto:anthony_clayden@clear.net.nz" target="_blank">anthony_clayden@clear.net.nz</a>> wrote:<o:p></o:p></p>
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--ghc 7.10 or 8.0.1<br>
<br>
{-# LANGUAGE DataKinds, KindSignatures, GADTs,<br>
MultiParamTypeClasses,<br>
FunctionalDependencies, FlexibleInstances,<br>
UndecidableInstances,<br>
NoOverlappingInstances #-}<br>
<br>
class TypeEq a a' (b :: Bool) | a a' -> b<br>
<br>
instance (b ~ True) => TypeEq a a b<br>
instance (b ~ False) => TypeEq a a' b<br>
<br>
Those two instance heads are nearly identical, surely they<br>
overlap?<br>
And for a type-level type equality test, they must be<br>
unifiable.<br>
But GHC doesn't complain.<br>
<br>
If I take off the FunDep, then GHC complains.<br>
<br>
AFAICT none of those extensions imply Overlaps,<br>
but to be sure I've put NoOverlapping.<br>
<br>
<br>
AntC<br>
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