[GHC] #15759: GHC doesn't use fixity in type instances

17 Oct 2018

      #15759: GHC doesn't use fixity in type instances
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If I say

 {{{#!hs
 data a * b
 infixl 7 *

 type family a + b
 infixl 6 +

 type instance Int * Bool + Double = Float
 }}}

 (with `-XNoStarIsType` -- `*` is not the issue here)

 I get

 {{{
     • Illegal family instance for ‘*’
         (* is not an indexed type family)
     • In the type instance declaration for ‘*’
 }}}

 But that's wrong. The `*` should bind tighter than the `+`, meaning this
 is an instance for `+`, not `*`. This also fails for closed type families:

 {{{#!hs
 data a * b
 infixl 7 *

 type family a + b where
   Int * Bool + Double = Float
 infixl 6 +
 }}}

 Interestingly, a ''class'' instance works here.

 This did not bite me "in the wild", but came up while reading the GHC
 source code. Still, it is a real bug.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15759
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