Re: [GHC] #14059: COMPLETE sets don't work at all with data family instances

29 May 2018

      #14059: COMPLETE sets don't work at all with data family instances
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Unfortunately, this means that the non-data-family instance version has
 also //regressed//. That is to say, this program:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 {-# OPTIONS_GHC -Wincomplete-patterns #-}
 module Foo where

 data SBool (z :: Bool) where
   SFalse :: SBool False
   STrue  :: SBool True

 pattern STooGoodToBeTrue :: forall (z :: Bool). ()
                          => z ~ True
                          => SBool z
 pattern STooGoodToBeTrue = STrue
 {-# COMPLETE SFalse, STooGoodToBeTrue #-}

 wibble :: SBool z -> Bool
 wibble STrue = True

 wobble :: SBool z -> Bool
 wobble STooGoodToBeTrue = True
 }}}

 Used to give the right warning in GHC 8.2 (as shown in the original
 description), but on GHC 8.4, it now demonstrates the same problem as in
 the version with data families:

 {{{
 $ /opt/ghc/8.4.2/bin/ghci Foo.hs
 GHCi, version 8.4.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Foo              ( Foo.hs, interpreted )

 Foo.hs:20:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘wibble’: Patterns not matched: SFalse
    |
 20 | wibble STrue = True
    | ^^^^^^^^^^^^^^^^^^^

 Foo.hs:23:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘wobble’: Patterns not matched: _
    |
 23 | wobble STooGoodToBeTrue = True
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I suppose the uniformity is comforting, at least...

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14059#comment:6
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