[GHC] #14179: "Conflicting family instance" error pretty prints data family instances poorly

1 Sep 2017

      #14179: "Conflicting family instance" error pretty prints data family instances
poorly
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  TypeFamilies   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This can be observed on GHC 8.0.1 or later:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 data family   Fam a
 data instance Fam Int
 data instance Fam Int
 }}}

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:5:15: error:
     Conflicting family instance declarations:
       Fam Int = -- Defined at Bug.hs:5:15
       Fam Int = -- Defined at Bug.hs:6:15
   |
 5 | data instance Fam Int
   |               ^^^
 }}}

 Notice how GHC attempts to pretty-print the RHS by using a `=` symbol, but
 there is no RHS!

 This gets even more confusing with this program, which requires GHC HEAD:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 data family   Fam :: k -> *
 data instance Fam :: * -> *
 data instance Fam :: * -> *
 }}}
 {{{
 $ ghc5/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.3.20170818: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:6:15: error:
     Conflicting family instance declarations:
       Fam = -- Defined at Bug.hs:6:15
       Fam = -- Defined at Bug.hs:7:15
   |
 6 | data instance Fam :: * -> *
   |               ^^^
 }}}

 Not only is there no RHS, but the kind signatures of each instance aren't
 printed either. This is an issue, since the entire reason why the two
 instances conflict are because of their kinds!

 We should rethink how data family instances are pretty printer here to
 avoid this issue.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14179
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