Re: [GHC] #12102: “Constraints in kinds” illegal family application in instance (+ documentation issues?)

27 Jul 2017

      #12102: “Constraints in kinds” illegal family application in instance (+
documentation issues?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13780            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 I'm confused then on what this ticket is about. Is the plan to keep this
 feature and fix the bugs reported in the original comment?

 FWIW, I noticed that even after commit
 c2417b87ff59c92fbfa8eceeff2a0d6152b11a47, which fixed several ugly pretty-
 printer bugs that caused things like `'GHC.Types.Eq#` to be printed, a
 similar pretty-printer bug in this ticket is still //not// fixed on GHC
 HEAD. For instance, this program:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}

 import Data.Kind
 import GHC.TypeLits hiding (type (*))

 type family IsTypeLit a where
   IsTypeLit Nat    = 'True
   IsTypeLit Symbol = 'True
   IsTypeLit a      = 'False

 data T :: forall a. (IsTypeLit a ~ 'True) => a -> * where
   MkNat    :: T 42
   MkSymbol :: T "Don't panic!"

 instance Show (T 42) where
 }}}

 Still spits out `'GHC.Types.Eq#`:

 {{{
 $ ghc5/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.3.20170725: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:21:10: error:
     • Illegal type synonym family application in instance:
         T ('Data.Type.Equality.C:~ ('GHC.Types.Eq# <>)) 42
     • In the instance declaration for ‘Show (T 42)’
    |
 21 | instance Show (T 42) where
    |          ^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12102#comment:11
GHC http://www.haskell.org/ghc/
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