[GHC] #8707: Kind inference fails in data instance definition

[GHC]

#8707: Kind inference fails in data instance definition
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        Reporter:  goldfire          |            Owner:  jstolarek
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by jstolarek):

 > 2) The error message is not particularly helpful. GHC 7.6.3 had a more
 verbose, but more helpful message.

 Code responsible for this difference is here
 [[GhcFile(compiler/types/TypeRep.lhs#L711)]] in the function `pprTcApp`:

 {{{
   | Just dc <- isPromotedDataCon_maybe tc
   , let dc_tc = dataConTyCon dc
   , isTupleTyCon dc_tc
   , let arity = tyConArity dc_tc    -- E.g. 3 for (,,) k1 k2 k3 t1 t2 t3
         ty_args = drop arity tys    -- Drop the kind args
 , ty_args `lengthIs` arity        -- Result is saturated
   = pprPromotionQuote tc <>
     (tupleParens (tupleTyConSort dc_tc) $
      sep (punctuate comma (map (pp TopPrec) tys)))
 }}}

 The only question is how we want the arguments printed? GHC 7.6 prints
 them like this:

 {{{
 Data constructor `SFalse' returns type `SingDF
                                           Bool k ('(,) Bool (k -> *)
 'False (Ctor k))'
   instead of an instance of its parent type `SingDF Bool Bool a'
 }}}

 GHC HEAD prints tuple constructors in the standard notation (not prefix as
 seen in GHC 7.6 output):

 {{{
 Data constructor ‘SFalse’ returns type ‘SingDF
                                           Bool k '(Bool, k -> *, 'False,
 Ctor k)’
   instead of an instance of its parent type ‘SingDF Bool Bool a’
 }}}

 But printing kinds as part of a tuple is misleading. Any suggestions how
 should the output look like?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8707#comment:3>
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