[GHC] #12976: GHCi displays the kinds of unboxed tuples incorrectly

14 Dec 2016

      #12976: GHCi displays the kinds of unboxed tuples incorrectly
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This can be reproduced in GHCi 8.0.2 or HEAD. First, a preamble:

 {{{
 $ inplace/bin/ghc-stage2 --interactive
 GHCi, version 8.1.20161208: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 λ> :set -fprint-explicit-runtime-reps -fprint-explicit-kinds -fprint-
 explicit-foralls -XUnboxedTuples -XTemplateHaskell -XMagicHash
 λ> :m + GHC.Exts Language.Haskell.TH Language.Haskell.TH.Lib
 Language.Haskell.TH.Syntax
 }}}

 The kind of `(#,#)` is reported correctly:

 {{{
 (#,#) :: forall (k0 :: RuntimeRep) (k1 :: RuntimeRep).
          TYPE k0 -> TYPE k1 -> TYPE 'UnboxedTupleRep
 }}}

 But when you give it an argument, it seems to default `TYPE k1` to `*` for
 some reason!

 {{{
 λ> :k (#,#) Int#
 (#,#) Int# :: * -> TYPE 'UnboxedTupleRep
 }}}

 This is a blatant lie, as this kind-checks:

 {{{
 λ> :k (#,#) Int# Int#
 (#,#) Int# Int# :: TYPE 'UnboxedTupleRep
 }}}

 The effect is even more noticeable when you throw `TemplateHaskell` into
 the mix:

 {{{
 λ> :k $(unboxedTupleT 2)
 $(unboxedTupleT 2) :: * -> * -> TYPE 'UnboxedTupleRep
 λ> :k $(unboxedTupleT 2) Int#
 $(unboxedTupleT 2) Int# :: * -> TYPE 'UnboxedTupleRep
 λ> :k $(conT (unboxedTupleTypeName 2))
 $(conT (unboxedTupleTypeName 2)) :: * -> * -> TYPE 'UnboxedTupleRep
 λ> :k $(conT (unboxedTupleTypeName 2)) Int#
 $(conT (unboxedTupleTypeName 2)) Int# :: * -> TYPE 'UnboxedTupleRep
 }}}

 All of these kinds should be reporting the `RuntimeRep`s that GHC is
 //actually// using under the hood.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12976
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