[GHC] #10179: Kinds missing from types in ghci

[GHC]

#10179: Kinds missing from types in ghci
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        Reporter:  br1               |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  GHCi              |                 Version:  7.10.1-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Good point.  The question is this: '''when pretty-printing a type, when
 and how should we display kind annotations?'''. Currenty in GHCi we by-
 default suppress foralls and kind annotations.

 (NB: by the time we get to "pretty print this type" we've lost touch with
 the original source-language type signature, and in any case the same
 rules should apply for inferred types.)

 For example, this is what happens currently (without `-fprint-explicit-
 foralls` and `-fprint-explicit-kinds`):
 {{{
 Type                                                     Display
 -------------------------------------------------------------------------------
 1. f :: forall (a::*). a->a                              f :: a->a

 2. f :: forall (m::*->*) (a::*). m a -> a                f :: m a -> a

 3. f :: forall (k::BOX) (m::k->*) (a::k). m a -> m a     f :: m a -> m a
 ???

 4. f :: forall (m::*->*) (a::*). m a -> m a              f :: m a -> m a
 ???
 }}}
 Note that
  * In (2) the kinds are forced by the rest of the type `m a -> a`
  * In (3), assuming `-XPolyKinds` the type on the left is the most kind-
 polymorphic type compatible with the type on the right.
  * In (4) the kind polymoprhism has been squashed out, as it has in the
 ticket description.

 The trouble is that (3) and (4) are displayed the same, even though they
 are different.

 It's hard to even say exactly what we want here!  Perhaps something like
 "print the kind-attributed foralls explicitly, unless the kinds are
 implied by the most-kind-polymorphic reading of the signature"; but that's
 quite complicated.  Ideas?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10179#comment:2>
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