[GHC] #12029: Notify user to import * from Data.Kind with TypeInType on

[GHC]

#12029: Notify user to import * from Data.Kind with TypeInType on
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  GHCi              |              Version:  8.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by johnleo):

 The "NB" was added in this patch: https://phabricator.haskell.org/D1610
 I don't see a corresponding trac ticket.  It seems there was some doubt as
 to whether this was the best course.

 There are actually a number of inconsistent behaviors with `*` and `★`
 which could be addressed.  For one thing if you import `*` from Data.Kind
 you don't get `★` also which might be confusing since one is supposed to
 be the Unicode version of the other, but maybe this is okay.

 Another is that you do `:set -fprint-unicode-syntax` then `:k` displays
 kinds as `★` whereas `:i` displays kinds as `*`.

 The NB for `*` (it works the same way for `★`) only shows up if you have
 no other definition for `*` in the kind namespace.  So for example if you
 define (with `TypeOperators`)
 {{{
 data (f * g) e = Inl1 (f e) | Inr1 (g e)
 data (f ★ g) e = Inl2 (f e) | Inr2 (g e)
 }}}
 you will now get
 {{{
 ghci> :k (*)
 (*) ∷ ∀ {k}. (k → ★) → (k → ★) → k → ★
 ghci> :k (★)
 (★) ∷ ∀ {k}. (k → ★) → (k → ★) → k → ★
 }}}
 with no mention of the missing kind `*` or `★`.  If you next import them
 you'll get an error for an ambiguous occurrence.

 I suppose the NB was added to handle a very specific case in which a naive
 user doing nothing sophisticated was trying to find the kind of `*` and
 was surprised it didn't exist.  Perhaps it would be equally surprising to
 find it not showing up with `:i`, but as this is more of a search over all
 uses I'd be inclined not to bother adding this special note.  In addition
 I'd recommend dropping the NB change in 8.2 to make the behaviors
 consistent, since the requirement to import `*` from Data.Kind is clearly
 documented and people will have had time to absorb the change by then.
 However I'm happy to hear other opinions about either of these proposals.

 Finally, since `*` and `★` are both deprecated should we not be using
 `Type` from 8.2 onward?  As in
 {{{
 ghci> :k (*)
 (*) ∷ Type
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12029#comment:1>
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