[GHC] #13962: GHCi allows unsaturated type family

[GHC]

#13962: GHCi allows unsaturated type family
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12089            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 GHC is behaving according to its manual. See
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html
 #ghci-cmd-:kind this section].

 We made a special case for `:kind` some years ago to allow queries like
 `:kind Map`. The user sensibly wants `Map`'s kind, and we should give it
 to them. So the TF-saturation requirement is dropped in `:kind`. I support
 this design choice.

 On the other hand, it's very suspect in `:kind!`. It's surprising to me,
 too, that this gives the right result. But, after all, TF evaluation makes
 fine sense with unsaturated type families -- it's just type inference that
 gets muddled. So: we could keep the current behavior (which doesn't seem
 to be hurting anyone), or restrict the behavior of `kind!` to disallow
 partially applied type families. Maybe an intermediate approach is to keep
 the current behavior but issue a warning in the event of a partially
 applied family.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13962#comment:3>
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