Re: [GHC] #16013: :kind! accepts unsaturated type aliases

16 Jan 2019

      #16013: :kind! accepts unsaturated type aliases
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        Reporter:  dmwit             |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  ghci/should_fail/T16013
      Blocked By:                    |             Blocking:
 Related Tickets:  #13962            |  Differential Rev(s):  Phab:D5471,
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/90
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Comment (by Ryan Scott ):

 In [changeset:"6b70cf611e5ddc475edaa54b893d20990699ddb8/ghc" 6b70cf6/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="6b70cf611e5ddc475edaa54b893d20990699ddb8"
 Be pickier about unsaturated synonyms in :kind

 Summary:
 We currently permit any and all uses of unsaturated type
 synonyms and type families in GHCi's `:kind` command, which allows
 strange interactions like this one:

 ```
...
:set -XTypeFamilies -XPolyKinds
type family Id (a :: k)
type instance Id a = a
type Foo x = Maybe
:kind! Id Foo

This is probably a stretch too far, so this patch moves to disallow
unsaturated synonyms that aren't at the top level (we still want to
allow `:kind Id`, for instance). We do this by augmenting `GhciCtxt`
with an additional `Bool` field to indicate if we are at the
outermost level of the type being passed to `:kind` or not. See
`Note [Unsaturated type synonyms in GHCi]` in `TcValidity` for the
full story.

Test Plan: make test TEST=T16013

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: simonpj, goldfire, rwbarton, carter

GHC Trac Issues: #16013

Differential Revision: https://phabricator.haskell.org/D5471
}}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16013#comment:13>
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