[GHC] #16326: Implement visible dependent quantification

[GHC]

#16326: Implement visible dependent quantification
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  task              |               Status:  closed
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler          |              Version:  8.7
      Resolution:  fixed             |             Keywords:  GHCProposal,
                                     |  VisibleDependentQuantification
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  dependent/should_compile/T16326_Compile{1,2},
                                     |  dependent/should_fail/T16326_Fail{1,2,3,4,5,6,7,8,9,10,11,12},
                                     |  th/T16326_TH
      Blocked By:                    |             Blocking:
 Related Tickets:  #15658            |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/378
-------------------------------------+-------------------------------------

Comment (by Marge Bot <ben+marge-bot@…>):

 In [changeset:"c26d299dc422f43b8c37da4b26da2067eedcbae8/ghc" c26d299/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c26d299dc422f43b8c37da4b26da2067eedcbae8"
 Visible dependent quantification

 This implements GHC proposal 35
 (https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0035
 -forall-arrow.rst)
 by adding the ability to write kinds with
 visible dependent quantification (VDQ).

 Most of the work for supporting VDQ was actually done _before_ this
 patch. That is, GHC has been able to reason about kinds with VDQ for
 some time, but it lacked the ability to let programmers directly
 write these kinds in the source syntax. This patch is primarly about
 exposing this ability, by:

 * Changing `HsForAllTy` to add an additional field of type
   `ForallVisFlag` to distinguish between invisible `forall`s (i.e,
   with dots) and visible `forall`s (i.e., with arrows)
 * Changing `Parser.y` accordingly

 The rest of the patch mostly concerns adding validity checking to
 ensure that VDQ is never used in the type of a term (as permitting
 this would require full-spectrum dependent types). This is
 accomplished by:

 * Adding a `vdqAllowed` predicate to `TcValidity`.
 * Introducing `splitLHsSigmaTyInvis`, a variant of `splitLHsSigmaTy`
   that only splits invisible `forall`s. This function is used in
   certain places (e.g., in instance declarations) to ensure that GHC
   doesn't try to split visible `forall`s (e.g., if it tried splitting
   `instance forall a -> Show (Blah a)`, then GHC would mistakenly
   allow that declaration!)

 This also updates Template Haskell by introducing a new `ForallVisT`
 constructor to `Type`.

 Fixes #16326. Also fixes #15658 by documenting this feature in the
 users' guide.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16326#comment:6>
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