[GHC] #10094: Template Haskell cannot represent type wildcards

[GHC]

#10094: Template Haskell cannot represent type wildcards
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        Reporter:  adamgundry        |                   Owner:
            Type:  feature request   |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.10.1-rc1
      Resolution:  duplicate         |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9879, #10548     |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"49373ffe4cbc87b46468d2372e850138e151a9ae/ghc"
 49373ffe/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="49373ffe4cbc87b46468d2372e850138e151a9ae"
 Support wild cards in TH splices

 - Declaration splices: partial type signatures are fully supported in TH
   declaration splices.

   For example, the wild cards in the example below will unify with `Eq
 a`
   and `a -> a -> Bool`, as expected:

 ```
 [d| foo :: _ => _
     foo x y = x == y |]
 ```

 - Expression splices: anonymous and named wild cards are supported in
   expression signatures, but extra-constraints wild cards aren't. Just
   as is the case for regular expression signatures.

 ```
 [e | Just True :: _a _ |]
 ```

 - Typed expression splices: the same wildcards as in (untyped)
   expression splices are supported.

 - Pattern splices: TH doesn't support type signatures in pattern
   splices, consequently, partial type signatures aren't supported
   either.

 - Type splices: partial type signatures are only partially supported in
   type splices, specifically: only anonymous wild cards are allowed.

   So `[t| _ |]`, `[t| _ -> Maybe _ |]` will work, but `[t| _ => _ |]` or
   `[| _a |]` won't (without `-XNamedWildCards`, the latter will work as
   the named wild card is treated as a type variable).

   Normally, named wild cards are collected before renaming a (partial)
   type signature. However, TH type splices are run during renaming, i.e.
   after the initial traversal, leading to out of scope errors for named
   wild cards. We can't just extend the initial traversal to collect the
   named wild cards in TH type splices, as we'd need to expand them,
   which is supposed to happen only once, during renaming.

   Similarly, the extra-constraints wild card is handled right before
   renaming too, and is therefore also not supported in a TH type splice.
   Another reason not to support extra-constraints wild cards in TH type
   splices is that a single signature can contain many TH type splices,
   whereas it mustn't contain more than one extra-constraints wild card.
   Enforcing would this be hard the way things are currently organised.

   Anonymous wild cards pose no problem, because they start without names
   and are given names during renaming. These names are collected right
   after renaming. The names generated for anonymous wild cards in TH
   type splices will thus be collected as well.

   With a more invasive refactoring of the renaming, partial type
   signatures could be fully supported in TH type splices. As only
   anonymous wild cards have been requested so far, these small changes
   satisfying this request will do for now. Also don't forget that a TH
   declaration splices support all kinds of wild cards.

 - Extra-constraints wild cards were silently ignored in expression and
   pattern signatures, appropriate error messages are now generated.

 Test Plan: run new tests

 Reviewers: austin, goldfire, adamgundry, bgamari

 Reviewed By: goldfire, adamgundry, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1048

 GHC Trac Issues: #10094, #10548
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10094#comment:4>
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