[GHC] #13322: Pattern synonyms in hs-boot files

[GHC]

#13322: Pattern synonyms in hs-boot files
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  hs-boot        |  Operating System:  Unknown/Multiple
  backpack                           |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Pattern synonyms are not permitted in hs-boot files:

 {{{
 ezyang at sabre:~/bun$ ghc-head -c A.hs-boot

 A.hs-boot:3:5: error:
     Misplaced pattern synonym signature: pattern X :: Bool
   |
 3 |     pattern X :: Bool
   |     ^^^^^^^^^^^^^^^^^

 A.hs-boot:3:13: error:
     The pattern synonym signature for ‘X’ lacks an accompanying binding
   |
 3 |     pattern X :: Bool
   |
 }}}

 There isn't really any reason why they shouldn't be supported.

 One thing to check closely: if we bundle a pattern synonym declared in an
 hs-boot file with another type, in an hs-boot file, what happens? In
 particular, does an hs file which doesn't bundle the pattern synonym and
 the type valid?

 If we add support for them, make sure that Backpack signatures handle it
 properly. In particular, suppose we have:

 {{{
 signature A(T(X)) where
   data T
   pattern X :: T
 }}}

 Then, when we instantiate A, we must be careful to NOT blindly reuse the
 implementing module of T for X (as we do today in `uAvailInfo` in
 `NameShape`), since T and X may be defined in different modules.

 I'm not likely to implement this unless someone asks.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13322>
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