Re: [GHC] #10653: PatternSynonyms should be imported/exported as part of the wildcard notation

18 Jul 2015

      #10653: PatternSynonyms should be imported/exported as part of the wildcard
notation
-------------------------------------+-------------------------------------
        Reporter:  gridaphobe        |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:  pattern
                                     |  synonyms
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by gridaphobe):

 Replying to [comment:3 goldfire]:
...
If `Q` belongs to pairs, then is it ever imported with `(..)` attached
 to some type? I would guess "no".
Well I suppose it would be possible, since

 {{{
 import GHC.Tuple ((,)(..))
 }}}

 is a valid import declaration.

 Here's how I imagine this working, if a module `M` exports a type `T` and
 a pattern `P` whose result type is headed by `T`, e.g.

 {{{
 module M (T(..), pattern P) where

 data T = T Int Int

 pattern P x = T x 0
 }}}

 then a client module that uses a wildcard import, e.g.

 {{{
 module B where
 import M ( T(..) )
 }}}

 should see `P` as well.

 This rule would avoid any sort of spooky action at a distance, where `T`
 and `P` are defined in separate modules, because the wildcard would only
 depend on a single interface file.

 That being said, I'm perfectly happy with goldfire's suggestion too :)
...
What if there were a way to declare that a pattern is attached to a
 type? For example:
{{{
module A ( A(.., pattern A1) ) where ...
}}}
By adding `A1` to the export list of the type `A`, then it is imported
 with `A` as well. We would probably want a check that `A1`'s result type
 is indeed headed by `A`. This seems to make sense in re-exports as well,
 in case the pattern synonyms and original datatype are defined in
 different modules.
One question though, if I have a client module

 {{{
 module B ( A(..) ) where
 import A ( A(..) )
 }}}

 does `B` now export `A1` under your scheme, or does it need to explicitly
 add `pattern A1` to the export declaration?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10653#comment:4
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