[GHC] #12428: Allow pattern synonyms to optionally carry coerceability

[GHC]

#12428: Allow pattern synonyms to optionally carry coerceability
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:3 simonpj]:
 > Could you give a specific, concrete example?

 Suppose I have a module that defines a newtype

 {{{#!hs
 newtype Foo a = Foo a
 }}}

 Suppose this module has a zillion users, and at some point its maintainer
 decides, for internal reasons, to change the definition of `Foo` to

 {{{#!hs
 newtype Foo a = Foo (Bar a)
 }}}

 where `Bar` is another newtype. She can expose a ''similar'' interface by
 hiding the `Foo` constructor and instead exporting a pattern synonym

 {{{#!hs
 pattern Foo a = Foo (Bar a)
 }}}

 But the interface is not quite the same! If someone has a module that
 imports hers and then uses the `Coercible a (Foo a)` instance, it will
 simply stop working now that the actual `Foo` constructor is no longer
 exported.

 What I'm proposing is that pattern synonym writers who can see the actual
 constructor should be able to "bless" their pattern synonyms with its
 magical import/export power, and that this should be available
 transitively. I ''don't'' want it to be automatic, because sometimes I
 explicitly want to use a pattern synonym to block coercions.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12428#comment:4>
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