#12428: Allow pattern synonyms to optionally carry coerceability -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler