[GHC] #2135: Warn if functions are exported whose types cannot be written

[GHC]

#2135: Warn if functions are exported whose types cannot be written
--------------------------------+-------------------------------------------
    Reporter:  dons             |       Owner:          
        Type:  feature request  |      Status:  new     
    Priority:  normal           |   Component:  Compiler
     Version:  6.8.2            |    Severity:  normal  
    Keywords:  warnings         |    Testcase:          
Architecture:  Unknown          |          Os:  Unknown 
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 It should be possible to warn if a function is exported from a module
 whose type is hidden, and thus the function's type can't be written down
 directly by the user.

 With the proliferation in type level programming in Haskell (and its use
 in libraries), situations
 can arise where functions are exported from a module which can be used,
 but whose type cannote be written, but where the type is complex enough
 that it really is required to write it down for the code to compile. This
 happened recently in the takusen library.

 A warning for this situation would help authors of libraries avoid this
 situation.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2135>
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