[GHC] #5910: Holes with other constraints

[GHC]

#5910: Holes with other constraints
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    Reporter:  xnyhps                   |       Owner:                                   
        Type:  feature request          |      Status:  new                              
    Priority:  normal                   |   Milestone:                                   
   Component:  Compiler (Type checker)  |     Version:  7.5                              
    Keywords:  holes                    |          Os:  Unknown/Multiple                 
Architecture:  Unknown/Multiple         |     Failure:  Incorrect warning at compile-time
  Difficulty:  Unknown                  |    Testcase:                                   
   Blockedby:                           |    Blocking:                                   
     Related:                           |  
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Changes (by simonpj):

  * difficulty:  => Unknown


Comment:

 Some brief responses. We may want a Skype chat.

  * I think the Right Way to do this is to add a new form of constraint, as
 you have done.

  * I do not see why you need `tcl_holes` in the `TcLclEnv`.

  * The place to report the unsolved holes is '''not''' in `TcRnDriver`,
 but rather in `TcErrors`.  It is a thing of beauty. The constraint solver
 tries to solve the constraints, ''returning an unsolved constraint''.  The
 latter is passed to `reportUnsolved` and '''that''' is the place to report
 any unsolved holes.

  * Moreover, `TcErrors` ''already has'' a kind of error prioritisation: it
 does not report ambiguity errors if it any more important errors (such as
 holes) happen first: see `tryReporters` in `reportInsolsAndFlats`.  So
 that will deal nicely with the problem you describe.

  * Finally, here's a fresh idea.  If you store the `TcTypeEnv` in the
 `CtOrigin` of the hole, then when you report the hole as an error you can
 look at the type environment (better zonk it first) and print the types of
 all locally bound identifiers.  Or some. Or whatever.

 So I think you can make your implementation able to do more, and yet have
 less code!

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5910#comment:2>
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