[GHC] #7812: Ambiguity check too eager with unconstrained type variable

[GHC]

#7812: Ambiguity check too eager with unconstrained type variable
----------------------------------------+-----------------------------------
    Reporter:  goldfire                 |       Owner:                  
        Type:  bug                      |      Status:  new             
    Priority:  normal                   |   Milestone:                  
   Component:  Compiler (Type checker)  |     Version:  7.7             
    Keywords:  AmbiguityCheck           |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple         |     Failure:  None/Unknown    
  Difficulty:  Unknown                  |    Testcase:                  
   Blockedby:                           |    Blocking:                  
     Related:                           |  
----------------------------------------+-----------------------------------
Changes (by simonpj):

  * difficulty:  => Unknown


Comment:

 Unlike #7804, this one is by design. Suppose you said
 {{{
 bar :: SBool b -> [F b a]
 bar = foo
 }}}
 You'd expect that to work, right?  Same type signature!  But it doesn't,
 even in 7.6:
 {{{
 T7812.hs:17:7:
     Couldn't match type `F b a' with `F b a0'
     NB: `F' is a type function, and may not be injective
     The type variable `a0' is ambiguous
     Possible fix: add a type signature that fixes these type variable(s)
     Expected type: SBool b -> [F b a]
       Actual type: SBool b -> [F b a0]
     In the expression: foo
     In an equation for `bar': bar = foo
 }}}
 People complained about this, especially when they got an inferred type,
 tried to add it as a type signature, and had the program rejected.

 Your example is deeply strange too; if you put a singleton list you might
 well be stuck.  And you could just make your type signature polymorphic in
 the list
 {{{
 foo :: SBool b -> [c]
 }}}
 In short, I can't see how to help here.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7812#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler