[GHC] #7857: GHC says there are unsafe overlapping instances, but only one instance applies

[GHC]

#7857: GHC says there are unsafe overlapping instances, but only one instance
applies
-------------------------------+--------------------------------------------
  Reporter:  dmwit             |          Owner:                  
      Type:  bug               |         Status:  closed          
  Priority:  normal            |      Milestone:                  
 Component:  Compiler          |        Version:  7.6.1           
Resolution:  fixed             |       Keywords:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown      |     Difficulty:  Unknown         
  Testcase:                    |      Blockedby:                  
  Blocking:                    |        Related:                  
-------------------------------+--------------------------------------------
Changes (by simonpj):

  * status:  new => closed
  * difficulty:  => Unknown
  * resolution:  => fixed


Comment:

 Thanks.  Happily, with HEAD I get
 {{{
 T7857.hs:8:11:
     Could not deduce (PrintfType s0) arising from a use of ‛printf’
     from the context (PrintfArg t)
       bound by the inferred type of g :: PrintfArg t => t -> s
       at T7857.hs:8:1-21
     The type variable ‛s0’ is ambiguous
     Note: there are several potential instances:
       instance [safe] (PrintfArg a, PrintfType r) => PrintfType (a -> r)
         -- Defined in ‛Text.Printf’
       instance [safe] PrintfType (IO a) -- Defined in ‛Text.Printf’
       instance [safe] IsChar c => PrintfType [c]
         -- Defined in ‛Text.Printf’
     In the second argument of ‛($)’, namely ‛printf "" i’
     In the expression: f $ printf "" i
     In an equation for ‛g’: g i = f $ printf "" i
 }}}
 which seems very plausible.

 So I propose to close this as fixed; I don't think it's worth messing with
 the 7.6 branch.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7857#comment:1>
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