[GHC] #12476: Error building HList

[GHC]

#12476: Error building HList
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        Reporter:  DanielWaterworth  |                Owner:
            Type:  bug               |               Status:  infoneeded
        Priority:  normal            |            Milestone:  8.0.2
       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:                    |
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Changes (by bgamari):

 * status:  new => infoneeded


Comment:

 Well, this doesn't quite compile yet but then again it also doesn't
 compile with 7.10. DanielWaterworth, can you confirm that this is
 expected?
 {{{
 Data/HList/Dredge.hs:78:27: error:
     • Ambiguous type variables ‘x1’, ‘k1’, ‘l1’ arising from a use of
 ‘dredge’
       prevents the constraint ‘(EnsureLabel
                                   x1 (Label l1))’ from being solved.
       Relevant bindings include
         label :: x1 (bound at Data/HList/Dredge.hs:78:9)
         dredge' :: x1 -> p1 a1 (f1 a1) -> p1 s1 (f1 s1)
           (bound at Data/HList/Dredge.hs:78:1)
       Probable fix: use a type annotation to specify what ‘x1’, ‘k1’, ‘l1’
 should be.
       These potential instances exist:
         instance forall k (x :: k). EnsureLabel (Label x) (Label x)
           -- Defined at Data/HList/Labelable.hs:204:10
         instance forall k (x :: k). EnsureLabel (Proxy x) (Label x)
           -- Defined at Data/HList/Labelable.hs:207:10
         ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘isSimple’, namely ‘(dredge label)’
       In the expression: isSimple (dredge label)
       In an equation for ‘dredge'’:
           dredge' label = isSimple (dredge label)

 Data/HList/Dredge.hs:90:29: error:
     • Ambiguous type variables ‘x0’, ‘k0’, ‘l0’ arising from a use of
 ‘dredgeND’
       prevents the constraint ‘(EnsureLabel
                                   x0 (Label l0))’ from being solved.
       Relevant bindings include
         label :: x0 (bound at Data/HList/Dredge.hs:90:11)
         dredgeND' :: x0 -> p0 a0 (f0 a0) -> p0 s0 (f0 s0)
           (bound at Data/HList/Dredge.hs:90:1)
       Probable fix: use a type annotation to specify what ‘x0’, ‘k0’, ‘l0’
 should be.
       These potential instances exist:
         instance forall k (x :: k). EnsureLabel (Label x) (Label x)
           -- Defined at Data/HList/Labelable.hs:204:10
         instance forall k (x :: k). EnsureLabel (Proxy x) (Label x)
           -- Defined at Data/HList/Labelable.hs:207:10
         ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘isSimple’, namely ‘(dredgeND label)’
       In the expression: isSimple (dredgeND label)
       In an equation for ‘dredgeND'’:
           dredgeND' label = isSimple (dredgeND label)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12476#comment:3>
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