[GHC] #12542: Unexpected failure.. (bug?)

[GHC]

#12542: Unexpected failure.. (bug?)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 From [http://www.cse.chalmers.se/~emax/documents/axelsson2012generic-
 slides.pdf A Generic Abstract Syntax Model for Embedded
 Languages]([http://www.cse.chalmers.se/~emax/documents/axelsson2012generic.pdf
 paper])

 {{{#!hs
 infixr :->
 infixl 1 :$
 data Full a
 data a :-> b

 data AST dom sig where
   Sym  :: dom sig -> AST dom sig
   (:$) :: AST dom (a :-> sig) -> AST dom (Full a) -> AST dom sig

 class Binding dom where
   viewVar :: dom a         -> Maybe Integer
   viewBnd :: dom (a :-> b) -> Maybe Integer

 freeVars :: Binding dom => AST dom a -> [Integer]
 freeVars = \case
   Sym (a -> Just v) -> [v]
   Sym (b -> Just v) :$ body -> undefined

   where
     (a, _) = (viewVar, undefined)
     (_, b) = (undefined, viewBnd)
 }}}

 and so does

 {{{#!hs
   where
     a = viewVar
     b = viewBnd
 }}}

 but

 {{{#!hs
   where
     (a, b) = (viewVar, viewBnd)
 }}}

 yields

 {{{
 tSvp.hs:20:5-31: error: …
     • Could not deduce (Binding dom0)
       from the context: Binding dom
         bound by the type signature for:
                    freeVars :: Binding dom => AST dom a -> [Integer]
         at /tmp/tSvp.hs:14:1-49
       or from: Binding dom2
         bound by the inferred type for ‘a’:
                    Binding dom2 => dom2 a1 -> Maybe Integer
         at /tmp/tSvp.hs:20:5-31
       The type variable ‘dom0’ is ambiguous
     • When checking that the inferred type
         a :: forall k k1 (dom :: * -> *) a (dom1 :: *
                                                     -> *) (a1 :: k) (b ::
 k1).
              (Binding dom1, Binding dom) =>
              dom a -> Maybe Integer
       is as general as its inferred signature
         a :: forall (dom :: * -> *) a.
              Binding dom =>
              dom a -> Maybe Integer
       In an equation for ‘freeVars’:
           freeVars
             = \case {
                 Sym (a -> Just v) -> [v]
                 Sym (b -> Just v) :$ body -> undefined }
             where
                 (a, b) = (viewVar, viewBnd)
 tSvp.hs:20:5-31: error: …
     • Could not deduce (Binding dom1)
       from the context: Binding dom
         bound by the type signature for:
                    freeVars :: Binding dom => AST dom a -> [Integer]
         at /tmp/tSvp.hs:14:1-49
       or from: Binding dom2
         bound by the inferred type for ‘b’:
                    Binding dom2 => dom2 (a1 :-> b) -> Maybe Integer
         at /tmp/tSvp.hs:20:5-31
       The type variable ‘dom1’ is ambiguous
     • When checking that the inferred type
         b :: forall k k1 (dom :: * -> *) a (dom1 :: *
                                                     -> *) (a1 :: k) (b ::
 k1).
              (Binding dom1, Binding dom) =>
              dom1 (a1 :-> b) -> Maybe Integer
       is as general as its inferred signature
         b :: forall k k1 (dom :: * -> *) (a :: k) (b :: k1).
              Binding dom =>
              dom (a :-> b) -> Maybe Integer
       In an equation for ‘freeVars’:
           freeVars
             = \case {
                 Sym (a -> Just v) -> [v]
                 Sym (b -> Just v) :$ body -> undefined }
             where
                 (a, b) = (viewVar, viewBnd)
 Compilation failed.
 }}}

 Is this intentional?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12542>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler