[GHC] #12124: Ambiguous type variable: it's a red herring!

27 May 2016

      #12124: Ambiguous type variable: it's a red herring!
-------------------------------------+-------------------------------------
           Reporter:  drb226         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 module Ret where

 data Whoops = Whoops Int Int

 foo :: Maybe Int
 foo = return (case Whoops 1 2 of
   Whoops a -> a
   _ -> 0)
 }}}

 Note that there is a pattern match error in the case statement, which the
 compiler correctly reports. However, it first reports the following red
 herring:

 {{{
 ret.hs:6:7: error:
     • Ambiguous type variable ‘m0’ arising from a use of ‘return’
       prevents the constraint ‘(Monad m0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘m0’ should be.
       These potential instances exist:
         instance Monad IO -- Defined in ‘GHC.Base’
         instance Monad Maybe -- Defined in ‘GHC.Base’
         instance Monad ((->) r) -- Defined in ‘GHC.Base’
         ...plus two others
         (use -fprint-potential-instances to see them all)
     • In the expression:
         return
           (case Whoops 1 2 of {
              Whoops a -> a
              _ -> 0 })
       In an equation for ‘foo’:
           foo
             = return
                 (case Whoops 1 2 of {
                    Whoops a -> a
                    _ -> 0 })
 }}}

 One would think that the context `foo :: Maybe Int` is sufficient for the
 compiler to realize that `return (blah)` implies that m0 = Maybe,
 regardless of the errors involved in the expression `(blah)`.

 ghc 7.10.3 does not report this red herring. One can get a similar red-
 herring type error in ghc 7.10.3 by replacing `return (...)` with `return
 $ ...` in the example above.

 The red herring also does *not* appear, in ghc 7.10.3 or in ghc 8.0.1, if
 you name the broken case expression with a let binding.

 {{{
 module Ret where

 data Whoops = Whoops Int Int

 foo :: Maybe Int
 foo = return boo where
   boo = case Whoops 1 2 of
     Whoops a -> a
     _ -> 0
 }}}

 There seems to be something fishy going on here.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12124
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