Ambiguous type variable: it's a red herring!

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.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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