
<interactive>:1:1: error:
    Ambiguous type variable ‘a0’ arising from a use of ‘foo’
    prevents the constraint ‘(Num a0)’ from being solved.
    Probable fix: use a type annotation to specify what ‘a0’ should be.
    These potential instances exist:
      instance Num Integer -- Defined in ‘GHC.Num’
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      ...plus two others
      ...plus 8 instances involving out-of-scope types
      (use -fprint-potential-instances to see them all)