
<interactive>:1:1: error: [GHC-39999]
    • 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.
      Potentially matching instances:
        instance Num Integer -- Defined in ‘GHC.Num’
        instance Num Double -- Defined in ‘GHC.Float’
        ...plus three others
        ...plus one instance involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: foo