tcfail040.hs:19:5: error: [GHC-39999]
    • Ambiguous type variable ‘a0’ arising from a use of ‘<<’
      prevents the constraint ‘(ORD a0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      Potentially matching instance:
        instance ORD (a -> b) -- Defined at tcfail040.hs:17:10
    • In the first argument of ‘(===)’, namely ‘(<<)’
      In the expression: (<<) === (<<)
      In an equation for ‘f’: f = (<<) === (<<)