T10999.hs:5:6: error:
    • Found type wildcard ‘_’ standing for ‘Ord a’
      Where: ‘a’ is a rigid type variable bound by
               the inferred type of f :: Ord a => () -> Set.Set a
               at T10999.hs:6:1-28
      To use the inferred type, enable PartialTypeSignatures
    • In the type signature: f :: _ => () -> _

T10999.hs:5:17: error:
    • Found type wildcard ‘_’ standing for ‘Set.Set a’
      Where: ‘a’ is a rigid type variable bound by
               the inferred type of f :: Ord a => () -> Set.Set a
               at T10999.hs:6:1-28
      To use the inferred type, enable PartialTypeSignatures
    • In the type signature: f :: _ => () -> _

T10999.hs:8:28: error:
    • Ambiguous type variable ‘b0’ arising from a use of ‘f’
      prevents the constraint ‘(Ord b0)’ from being solved.
      Relevant bindings include g :: [b0] (bound at T10999.hs:8:1)
      Probable fix: use a type annotation to specify what ‘b0’ should be.
      These potential instances exist:
        instance Ord a => Ord (Set.Set a) -- Defined in ‘Data.Set.Internal’
        instance Ord Ordering -- Defined in ‘GHC.Classes’
        instance Ord Integer
          -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
        ...plus 23 others
        ...plus three instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the second argument of ‘($)’, namely ‘f ()’
      In the second argument of ‘($)’, namely ‘Set.toList $ f ()’
      In the expression: map fst $ Set.toList $ f ()