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T10999.hs:5:6: error:
• Found extra-constraints 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 ‘() -> _’
In the type signature: f :: _ => () -> _
T10999.hs:8:28: error:
• Ambiguous type variable ‘b1’ arising from a use of ‘f’
prevents the constraint ‘(Ord b1)’ from being solved.
Relevant bindings include g :: [b1] (bound at T10999.hs:8:1)
Probable fix: use a type annotation to specify what ‘b1’ should be.
Potentially matching instances:
instance Ord a => Ord (Set.Set a) -- Defined in ‘Data.Set.Internal’
instance Ord Ordering -- Defined in ‘GHC.Classes’
...plus 24 others
...plus two 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 ()
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