Re: Typechecker / OverloadedStrings question 7.8 vs. 7.10

I think the error message could be made clearer simply by emphasizing the fact that type ambiguity over the lack of instances. Ambiguous type variable 't0' arising from a use of elem :: a -> t0 a -> Bool caused by the lack of an instance 'Data.String.IsString (t0 Char)' Either add a type annotation to dictate what 't0' should be based on one of the potential instances: instance Foldable (Either a) -- Defined in ‘Data.Foldable’ instance Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’ instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i) -- Defined in ‘Data.Foldable’ ...plus three others) or define the required instance 'Data.String.IsString (t0 Char)'.