Typechecker / OverloadedStrings question 7.8 vs. 7.10

[Phil Ruffwind]

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)'.