Why can I not make String an instance of a typeclass?

Question

Given:

data Foo =
  FooString String
  …

class Fooable a where --(is this a good way to name this?)
  toFoo :: a -> Foo

<

Answer 1

FlexibleInstances are not a good answer in most cases. Better alternatives are wrapping the String in a newtype or introduce a helper class like so:

class Element a where
   listToFoo :: [a] -> Foo

instance Element Char where
   listToFoo = FooString

instance Element a => Fooable [a] where
   toFoo = listToFoo

See also: http://www.haskell.org/haskellwiki/List_instance