Something

instance Serializable Char => Serializable [Char] where
    toString s = s
    fromString s = s

Actions.hs:24:0:
    Illegal instance declaration for `Serializable [Char]'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are distinct type *variables*
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Serializable [Char]'
Failed, modules loaded: none.