Predicate Logic in Haskell

I've been using the following data structure for the representation of propositional logic in Haskell:

data Prop 
    = Pred  String
    | Not   Prop
    | And   Prop Prop
    | Or    Prop Prop
    | Impl  Prop Prop
    | Equiv Prop Prop
    deriving (Eq, Ord)

Any comments on this structure are welcome.

However, now I want to extend my algoritms to handle FOL - predicate logic. What would be a good way of representing FOL in Haskell?

I've seen versions that are - pretty much - an extension of the above, and versions that are based on more classic context-free grammars. Is there any literature on this, that could be recommended?