gistfile1.hs

data Logic a where
    B :: Bool -> Logic Bool
    And :: Logic Bool -> Logic Bool -> Logic Bool
    Or  :: Logic Bool -> Logic Bool -> Logic Bool
    Imp  :: Logic Bool -> Logic Bool -> Logic Bool
    Not :: Logic Bool -> Logic Bool

imp :: Bool->Bool->Bool
imp False _ = True
imp True True = True
imp _ _ = False

eval :: Logic a -> a

eval (B a) = a
eval (And e1 e2) = (eval e1) && (eval e2)
eval (Or  e1 e2) = (eval e1) || (eval e2)
eval (Imp e1 e2) = imp (eval e1) (eval e2)
eval (Not e)     = not $ eval e 

eval $ (B True) `And` (B False)