{-# OPTIONS -fglasgow-exts #-}

module GenericSequent where

import ModalLogic

import CombLogic

import Data.List as List

--import Data.Map as Map

-- | Generate all possible clauses out of a list of atoms

allClauses :: [a] -> [Clause a]

allClauses x = case x of

h:t -> let addPositive c (Implies n p) = Implies n (c:p)

addNegative c (Implies n p) = Implies (c:n) p

in List.map (addPositive h) (allClauses t) ++

List.map (addNegative h) (allClauses t)

_ -> [Implies [] []]

-- | Extract the modal atoms from a formula

getMA :: Eq a => Boole a -> [Boole a]

getMA x = case x of

And phi psi -> (getMA phi) `List.union` (getMA psi)

Not phi -> getMA phi

At phi -> [At phi]

_ -> []

-- | Substitution of modal atoms within a "Boole" expression

subst :: Eq a => Boole a -> Clause (Boole a) -> Boole a

subst x s@(Implies neg pos) =

case x of

And phi psi -> And (subst phi s) (subst psi s)

Not phi -> Not (subst phi s)

T -> T

F -> F

_ -> if (elem x neg) then F

else if (elem x pos) then T

else error "GenericSequent.subst"

-- | Evaluation of an instantiated "Boole" expression

eval :: Boole a -> Bool

eval x = case x of

And phi psi -> (eval phi) && (eval psi)

Not phi -> not (eval phi)

T -> True

F -> False

_ -> error "GenericSequent.eval"

-- | DNF: disjunctive normal form of a Boole expression

dnf :: (Eq a) => Boole a -> [Clause (Boole a)]

dnf phi = List.filter (\x -> eval (subst phi x)) (allClauses (getMA phi))

-- | CNF: conjunctive normal form of a Boole expression

cnf :: (Eq a) => Boole a -> [Clause (Boole a)]

cnf phi = List.map (\(Implies x y) -> Implies y x) (dnf (Not phi))

-- | Generic Provability Function

provable :: (Eq a, Form a b c) => Boole a -> Bool

provable _ = True

{-

provable phi =

let unwrap (Subst x) = Map.elems x

in all (\c -> any (all provable) (List.map unwrap.snd (match c))) (cnf phi)

-}

-- | Generic Satisfiability Function

sat :: (Eq a, Form a b c) => Boole a -> Bool

sat phi = not $ provable $ neg phi

-- | Function for "normalizing" negation

neg :: Eq a => Boole a -> Boole a

neg phi = case phi of

Not psi -> psi

_ -> phi