gistfile1.hs

data Expr = Expr Bool
          | Conjunction Expr Expr
          | Negation Expr
          | Disjunction Expr Expr
          | Implication Expr Expr
          deriving (Eq, Show)
                   
nnf :: Expr -> Expr
nnf (Expr expr) = Expr expr
nnf (Conjunction expr1 expr2) = Conjunction (nnf expr1) (nnf expr2)
nnf (Disjunction expr1 expr2) = Disjunction (nnf expr1) (nnf expr2)
nnf (Negation (Negation expr)) = nnf expr
nnf (Negation (Conjunction expr1 expr2)) = (Disjunction (Negation (nnf expr1)) (Negation (nnf expr2)))
nnf (Negation (Disjunction expr1 expr2)) = (Conjunction (Negation (nnf expr1)) (Negation (nnf expr2)))
nnf (Implication expr1 expr2) = (Disjunction (Negation (nnf expr1)) (nnf expr2))
nnf (Negation (Implication expr1 expr2)) = nnf (Negation (Disjunction (Negation (nnf expr1)) (nnf expr2)))