On 11/1/07, Arnar Birgisson
I'm learning too and found this an interesting problem. Luke, is this similar to what you meant?
Heh, your program is almost identical to the one I wrote to make sure I wasn't on crack. :-)
data LS = Var String | Not LS | And LS LS | Or LS LS deriving Show
data Term = Pos String | Neg String deriving Show type Conj = [Term] type DNF = [Conj]
dnf :: LS -> DNF dnf (Var s) = [[Pos s]] dnf (Or l1 l2) = (dnf l1) ++ (dnf l2) dnf (And l1 l2) = [t1 ++ t2 | t1 <- dnf l1, t2 <- dnf l2] dnf (Not (Not d)) = dnf d dnf (Not (And l1 l2)) = (dnf $ Not l1) ++ (dnf $ Not l2) dnf (Not (Or l1 l2)) = [t1 ++ t2 | t1 <- dnf $ Not l1, t2 <- dnf $ Not l2]
These two are doing a little extra work: dnf (Not (And l1 l2)) = dnf (Or (Not l1) (Not l2)) dnf (Not (Or l1 l2)) = dnf (And (Not l1) (Not l2))
dnf (Not (Var s)) = [[Neg s]]
-- test cases x = (Or (And (Var "A") (Var "B")) (Or (And (Not $ Var "C") (Var "D")) (Var "E"))) y = (Not (And (Var "A") (Var "B"))) z = (Not (And (Not y) y))
Luke