A good way to approach this is data-structure-driven programming. You want a data structure which represents, and can _only_ represent, propositions in DNF. So:
data Term = Pos Var | Neg Var type Conj = [Term] type DNF = [Conj] Then write: dnf :: LS -> DNF The inductive definition of dnf is straightforward given this output type... Luke On 11/1/07, Jim Burton <[EMAIL PROTECTED]> wrote: > > I am trying to rewrite sentences in a logical language into DNF, and wonder > if someone would point out where I'm going wrong. My dim understanding of it > is that I need to move And and Not inwards and Or out, but the function > below fails, for example: > > > dnf (Or (And A B) (Or (And C D) E)) > And (Or A (And (Or C E) (Or D E))) (Or B (And (Or C E) (Or D E))) > > > data LS = Var | Not LS | And LS LS | Or LS LS > --convert sentences to DNF > dnf :: LS -> LS > dnf (And (Or s1 s2) s3) = Or (And (dnf s1) (dnf s3)) (And (dnf s2) (dnf s3)) > dnf (And s1 (Or s2 s3)) = Or (And (dnf s1) (dnf s2)) (And (dnf s1) (dnf s3)) > dnf (And s1 s2) = And (dnf s1) (dnf s2) > dnf (Or s1 s2) = Or (dnf s1) (dnf s2) > dnf (Not (Not d)) = dnf d > dnf (Not (And s1 s2)) = Or (Not (dnf s1)) (Not (dnf s2)) > dnf (Not (Or s1 s2)) = And (Not (dnf s1)) (Not (dnf s2)) > dnf s = s > > Thanks, > > Jim > > -- > View this message in context: > http://www.nabble.com/Disjunctive-Normal-Form-tf4733248.html#a13534567 > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
