[Haskell-cafe] what is the fastest way to extract variables from a
proposition?
Cetin Sert
cetin.sert at gmail.com
Wed Feb 20 16:29:19 EST 2008
-- proposition
data Prp a = Var a
| Not (Prp a)
| Or (Prp a) (Prp a)
| And (Prp a) (Prp a)
| Imp (Prp a) (Prp a)
| Xor (Prp a) (Prp a)
| Eqv (Prp a) (Prp a)
| Cns Bool
deriving (Show, Eq)
-- Here are to variable extraction methods
-- variable extraction reference imp.
-- Graham Hutton: Programming in Haskell, 107
vars_ :: Prp a → [a]
vars_ (Cns _) = []
vars_ (Var x) = [x]
vars_ (Not p) = vars_ p
vars_ (Or p q) = vars_ p ++ vars_ q
vars_ (And p q) = vars_ p ++ vars_ q
vars_ (Imp p q) = vars_ p ++ vars_ q
vars_ (Xor p q) = vars_ p ++ vars_ q
vars_ (Eqv p q) = vars_ p ++ vars_ q
-- variable extraction new * this is faster
vars :: Prp a → [a]
vars p = evs [p]
where
evs [] = []
evs (Cns _ :ps) = []
evs (Var x :ps) = x:evs ps
evs (Not p :ps) = evs (p:ps)
evs (Or p q:ps) = evs (p:q:ps)
evs (And p q:ps) = evs (p:q:ps)
evs (Imp p q:ps) = evs (p:q:ps)
evs (Xor p q:ps) = evs (p:q:ps)
evs (Eqv p q:ps) = evs (p:q:ps)
-- for : Not (Imp (Or (Var 'p') (Var 'q')) (Var p))
-- vars_: ['p','q','p']
-- vars : ['p','q','p']
-- order and the fact that 'p' appears twice being irrelevant:
-- is there an even faster way to do this?
--
-- Cetin Sert
-- www.corsis.de
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20080220/acaf6d5c/attachment.htm
More information about the Haskell-Cafe
mailing list