Re: [Haskell-cafe] what is the fastest way to extract variables from a proposition?

G'day all. Quoting Cetin Sert :

-- 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)

This is probably the fastest: vars :: Prp a -> [a] vars p = vars' p [] where vars' (Var a) = (a:) vars' (Not p) = vars' p vars' (Or l r) = vars' l . vars' r {- etc -} vars' (Cns _) = id Cheers, Andrew Bromage