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

[ajb at spamcop.net]

G'day all.

Quoting Cetin Sert <cetin.sert at gmail.com>:

> -- 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