[Haskell-cafe] Boolean formula: part2

[Dmitry Bogatov]

Hello!  I have following code:

buildFormulas :: [Variable] -> Int -> [Formula]
buildFormulas vars 0 = map Var vars
buildFormulas vars n = join $
    forM [0 .. n - 1 ] $ \i -> do
        x <- buildFormulas vars i
        y <- buildFormulas vars (n - i - 1)
        z <- buildFormulas vars (n - 1)
        let t1 = case z of
              (Negation _) -> []
              _ -> [Negation z]
        t1 ++ [conj x y, impl x y, disj x y]

It is correct, typechecks, but it is insanely slow (calculating
take 35 (buildFormulas [P, Q, R] 5) hangs my computer).

My guess is that it builds too much before returning first results.
Can you suggest more efficent way enumerate formulas of set of variables?




--
Best regards, Dmitry Bogatov <KAction at gnu.org>,
Free Software supporter, esperantisto and netiquette guardian.
GPG: 54B7F00D
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