On Wed, Oct 13, 2004 at 10:06:44AM +0200, Sander Evers wrote:
Question 2 ---------- How to arrange the above `lazy' output?
Your function addToSPair
addToSPair :: Char -> (String, String) -> (String, String) addToSPair c (xs, ys ) = (c:xs, ys )
does a strict pattern match on the pair constructor. It needs to know that its second argument is a pair before it produces anything of its result. If you use a lazy pattern match
addToSPair :: Char -> (String, String) -> (String, String) addToSPair c ~(xs, ys ) = (c:xs, ys )
instead, it doesn't need to evaluate this argument immediately.
!! This is great, thank you. This improves a lot. I never recalled of the tilda for several years. Excuse me for the stupidity, but I have a next naive question. As the types are resolved before the computation, as the above program shows, how can addToSPair expect anything besides a string pair? Why tilda is not a default? --------------------------------------------- repeating: bottomStr :: String bottomStr = let xs = xs in xs bottomSPair :: (String, String) bottomSPair = let p = p in p longToReach = 10^8 :: Int f1, f2, g1, g2 :: Char f1 = head ('a' : (if last [1 .. longToReach] > 0 then "b" else "c")) f2 = head ('a' : bottomStr) addToSPair :: Char -> (String, String) -> (String, String) addToSPair c (xs, ys ) = (c:xs, ys ) g1 = head $ fst $ addToSPair 'a' (if last [1 .. longToReach] > 0 then ("b", "") else ("c", "")) g2 = head $ fst $ addToSPair 'a' bottomSPair ----------------------------------------------------------------- The only doubt may be the matching of (xs, ys) against bottom :: (String, String) Is not this natural to have a result as (bottom :: String, bottom :: String) -- = xs = ys and compute further? What will occur if all the Haskell programmers set `~' before each data constructor in each pattern in their existing programs? As the types are recognized before the computation, the programs will remain safe and will become considerably faster. For example, the above program of g1 becomes a billion times faster. I wonder. Copy, please, the answer to mechvel@botik.ru ----------------- Serge Mechveliani mechvel@botik.ru