On Wed, Sep 24, 2008 at 1:14 PM, Creighton Hogg
Hi Haskellers, So as another step in my education, I wanted to ask if the following code at least feels 'morally' correct as a find/replace function, replacing each occurrence of the sub-list before with after in the list. unfoldr felt like the most natural higher order function to capture the recursion, but I imagine there are still slicker ways to have done this because I am creating an intermediate list-of-lists. However, doing it this way seems at least moderately speedy given that I was able to read in, find/replace, and write out a 120 MB file in 30 seconds on a fairly dinky linux box.
replace :: (Eq a) => [a] -> [a] -> [a] -> [a] replace before after = concat . unfoldr aux where aux [] = Nothing aux str | take l str == before = Just (after,drop l str) | otherwise = Just (take 1 str,drop 1 str) l = length before
Yeah, this code is pretty nice. concat is a foldr, so you have an elegant foldr . unfoldr at the top. IIRC foldr . unfoldr fuses, so you get efficiency that way. "take l str == before" is also known as "before `isPrefixOf` str", just to be slightly more idiomatic. This algorithm is quite inefficient, O(nm). There is probably a more efficient one that still only relies on Eq, I am not certain though.€€ Luke