On Tue, Feb 11, 2003 at 08:17:49PM +0300, antonmuhin ???? rambler.ru wrote:
Dear Haskellers!
Another letter from newbie :)
May you criticize my code for replacing substring in a string cited below? Any critics and comments are highly appreciated.
And some more questions:
1. Is there more functional way for text processing? For example, intuitively I feel that there is more functional approach for matching than one suggested below.type Match = String -> Maybe String
I think the following would be an example of a reasonably nice functional way of defining part of a regexp matching code. I think it would be relatively straightforward to extend it as much as you like. match here is the same as your match, and your replace old new s would be my replace (match old) new s. Looking back at what I've written, I'm thinking that perhaps the "Match" type should be a monad, since it seems to have monadic properties, and that might cut down on the amount of coding. Most notably, I bet the matchSequence function would just be the sequence function of a monad... type Match = String -> Maybe String -- replace replaces a matched regexp with a constant string. replace :: Match -> String -> String -> String replace m repl (c:cs) = case m (c:cs) of Nothing -> c : replace m repl cs Just r -> repl ++ replace m repl r replace _ _ "" = "" -- in a regexp a (i.e. the char a) matchchar :: Char -> Match matchchar m (c:cs) = if m == c then Just cs else Nothing -- in a regexp exp1exp2exp3 (i.e. in sequence) matchSequence :: [Match] -> Match matchSequence (m:ms) s = m s >>= matchSequence ms matchSequence [] s = Just s -- in a regexp abcd match :: String -> Match match s = matchSequence $ map matchchar s -- in a regexp (exp)* matchStar :: Match -> Match matchStar m s = case m s of Nothing -> Just s Just s' -> matchStar m s' -- in a regexp (exp)+ matchPlus :: Match -> Match matchPlus m s = case m s of Nothing -> Nothing Just s' -> matchStar m s' -- in a regexp (exp1|exp2|exp3) matchOr :: [Match] -> Match matchOr (m:ms) s = case m s of Nothing -> matchOr ms s Just s' -> Just s' matchOr [] s = Nothing -- In a regexp something like [abc] matchBrackets :: String -> Match matchBrackets s = matchOr $ map matchchar s -- David Roundy http://civet.berkeley.edu/droundy/