It is a little bit roundabout, but here's how you would do a direct translation: import Control.Monad.Writer import Data.Char ( toLower ) import Text.Regex.Posix ( (=~) ) replaceStuff :: String -> Writer [String] String replaceStuff "" = return "" replaceStuff cs = do let (pre, found, post) = cs =~ pattern tell ["found:" ++ found] moreStuff <- replaceStuff post return $ pre ++ transf found ++ moreStuff where pattern = "[A-E]" -- some regExp pattern transf = map toLower -- placeholder function main = print $ runWriter $ replaceStuff "this is thE Story of peter" Other than running runWriter, just remember everything that was pure now must be a monad, so return everything. Also when you recurse that is also now monadic. It now prints instead of putStrLn because it must return a tuple. Everything else is pretty much the same. On Wed, Mar 6, 2013 at 4:22 PM, Franco <franco00@gmx.com> wrote:

I am trying to get comfortble with the various monads, just to hone my tools for a better tomorrow.

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import Control.Monad.Writer

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import Data.Char ( toLower ) import Text.Regex.Posix ( (=~) )

So I decided to code a very small example to 'get into it'. The following code is extremely simple. Given a string, it will find all occourences of a specific regexp, replace them, and return the changed string back.

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replaceStuff :: String -> String replaceStuff "" = "" replaceStuff cs = let (pre, found, post) = cs =~ pattern in pre ++ transf found ++ replaceStuff post where pattern = "[A-E]" -- some regExp pattern transf = map toLower -- placeholder function

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main :: IO () main = putStrLn $ replaceStuff "this is thE Story of peter"

Yay, it works! Now let's talk about the Writer monad. I would like to have a mini log via the writer monad (could be anything , probably "x found and replaced with y")

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type Log = String

But after that I have no idea how to add the writer monad to my simple but recursive function. Type sig will be something like

replaceCols :: String -> Writer Log String

but I don't know how to replace this part of the non monadic code

pre ++ transf found ++ replaceStuff post

I wrote this before giving up

writer ((pre ++ found ++ ??whathere??), (found ++ " found\n")) >>

Probably I am no "getting it", so any help is appreciated! Thanks

-F

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