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
I am trying to get comfortble with the various monads, just to hone my tools for a better tomorrow.
import Control.Monad.Writer
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.
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
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")
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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