[Haskell-beginners] Converting a function to -> Writer w a
franco00 at gmx.com
Wed Mar 6 22:22:09 CET 2013
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
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