[Haskell-beginners] Converting a function to -> Writer w a
toad3k at gmail.com
Wed Mar 6 22:51:19 CET 2013
It is a little bit roundabout, but here's how you would do a direct
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 at gmx.com> wrote:
> I am trying to get comfortble with the various monads, just to hone my
> 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
> Beginners mailing list
> Beginners at haskell.org
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