[Haskell-beginners] Problem with Data.Map and IO...

[Markus Läll]

You want the Map to map from filenames to their contents?

Basically you need mapM or foldM, depending in where you want to use
getXmlContent. In the below code I used mapM to read the files into a
list of pairs, and fold over it (I changed insertRec to take a pair).

--
module Main where

import Data.List
import qualified Data.Map as M
import Control.Monad (mapM)

insertRec :: M.Map String String -> (String, String) -> M.Map String String
insertRec m (key, value) =
   if M.lookup key m == Nothing
      then M.insert key value m
      else m

insertRec2 m (key, value) =
   M.insertWith const key value m


createMap :: [(String, String)] -> M.Map String String
createMap lst = foldl' insertRec M.empty lst


getXmlContent :: FilePath -> IO (String,String)
getXmlContent filename = do
   contents <- readFile filename
   return (filename, contents)

main :: IO ()
main = do
 xmlfiles <- mapM getXmlContent ["a.xml","f.xml", "a.xml"]
 let ht = createMap xmlfiles
 print ht
--

You could also read the files in insertRec instead of in main -- that
would make insertRec return an "IO (M.Map String String)" instead of
just "M.Map String String". And then you have to use foldM in
createMap to use it, because the folding function is monadic now.

On 5/11/11, Michael Xavier <nemesisdesign at gmail.com> wrote:
> I believe let when you're in do notation is for pure functions. Since c is
> IO (String String), I think what you might try is:
>
> c <- getXmlContent e
> k <- fst c
> v  <- snd c
>
> Or even better, use pattern matching:
>
> (k, v) <- getXmlContent e
>
> Someone please step in if I'm misrepresenting how it's done.
>
>
> On Wed, May 11, 2011 at 11:05 AM, Manfred Lotz <manfred.lotz at arcor.de>wrote:
>
>> Hi there,
>> I've got a problem creating a map.
>>
>> I have a list of dataset names and adding those dataset names into a
>> map works fine like this:
>>
>> <----------------------------snip-------------------------------->
>> module Main where
>>
>> import Data.List
>> import qualified Data.Map as M
>>
>>
>> insertRec :: M.Map String String -> String -> M.Map String String
>> insertRec m e =
>>  let f = M.lookup e m in
>>  if  f == Nothing
>>  then
>>     M.insert e  "bla" m
>>  else
>>     m
>>
>>
>> createMap :: [String] -> M.Map String String
>> createMap lst =
>>  let m = M.empty in
>>  foldl' insertRec m  lst
>>
>>
>> main :: IO ()
>> main = do
>>  let xmlfiles = [ "a.xml","f.xml"]
>>  let ht = createMap xmlfiles
>>  print ht
>> <----------------------------snap-------------------------------->
>>
>>
>> However, actually I want to parse the xml files and adding content into
>> the map.
>>
>> For this I have a function
>> getXmlContent :: FilePath -> IO (String,String)
>>
>> and would like to do something like this:
>>
>> <----------------------------snip-------------------------------->
>> insertRec m e = do
>>  c <- getXmlContent e
>>  let k = fst c
>>  let v = snd c
>>  M.insert  k v  m
>> <----------------------------snap-------------------------------->
>>
>> Unfortunately, this doesn't work because of IO. I have no glue how to
>> get this done.
>>
>>
>> Any help appreciated.
>>
>>
>>
>> --
>> Thanks,
>> Manfred
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
>
>
> --
> Michael Xavier
> http://www.michaelxavier.net
>


-- 
Markus Läll