[Haskell-beginners] (Basic?) IO question

[Isaac Dupree]

The one that's doing nothing is returning type
IO (IO ()), which executes the outer IO (which is "c") and returns the 
inner IO (which is the contents of renderTrackInfosToFile2).

You want a different combinator.  Try playing with "join" and/or ">>=" 
(pronounced "bind") and you should be able to figure out ways to do it. 
  Try looking at the types of liftM and liftM2 and see if you can figure 
out why they don't work.  In ghci, try :t expression (:type expression) 
to find the type of any expression you input.  The Control.Monad 
documentation has types of lots of convenient operations (look at the 
synopsis which has all their types together) (all of these operations 
are written using the basic operations of Monads.)
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html

By the way, 'liftM' is equivalent to 'fmap', if you're ever wondering.

-Isaac

On 01/18/2012 01:35 PM, Guillaume Basse wrote:
> Hello,
>
> So I've been trying to print some information in a file. I have two
> versions of the same function: one of them doesn't work and I don't
> understand why.
> Here are the functions:
>
> # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
>
> -- This one works as intended when called from ghci
> renderTrackInfosToFile :: FilePath ->  String ->  IO [TrackInfos] ->  IO ()
> renderTrackInfosToFile filename sep ls = do nls<- ls
>                                                                  writeFile
> filename (intercalate "\n" $ map (renderTrackInfo sep) nls)
>                                                                  return ()
>
> -- This one does nothing when called from ghci
> renderTrackInfosToFile2 :: FilePath ->  String ->  [TrackInfos] ->  IO ()
> renderTrackInfosToFile2 filename sep ls = writeFile filename (intercalate
> "\n" $ map (renderTrackInfo sep) ls)
>
>
> renderTrackInfo :: String ->  TrackInfos ->  String
>
> # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
>
> Both functions load in ghci, and the following commands are issued:
>
>> -- c :: IO [TrackInfos]
>> liftM (renderTrackInfosToFile2 "blabla.txt" "|")  c -- this command seems
> to do absolutely nothing
>> renderTrackInfosToFile "blabla.txt" "|"  c -- this one works as intended
>
> # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
>
> And this is not the first time I have this sort of problem using liftM. I
> now that I'm missing something important here,
> can anyone explain me my mistake?
>
> Much thanks,
>
> Guillaume Basse
>
>
>
>
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