[Haskell-beginners] (Basic?) IO question

[David McBride]

When you go with something like:

putStrLn "asdf" :: IO ()

but when you go:

liftM putStrLn (return "asdf") :: Monad m => m (IO ())

These are totally different.  The first one is an IO action.  The
second one is a monad that returns an IO action.  When run in ghci,
ghci decides that the outer monad must be IO as well, so the type
becomes IO (IO ()).  When you run that action, all you get is an IO ()
which has not been run and as such will not execute or print.

I'm not sure what you are attempting to get with liftM, but just make
sure to check out the final types of your statements with the :t
command in ghci and you should be able to figure it out.

On Wed, Jan 18, 2012 at 1:35 PM, Guillaume Basse <gw.basse at gmail.com> 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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