RE: [Haskell-cafe] One liner?

[Mitchell, Neil]

Hi
 
You can translate this step by step.
 
main = do dc <- getDirectoryContents "./foo/"
          mapM_ putStrLn dc

Translating out the do notation
(http://www.haskell.org/haskellwiki/Keywords#do):

main = getDirectoryContents >>= \dc ->
       mapM_ putStrLn dc

Then we can chop out the dc argument, as its \x -> .... x, and can be
removed (eta reduction):

main = getDirectoryContents >>= 
       mapM_ putStrLn

And finally we just remove the newline:

main = getDirectoryContents >>= mapM_ putStrLn  

Alternatively, we can flip the >>= for =<< and write:

main = mapM_ putStrLn =<< getDirectoryContents

This is now one line, and mirrors how you would write the function if it
was pure using function composition.

Thanks

Neil
 

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________________________________

        From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir
        Sent: 02 October 2008 4:20 pm
        To: haskell-cafe@haskell.org
        Subject: [Haskell-cafe] One liner?
        
        

        Hi all,
        
        There's a common little situation I keep bumping up against. I
don't understand where I'm going wrong, so I've made a little example.
It's to do with binding a result to a variable name using "<-". This
code works fine:
        
        ----------------------------------------------
        module Main where
        
        import System.Directory (getDirectoryContents)
        
        main = do dc <- getDirectoryContents "./foo/"
                  mapM_ putStrLn dc
        ----------------------------------------------
        
        But if I try to avoid the use of the bind to "dc", I fail:
        
        ----------------------------------------------
        mapM_ putStrLn (getDirectoryContents "./foo/")
        ----------------------------------------------
        
        I've tried using map instead of mapM_, and inserted "return"s
here and there, but no luck. Can anyone tell me where and why I'm going
wrong? The error message is below.
        
        Cheers,
        Paul
        
        
        Couldn't match expected type `[String]'
               against inferred type `IO [FilePath]'
        In the second argument of `mapM_', namely
            `(getDirectoryContents "./foo/")'
        In the expression: mapM_ putStrLn (getDirectoryContents
"./foo/")
        In the definition of `main':
            main = mapM_ putStrLn (getDirectoryContents "./foo/")
        


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