RE: [Haskell-cafe] One liner?

2 Oct 2008

      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: haskell-cafe-bounces@haskell.org
[mailto:haskell-cafe-bounces@haskell.org] 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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RE: [Haskell-cafe] One liner?

Mitchell, Neil