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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