Hello Neil, Thursday, October 2, 2008, 7:26:23 PM, you wrote: shortly speaking, getDirectoryContents is an action (having "IO a" type) while second mapM_ argument should be a value returned by this action. by using dc variable or >>= operator, you can evaluate action and pass its result to mapM_. of course, after imperative languages experience, it's hard to see difference. take look at http://haskell.org/haskellwiki/IO_inside
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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