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 This material is sales and trading commentary and does not constitute investment research. Please follow the attached hyperlink to an important disclaimer <www.credit-suisse.com/emea/legal <outbind://31/www.credit-suisse.com/emea/legal> > ________________________________ 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/") ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ============================================================================== _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe