[Haskell-cafe] One liner?

Paul Keir pkeir at dcs.gla.ac.uk
Thu Oct 2 11:20:18 EDT 2008

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.


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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