Haskell recursion in do block don't work (recursion is in list conprehension)

Question

I'm pretty new to Haskell, and want to execute recursion in do block.

countLines :: String -> IO Int
countLines filePath = do
    isFile <- doesFileExist filePath
    if isFile
        then do contents <- readFile filePath
                print contents
                pure 0
        else do files <- getDirectoryContents filePath    
                [countLines(file) | file <- files] -- recursion here!
                pure 0

until i add this list comprehension, everything works fine but once i add this, i get the following error:

Main.hs:59:17: error:
    • Couldn't match type ‘[]’ with ‘IO’
      Expected type: IO (IO Int)
        Actual type: [IO Int]
    • In a stmt of a 'do' block: [countLines (file) | file <- files]
      In the expression:
        do files <- getDirectoryContents filePath
           [countLines (file) | file <- files]
           pure 0
      In a stmt of a 'do' block:
        if isFile then
            do contents <- readFile filePath
               print contents
               pure 0
        else
            do files <- getDirectoryContents filePath
               [countLines (file) | file <- files]
               pure 0
   |
59 |                 [countLines(file) | file <- files]
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
bash-3.2$

does any one know how to fix this?

You can convert a list of actions (like [ countFile(file) | file <- files ]) into an action that executes them all using the sequence function — luqui
– luqui, Commented May 2, 2020 at 3:27

willeM_ Van Onsem · Accepted Answer · 2020-05-01 15:49:54Z

2

The list comprehension is an expression of the [IO Int], not IO Int, while a list [] is a member of the Monad typeclass, it is not the one you are currently working with in the do block. You can use mapM_ :: (Foldable f, Monad m) => (a -> m b) -> f a -> m () to work with recursion here:

countLines :: String -> IO Int
countLines filePath = do
    isFile <- doesFileExist filePath
    if isFile
        then readFile filePath >>= print
        else getDirectoryContents filePath >>= mapM_ countLines
    pure 0

As @chi says, if you need to retrieve a list of the results, you should use mapM :: (Traversable f, Monad m) => (a -> m b) -> t a -> m (t b). This thus can you provide a list of the results of the monadic calls. You can for example sum these up to obtain the result. I leave that as an exercise.

edited May 1, 2020 at 15:49

answered May 1, 2020 at 14:28

willeM_ Van Onsem

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1 Comment

chi Over a year ago

I guess the OP actually wants mapM (or traverse / for) since they'll really need the returned Ints from the recursive calls.

OpenAI was the last straw · Accepted Answer · 2020-05-01 14:33:29Z

You're in the IO monad, so every line should produce an IO a, but your list comprehension produces... well, a list [a]. Thankfully, there is a function sequence_ :: (Foldable t, Monad m) => t (m a) -> m (), that will turn your [IO ()] into a single IO (), while running all of the IO actions in it. Thus your else block should look like this instead:

    else do files <- getDirectoryContents filePath    
            sequence_ [countLines(file) | file <- files]
            pure 0

As Willem mentions, it's possible to simplify these even more using mapM_ :: (Foldable f, Monad m) => (a -> m b) -> f a -> m ():

    else do files <- getDirectoryContents filePath    
            mapM_ countLines files
            pure 0

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