[Haskell-beginners] Reading Multiple Files and Iterate Function Application

[Lorenzo Isella]

On 10/11/2010 07:20 PM, Thomas Miedema wrote:
> Hi,
>
> replace
> let fl = getAllLengths nums
>
> by
> fl <- getAllLengths nums
>
> , since getAllLengths returns a monadic action.
>
> The author of the following book is much better at explaining why this
> is so than I am: http://learnonlineyouahaskell.com/
> <http://learnyouahaskell.com/>. May I suggest you read it cover to
> cover, it's really really good. It is probably the best way to learn
> Haskell at the moment, together with trying out code snippets like
> you're doing right now.
>
> Regards,
> Thomas
>
>
>
> On Mon, Oct 11, 2010 at 6:06 PM, Lorenzo Isella
> <lorenzo.isella at gmail.com <mailto:lorenzo.isella at gmail.com>> wrote:
>
>     Thanks a lot Daniel, but I am a bit lost (up to not long ago I did
>     not even know the existence of a control monad...and some
>     unstructured reading did not help).
>     Some online research about mapM and fmap led me here
>     http://en.wikibooks.org/wiki/Haskell/Category_theory
>     and I think I am a bit astray at this point ;-)
>
>     Why does my "simple" snippet below raise a number of errors?
>
>     Cheers
>
>     Lorenzo
>
>
>     import Data.Ord
>
>     import Data.List
>
>     main :: IO ()
>
>     main = do
>
>       let nums=[1,2]
>
>       let fl = getAllLengths nums
>
>       putStrLn "fl is, "
>       print fl
>
>
>
>     filename :: Int -> FilePath
>     filename i = "file" ++ show i ++ ".dat"
>
>     fileLength :: FilePath -> IO Int
>     fileLength file = fmap length (readFile file)
>
>     getAllLengths :: [Int] -> IO [Int]
>     getAllLengths nums = mapM (fileLength . filename) nums
>
>
>
>     On 10/11/2010 05:21 PM, Daniel Fischer wrote:
>
>         On Monday 11 October 2010 16:56:58, Lorenzo Isella wrote:
>
>             Dear All,
>             Another I/O question.
>             Let us say that you are given a list of files file1.dat,
>             file2.dat...file10.dat and so on (i.e. every file is indexed
>             by a number
>             and every file is a single column where every entry is a
>             string without
>             spaces).
>             In the snippet below I read file1.dat, convert it to a list
>             and then
>             print out its length.
>             Now, how can I iterate the process on file1.dat, file2.dat
>             and file3.dat
>             and store the lengths in a list?
>
>
>         fileLength :: FilePath ->  IO Int
>         fileLength file = fmap length (readFile file)
>
>         filename :: Int ->  FilePath
>         filename i = "file" ++ show i ++ ".dat"
>
>         getAllLengths :: [Int] ->  IO [Int]
>         getAllLengths nums = mapM (fileLength . filename) nums
>
>
>         If you want something other than the character count, instead of
>         fileLength
>         use e.g.
>
>         countLines :: FilePath ->  IO Int
>         countLines file = fmap (length . lines) (readFile file)
>
>         or whatever you're interested in.
>
>         Another nice thing is often forM (from Control.Monad)
>
>         forM nums $ \i ->  do
>             let filename = "file" ++ show i ++ ".dat"
>             contents<- readFile filename
>             let result = function contents
>             doSomethingOrNot
>             return result
>
>             I would like to map the file reading and following
>             operations on the
>             list [1,2.3], but that is giving me a headache.
>             It is relatively easy to create the file name
>
>             filename="file"++(show i)++".dat"   , for i=1,2,3
>
>             but it the the iteration part that is giving me troubles.
>             Any suggestion is appreciated.
>             Cheers
>
>             Lorenzo
>
>
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>
>

Thanks Thomas. Yep, I do need some extra reading unfortunately.
One question: if I was to apply a function on many files file1, 
file2...using e.g. Python, this would be my pipeline
read file1
do stuff on file 1

read file2
do stuff on file 2

......

Now, due to the laziness of haskell, can I here resort to this approach

read file1, file2... into a single list

map (do-my-stuff) on list

As far as I understand, this should not result e.g. into a huge RAM 
consumptions since files are read and processed only when needed (hence 
one at the time).
Am I on the right track?
Cheers

Lorenzo