On Mon, Oct 11, 2010 at 6:06 PM, Lorenzo Isella <lorenzo.isella@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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