On Monday 11 October 2010 18:06:50, Lorenzo Isella 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).
I think there's a misunderstanding here. Control is the top-level name for stuff related to control flow, like Control.Concurrent for concurrency combinators, Control.Parallel (and Control.Parallel.Strategies) for parallelism combinators. Monads (some) are also related to control flow, so the monad stuff that's not available from the Prelude lives in Control.Monad and Control.Monad.Whatever (Control.Monad.State, Control.Monad.Writer, ...) Control.Monad (part of the standard libraries) provides a lot of general functions for working with Monads, among them forM (which is "flip mapM").
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
That means fl is the IO-action which gets the file lengths. You want the result, thus fl <- getAllLengths nums to bind the result of that action to the name fl.
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