Provide iteratee-based IO as described in Oleg Kiselyov's paper 'http://okmij.org/ftp/Haskell/Iteratee/'.

Oleg's original code uses lists to store buffers of data for reading in the iteratee. This package allows the use of arbitrary types through use of the ListLike type class.

Iteratees can be thought of as stream processor combinators. Iteratees are combined to run in sequence or in parallel, and then processed by enumerators. The result of the enumeration is another iteratee, which may then be used again, or have the result obtained via the run function.

 -- count the number of bytes in a file, reading at most 8192 bytes at a time
 import Data.Iteratee as I
 import Data.Iteratee.IO
 import Data.ByteString
 
 byteCounter :: Monad m => Iteratee ByteString m Int
 byteCounter = I.length
 
 countBytes = do
   i' <- enumFile 8192 "/usr/share/dict/words" byteCounter
   result <- run i'
   print result

Iteratees can be combined to perform much more complex tasks. The iteratee monad allows for sequencing iteratee operations.

 iter2 = do
   I.drop 4
   I.head

In addition to enumerations over files and Handles, enumerations can be programmatically generated.

 get5thElement = enumPure1Chunk [1..10] iter2 >>= run >>= print

Iteratees can also work as stream transformers, called Enumeratees. A very simple example is provided by filter. When working with enumeratees, it's very common to collaps the nested iteratee with joinI.

This function returns the 5th element greater than 5.

 iterfilt = joinI $ I.filter (>5) iter2
 find5thOver5 = enumPure1Chunk [10,1,4,6,7,4,2,8,5,9::Int] iterfilt >>= run >>= print

Another common use of iteratees is takeUpTo, which guarantees that an iteratee consumes a bounded number of elements. This is often useful when parsing data. You can check how much data an iteratee has consumed with enumWith

 iter3 :: (Num el, Ord el, Monad m) => Iteratee [el] m (el,Int)
 iter3 = joinI (I.takeUpTo 100 (enumWith iterfilt I.length))

Many more functions are provided, and there are many other useful ways to combine iteratees and enumerators.