Is there an Iteratee-like concept which pulls data from multiple sources?

Question

It is possible to pull on demand from a number (say two for simplicity) of sources using streams (lazy lists). Iteratees can be used to process data coming from a single source.

Is there an Iteratee-like functional concept for processing multiple input sources? I could imagine an Iteratee whose state signals from which source does it want to pull.

Answer 1

To do this using pipes you nest the Pipe monad transformer within itself, once for each producer you wish to interact with. For example:

import Control.Monad
import Control.Monad.Trans
import Control.Pipe

producerA, producerB :: (Monad m) => Producer Int m ()
producerA = mapM_ yield [1,2,3]
producerB = mapM_ yield [4,5,6]

consumes2 :: (Show a, Show b) =>
    Consumer a (Consumer b IO) r
consumes2 = forever $ do
    a <- await       -- await from outer producer
    b <- lift await  -- await from inner producer
    lift $ lift $ print (a, b)

Just like a Haskell curried function of multiple variables, you partially apply it to each source using composition and runPipe:

consumes1 :: (Show b) => Consumer b IO ()
consumes1 = runPipe $ consumes2 <+< producerA

fullyApplied :: IO ()
fullyApplied = runPipe $ consumes1 <+< producerB

The above function outputs when run:

>>> fullyApplied
(1, 4)
(2, 5)
(3, 6)

This trick works for yielding or awaiting to any number of pipes upstream or downstream. It also works for proxies, the bidirectional analogs to pipes.

Edit: Note that this also works for any iteratee library, not just pipes. In fact, John Milikin and Oleg were the original advocates for this approach and I just stole the idea from them.

Answer 2

We're using Machines in Scala to pull in not just two, but an arbitrary amount of sources.

Two examples of binary joins are provided by the library itself, on the Tee module: mergeOuterJoin and hashJoin. Here is what the code for hashJoin looks like (it assumes both streams are sorted):

/**
 * A natural hash join according to keys of type `K`.
 */
def hashJoin[A, B, K](f: A => K, g: B => K): Tee[A, B, (A, B)] = {
  def build(m: Map[K, A]): Plan[T[A, B], Nothing, Map[K, A]] = (for {
    a  <- awaits(left[A])
    mp <- build(m + (f(a) -> a))
  } yield mp) orElse Return(m)
  for {
    m <- build(Map())
    r <- (awaits(right[B]) flatMap (b => {
      val k = g(b)
      if (m contains k) emit(m(k) -> b) else Return(())
    })) repeatedly
  } yield r
}

This code builds up a Plan which is "compiled" to a Machine with the repeatedly method. The type being built here is Tee[A, B, (A, B)] which is a machine with two inputs. You request inputs on the left and right with awaits(left) and awaits(right), and you output with emit.

There is also a Haskell version of Machines.