Control.Monad.Catana

Computation type:

Computations that both consume and produce elements lazily with support for advanced control flow using continuations

Binding Strategy:

Binding a function to a monadic value produces a continuation which passes the unconsumed input and the combined output function in turn to the next continuation.

Useful for:

Lazily processing a list with complex control structures.

Zero and plus:

None.

Example type:

Catana i o b a

The Catana monad represents computations that are both catamorphisms and anamorphisms; they both consume and produce values. In addition, the Catana monad represents the computation in continuation-passing style, and implements callCC.

The Catana monad

data Catana i o b a

Constructors

Catana

Fields

runCatana :: (a -> Step i o b) -> Step i o b

Instances

Monad (Catana i o b)

Functor (Catana i o b)

Applicative (Catana i o b)

MonadCont (Catana i o b)

consume :: Catana i o a i

Consumes an element from the input list, returning it If there is no more input, the chain of continuations ends immediately; no more computations will be processed

top :: Catana i o a i

Returns the next input without consuming it

push :: i -> Catana i o a ()

Pushes x into the input

produce :: o -> Catana i o a ()

Produces x in the output

stop :: b -> Catana i o b a

Stops computation, ending the continuation chain

evalCatana :: Catana i o a a -> [i] -> [o]

Converts a Catana monad into a function over lists

evalCatana' :: Catana i o a a -> [i] -> (Maybe a, [o])

Evaluates a Catana monad over a list returning the result and output

parallelB :: Catana i o a a -> Catana i o b b -> Catana i o c (a, b)

Combine two monads to run in parallel, consuming the same input

parallelE :: Catana i o a a -> Catana i o b b -> Catana i o c (Either a b)

Combine two monads to run in parallel, consuming the same input, stopping when either of them finish.

serial :: Catana io o a a -> Catana i io b b -> Catana i o c (Either a b)

Combine two monads to run in serial, the first consuming the output of the second

Example 1: Usage of the Catana monad

An example of complex control structure using Catana:

catanaExample1 =
  forever $ do
    z <- callCC $ \exit -> do
      (x, y) <- (,) <$> consume <*> consume
      produce (x + y)
      produce (x * y)
      if x > 1
        then exit 0
        else do
          produce (x - y)
          produce (x / y)
          return 1
    produce z

Catana monads can be converted into a function over lists using evalCatana.

 evalCatana catanaExample1 [1..4]
 -- result: [3.0,2.0,-1.0,0.5,1.0,7.0,12.0,0.0] 

An example using serial and parallel data flow:

catanaExample2 = sq `serial` (ev `parallelB` od)
  where od = forever $ do
               x <- consume
               when (odd x) .
                 produce $ x * 2
        ev = forever $ do
               x <- consume
               when (even x) .
                 produce $ x + 3
        sq = forever $ do
               x <- consume
               produce x
               produce $ x ^ 2

 let l = 1 : evalCatana catanaExample2 l
 take 10 l
 -- result: [1,2,4,5,25,7,49,10,100,50]