Stupid question: Is it related to Arrows?
Not really. You might say it's more general than arrows, because the streaming components are not restricted to a single input and single output type. On the other hand, they are all specific to stream processing, much like Fudgets and Yampa which are arrow-based.
Arrows use tuple values for multiple inputs and outputs. Actually I'm using arrows this way for signal processing.
I suppose the Transducer data type [1] could be made an Arrow instance, which would let me rename the 'connect' combinator [2] to (>>>). I'll have a look at Yampa to see if I can harvest more combinator names, thank you!
After some investigation, I've concluded that Transducer cannot be made an instance of Arrow in any way that's both practical and general. The reason is that a Transducer converts a finite stream to another finite stream of arbitrary length. If we ignore effects, it's like a function of type [a] -> [b]. When trying to define an instance of Arrow for
newtype Transducer a b = Transducer ([a] -> [b])
the arr and (>>>) methods are trivial:
instance Arrow Transducer where arr f = Transducer (map f) Transducer t1 >>> Transducer t2 = Transducer (t2 . t1)
but there is no satisfactory definition for the method first. A sensible definition, IMO, would have to give rise to a pairwise (***), so if we have two transducers t1 and t2 which respectively convert [a,b] to [c,d] and [1,2] to [3,4], t1 *** t2 would have to map [(a,1), (b,2)] to [(c,3), (d,4)]. There is no general way to define such a method first for the Transducer data type above. One can try appending the right-hand sides of the input to a queue and adding them to the outputs of the argument transducer:
first (Transducer t) = Transducer (uncurry (zip . t) . unzip)
but this works only for injective transducers that map each new input item to exactly one output item. If the input stream has finite length, the input and output stream must be of equal length. In his paper "Generalising Monads to Arrows" from 1998, John Hughes defines a similar Arrow instance for stream processors operating on infinite streams which theoretically works for non-injective transforms, but leaks infinite space and can't be adopted to finite stream processors. HXT defines a different kind of an Arrow instance similar to the list monad instance. This approach can combine non-injective transforms, but in it the t1 *** t2 example above would map the input [(a,1), (b,2)] to [(c,3), (c,4), (d,3), (d,4)]. While this is a simple and lawful Arrow instance, I don't find it useful for stream processing. Yampa and other Functional Reactive Programming libraries associate a unique timestamp to each stream item. Method first can use this timestamp to associate input and output items even if the transducer leaves out some output items, and if the time stamps are ordered in each stream I guess it could also handle extra output items unrelated to any input. I'm not certain to what extent the FRP transducers (i.e., signal processors) are allowed to do this. To summarize: while I'd love the benefits of satisifying an Arrow instance, the loss of generality seems too high for me. SCC transducers work on finite streams of any values, and their output stream need not have any resemblance to the input stream. I may define an Arrow-conformant subclass of Transducer in a future version of SCC, perhaps operating on streams with timestamped values.
[1]
http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concurr...
[2]
http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concurr...
In whatever way it is related to arrows, maybe you can mention it on the project page.
I'll copy this message there.