> any implementable function has to be one whose first k outputs are determined by at most the first k inputs -- i.e., you can't look into the future. That is, these functions have to be causal.
def f(input_stream):
i = next(input_stream)
j = next(input_stream)
yield i + j
f(input_stream)
This function produces k outputs when given 2*k inputs, so it's either acausal or impossible to execute. Right?
For GP’s stream-transformer / state-machine equivalence to work, you need to sprinkle option types throughout so each input yields some kind of output, even if empty. So more like
def co():
i = yield None # hurray for off-by-one streams
j = yield None
while True:
i = yield i + j
j = yield None
This won’t help if the output stream produces more than one output item from each input item. You could sprinkle lists instead, but in reality multiple simultaneous events have always been a sore point for FRP—in some libraries you can have them (and that’s awkward in some cases), in some you can’t (and that’s awkward in others).
But I am not criticizing his stream-transformer / state-machine equivalence, I am just curious why he thinks functions of Stream A -> Stream B have to produce exactly 1 output for exactly 1 input.
Now, I know that Haskell in its pre-monad days used to have main have signature [Response] -> [Request]: the lists being lazy, they're essentially streams. Each Request produced by the main would result in a Response being provided to it by the runtime. This model actually has to be strictly 1-to-1, and indeed, it was so easy to accidentally deadlock yourself that switching to IO monad was quite welcomed, according to SPJ in his "Tackling the Awkward Squad" paper.
I guess what I wanted to say was that to me (given that the comment was presumably targeted at people who already know how all of FRP, stream transformers, and state transducers work, or can at least make a good guess) it was within the limits of acceptable sloppiness to mix up (State, Input) -> (State, Output), (State, Input) -> (State, Maybe Output), and (State, Input) -> (State, [Output]), or the equivalent on the stream transformation side. The point of the comment does not really depend on what you choose here.
