How to query a URL every timestep using Functional Reactive Programming?

Question

I'm wondering, how would you write a program using Functional Reactive Programming which, every x timesteps, retrieves a JSON object from a given URL?

I'm looking in the Elm framework, but I'm open to more general solutions. I have a method

send : Signal (Request a) -> Signal (Response String)

i.e. it takes a HTTP Request wrapped in a signal, and returns a response string, wrapped in a signal.

Right now, I have a "next state" function which takes input signals and creates a new game state. These are wrapped up using foldp. One of the input signals is the Response from the HTTP request. However, when I run it, the query runs only once, not every timestep. How can I fix this?

EDIT: Here's how I would solve this using non-FRP (imperative style):

while True:
    myJson = send postRequest url
    --do stuff with myJSON
    sleep(timestep)

i.e. just query the url every so often, infinitely looping.

Answer 1

From Elm docs you will find:

every : Time -> Signal Time

lift : (a -> b) -> Signal a -> Signal b

get : String -> Request String

send : Signal (Request a) -> Signal (Response String)

Above functions can be used to do what you need:

send $ lift (\_ -> get myURL) $ every (10 * seconds) which will be of type Signal (Response String)

I haven't tested the code but I hope this gives you the idea.