Create a Signal from a List

Question

Is it possible to create a Signal from a List? Essentially what I want is something with the signature List a -> Signal a. I know that a Signal represents a time-varying value and so something like this doesn't actually make any sense (i.e. I can't think of a reason to use it in production code).

I could see applications of it for testing though. For example, imagine some function which depended on the past values of a Signal (via foldp for instance) and you wanted to make assertions about the state of the system given the signal had received values x, y, and z.

Note that there wouldn't have to be anything special about the Signal denoting that it only would ever receive a fixed number of values. I'm thinking of it more like: in production you have a Signal of mouse clicks, and you want to test that from a given starting position, after a given set of clicks, the system should be in some other known state. I know you could simulate this by calling the function a fixed number of times and feeding the results back in with new values, I'm just wondering if it is possible.

Answer 1

I guess it's possible. You use a time-based signal, and map the values from the list over it:

import Time
import Graphics.Element exposing (show)

list = [1..10]


signalFromList : List a -> Signal a
signalFromList list =
  let
    (Just h) =
      List.head list

    time =
      Time.every Time.second

    maybeFlatMap =
      flip Maybe.andThen

    lists =
      Signal.foldp (always <| maybeFlatMap List.tail) (Just list) time
  in
    Signal.filterMap (maybeFlatMap List.head) h lists

main = Signal.map show <| signalFromList list

However!

It shouldn't be hard to do testing without signals. If you have a foldp somewhere, in a test you can use List.foldl over a list [x,y,z] instead. That should give you the ability to look at the state of your program after inputs x, y, z.