Why doesn't this Elm code work? (Example provided with expected output)

Question

I'm trying to make an elm program that outputs the following pattern :

3 4 2 3 2 4 ...

Here is my Elm Code :

main =  lift asText agent

delta = lift (\x -> -1) (fps 1)

agent = foldp update 3 delta

update : Int -> Int -> Int
update x y = x + y  + (threshold x)

threshold : Int -> Int

threshold x = if (x < 3) then x
              else 0

This is what I think the code should be saying

 3 + -1 + 0 = 2
 2 + -1 + 3 = 4
 4 + -1 + 0 = 3
 3 + -1 + 0 = 2 ... etc

However this is not the output. I think I'm confused as to how the signals update ...

Answer 1

There are a few things keeping your code from giving you 3 then 2 then 4 then 3 again. (Which is not exactly what you describe at the start of your question, but you do at the finish so I'll go with that. )

Argument order of update

If you look at the type of foldp,

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

you'll notice that the last value calculated is the second argument to the update function you supply. So if you make your update function

update y x = x + y  + (threshold x)

then you should be closer to what you wanted you function to be.

Meaning of threshold

threshold returns x when x < 3, so when the last x was 2, it'll return 2, so you'll get:

3 + -1 + 0 = 2
2 + -1 + 2 = 3
3 + -1 + 0 = 2
2 + -1 + 2 = 3

Now I'm not sure what you're after but if you add a +1 to threshold you should get close to the output you were expecting.

Conclusion

With these changes your code would look like:

main =  lift asText agent

delta = lift (\x -> -1) (fps 1)

agent = foldp update 3 delta

update : Int -> Int -> Int
update y x = x + y  + (threshold x)

threshold : Int -> Int

threshold x = if (x < 3) then x+1
              else 0