Why not foldp?

[yakir12]

In Reactive there's foldp which is useful for accumulating state over updates (https://juliagizmos.github.io/Reactive.jl/#maintaining-state). I found it pretty useful. Right now I accomplish this with:

u = Observable(0)
oldu = Ref(u[])
p = Observable(0)
map!(p, u) do newu
  s = oldu[] + newu
  oldu[] = newu
  p[] + s
end

which seems a bit cumbersome considering what it could be:

p = foldp(+, u)