Mechanism to early-exit reactivity

[fonsp]

We now have @use_memo from https://github.com/JuliaPluto/PlutoHooks.jl , which allows me to memoize computations:

Schermopname.2021-12-07.om.13.04.25.mov

This is great! But!

Even though the calculation of result is being memoized, it is still being redefined every time x changes, triggering a reactive run. If I use result throughout my notebook (doing further analysis), I now need to put @use_memo around everything that depends on result. This came up in my Catalyst.jl project and in #1706.

To solve this, we need a mechanism for notebook code to tell Pluto to early-exit reactivity. If we do this based on === equality to the last value, then it resembles the runtime of systems like https://www.youtube.com/watch?v=5GGl3TojN2o and https://github.com/nextjournal/clerk . We can't simply enable this by default, because there are really cool notebooks that depend on symbolic-based triggers (like https://www.youtube.com/watch?v=6Qs5EXDpZBI).

A slightly related issue is #1696 , though we can also just implement that without this generality

[fonsp]

@purevalue macro

I imagine that we could do something like:

@bind x Slider(1:100)

z = x ÷ 20

result = @purevalue(@use_memo([z]) do
	expensive_calculation(z)
end)

Which tells Pluto that result is "pure": if its value did not change, it should not trigger reactivity.

Both @purevalue and @use_memo are low-level, but I imagine that we can combine the two into a single macro in PlutoLinks.jl:

result = @pure expensive_calculation(z)

where we automatically find dependencies using ExpressionExplorer (we copy-paste it into PlutoLinks.jl), or you can give the dependencies manually to allow tricks:

result = @pure expensive_calculation(z + x) [z]

(Though @pure is already taken by Base.@pure...)

[Pangoraw]

What should happen when a cell contain both pure and unpure variables ?

begin
  y = rand(x)
  z = @pure expensive_calculation(x)
end

Maybe the reactive run can stop all runs to the :z edges but not on :y ?

[fonsp]

Yes! It should apply to the edges!