Question about reactivity and side-effects

[breckognize]

Hi, I recently came across Pluto and it's reactive updates - it's super cool! I'm trying to understand how the feature works with operations that have side-effects. I'm new to Julia, so I apologize if the answer here lies with Julia and not Pluto.

I created a notebook like this:

Cell 1: x = Dict("key" => 0)
Cell 2: x["key"] = 1
Cell 3: x["key"] + 1

First, I ran cells 1, 2, and 3 and get outputs Dict("key2" => 0), 1, 2 respectively (as expected).

Then I changed cell 1 to: x = Dict("key" => 42) and reran it. Cell 3 didn't update. Then I manually re-ran cell 3 and it still didn't update.

Removing the assignment in cell 2 and repeating the above steps produces the expected behavior.

My question is - what's going on under the hood? Is Pluto caching something? Is the guidance "Reactivity breaks if you do something with side effects - so don't do that."?

[fonsp]

Hey! Take a look at #564 and https://www.youtube.com/watch?v=IAF8DjrQSSk for some background.

In your example, Cell 1 is seen as a definition of x, and cells 2&3 are referencing x, and this is all the reactivity information that Pluto has. (Pluto's reactivity is syntax-based, not value-based.) This means that whenever you run cell 1, Pluto will also run cells 2 & 3. Pluto will probably run cell 2 before cell 3, because cell order is used as a tie-breaker in our algorithm.

So when you changed cell 1 to: x = Dict("key" => 42) and reran it, this triggered cell 2 to run (setting the value to 1), and then cell 3, returning 2.