The backstage of reactivity

by Nick aka malyvsen

Hey there 👋 In this notebook, I'll try to convince you that you could have made Pluto yourself!

This notebook is part of PlutoCon 2021. It's best served together with my presentation 🎥

Expression objects

If you write a colon : in front of some code, you get an expression object representing the syntax tree.

This object has a .head and .args, which represent a node in the syntax tree and its children. In this case, .head is the assignment operator = and the children are the left-hand side (the variable being assigned to) and the right-hand side (the value being assigned).

The :toucan you're seeing here is a symbol object - it represents a variable name and can easily be turned into a string:

Also, check this out: args[2] is an expression object in itself!

Now for a little it of Julia magic: 2 + can is a function call to the function +:

.args[1] is the function being called, and the rest of .args is its arguments. So + is secretly a function!

One last thing about expression objects - you can easily make them out of strings like so:

What variables does a cell use?

Let's write a function, references, which tells us exactly that!

What variables does a cell assign to?

When does one cell depend on another?

We'll write a function called depends_on which tells us whether cell a depends on cell b, i.e. whether b needs to be ran before a.

It's pretty easy! Try to figure it out before you proceed 🧙🏼 Here's how our function should behave:

Tell me how it's done!

In what order should we run cells?

Julia's sort function can help here - it takes a "less than" argument, lt, which should be a function that returns true if its first argument should come before its second argument, and false otherwise.

Actually running cells

Let's use Julia's Core.eval to run cells in the correct order.

The first argument to Core.eval is the module to evaluate the expression in - we'll just use Main. The second argument is an expression object.

We're almost there! The cells below are quite meta, so Pluto might have trouble updating them - please help it 😇

Bam, we did it! I hope Pluto is still magic 🪄 after you got a sneak peek behind the curtain. Adieu!