Typing of object/array pattern components via ::

[edemaine]

Fixes #126 by implementing a long-sought syntax for typing individual object properties and array elements in left-hand-side patterns (including function arguments and assignment). Basically, :: type can come at the end of any existing object property or array element. For example:

function Component({
  name: [first:: string, last:: string, ...rest:: string[]],
  counter:: number
  setCounter: sc:: (number) => void
})
↓↓↓
function Component({
  name: [first, last, ...rest],
  counter,
  setCounter: sc
}: {
  name: [ string, string,... string[],],
  counter: number,
  setCounter: (number) => void,}){}

Why this notation?

• name:: type: value is ambiguous: it could be interpreted as name:: {type: value} (typing as object). (I tried to include this as an option at first, but ran into this alternate parse.)
• name: value: type is similarly ambiguous: it could be interpreted as name: {value: type} (complex destructuring).

I admit I still often type the wrong number of :s when writing examples, but I imagine we'll get used to it. (Also open to better proposals for notation.)

I still need to write actual docs, but hopefully the example above and the tests give a reasonable idea.

[STRd6]

Looks good, I like the feature! The explanation of the challenges with the other approaches makes sense to me.

I added a few nitpicks but don't let them hold this up if it's too tedious.

[STRd6]

We should only add trailing commas as authored in the source. It's not necessary for this to merge but ideally trailing commas would always be as authored (some parts of the parser currently do this better than others).

[edemaine]

Thanks for encouraging this; it turned out to be pretty easy to improve upon using delim properties (just has to add them in one case). It's still not exactly perfect; there's an added comma at the end of an object when a ...rest part got extracted out so it wouldn't be necessary. But it seems close enough.

Leaving better preservation of whitespace to future work. It's pretty good now, but definitely not perfect.