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Be taught the contrivance to Device a Luau: Augmenting Lua’s Syntax With Kinds

For a in reality long time, Lua 5.1 modified into once the language of selection for Roblox. As we grew, so too did the demand of for greater tooling enhance as properly as a extra performant VM. To acknowledge this, we started the initiative to rebuild our Lua stack named “Luau” (pronounced /lu-wow/), with the honest of encompassing the choices programmers question a contemporary language to provide – which contains a fashion checker, a brand recent linter framework, and a sooner interpreter, lawful to title about a.

To fabricate all of that doubtless, we needed to rewrite most of our stack from scratch. The problem is that the Lua 5.1 parser is tightly coupled with bytecode generation, and that’s insufficient for our wants. We’re attempting with a idea to traverse the AST for deeper evaluation, so we need a parser to provide that syntax tree. From there, we’re free to construct any operations we get to construct on that AST.

As success would get it, there modified into once an present Lua 5.1 parser lying about in Studio handiest ragged for overall linting plod. That made it very easy for us to adopt that parser and extend it to acknowledge Luau-reveal syntax, which thereby minimized the doubtless risk of fixing the following parse in some delicate contrivance. A notable ingredient because one of our sacred values at Roblox is backward compatibility. We get millions of traces of Lua code already written and we are dedicated to guaranteeing that they continue to work without end.

So with these factors in mind, the requirements are particular. We get to:

  • defend particular of grammar quirks that require backtracking
  • get an efficient parser
  • defend ahead-treasure minded syntax
  • remain backward treasure minded with Lua 5.1

Sounds easy, lawful?

How the type inference engine influenced syntax decisions

To launch, we get to appreciate some context about how we arrived in this scenario. We chose these syntaxes because they’re already at once familiar to the majority of programmers, and are in reality substitute normal. You don’t get to learn the rest recent.

There are a whole lot of locations where Luau permits you to jot down such form annotations:

  • native foo: string
  • honest add(x: quantity, y: quantity): quantity … conclude
  • form Foo = (quantity, quantity) -> quantity
  • native foo = bar as string

Adding syntax to annotate your bindings is amazingly crucial for the type inference engine to greater understand the intended typings. Lua is a in reality extremely fantastic language that permits you to overload virtually every operator within the language. Without a technique to annotate what things are, we can’t even confidently state that the expression x + y is going to provide a quantity!

Form solid expression

Something we in reality treasure from TypeScript is what they name a fashion assertion. It’s in overall a technique so as to add additional form knowledge to a program for the checker to study. In TypeScript, the syntax is:

bar as string

Sadly, when we tried this out, we had been in for a grisly surprise: this breaks present code! Surely one of our customers’ games had a honest named as. Their scripts therefore incorporated snippets treasure:

native x = y

as(w, z) — Anticipated ‘->’ when parsing honest form, obtained

We seemingly can also get made this work, had been it now now not for one additional complication: we wanted our parser to work with handiest a single token of lookahead. Performance is crucial to us, and allotment of writing a in reality extremely performant parser is minimizing the volume of backtracking it has to construct. It wouldn’t be efficient for our parser to get to scan ahead and backward arbitrarily some distance to resolve what an expression in reality manner.

It also looks that TypeScript can thank JavaScript’s automated semicolon insertion rule for making this work for free. For these who write this snippet in TypeScript/JavaScript, it will insert semicolons on every line, inflicting it to be parsed as two separate statements. Whereas if it had been on a single line, it is a syntax error on the as token in JavaScript, but a sound form assertion expression in TypeScript. As a result of Lua doesn’t construct this, nor does it implement semicolons, it has to overview out to parse for every longest doubtless assertion even within the occasion that they span across quite a bit of traces.

let x = y

as(w, z)

Luau’s normal form solid expression modified into once now now not backward treasure minded although it had the performance we wanted. Regrettably, this broke our promise of Luau being a superset of Lua 5.1, so we are able to’t construct it with out some additional constraints corresponding to requiring parentheses in certain contexts!

Form arguments in honest calls

One other uncomfortable ingredient in Lua’s grammar prevents us from adding form arguments to honest calls with out introducing one more ambiguity:

return someFunction(c)

It’ll also mean two diverse things:

  • overview someFunction < A and B > c, and return the implications
  • name and return someFunction with two form arguments A and B, and an argument of c

This ambiguity handiest occurs within the context of an expression checklist. It’s now now not in reality a mammoth say in TypeScript and C# because they both get the helpful thing about compiling ahead of time. As a result of this truth, they can both afford to use some cycles attempting to try to disambiguate this expression down to one of many 2 choices.

Whereas evidently we are able to also construct the same thing, corresponding to applying heuristics at some level of parsing or form checking, we in reality can’t. Lua 5.1 has the skill to dynamically inject globals into any atmosphere, and that can perhaps demolish this heuristic. We also flat out construct now now not get that attend because we need with a idea to generate bytecode as posthaste as doubtless for all clients to launch interpreting.

Form alias assertion

Parsing this kind alias assertion is now now not a breaking change because it’s already invalid Lua syntax:

form Foo = quantity

What we construct is easy. We parse a foremost expression which handiest finally ends up parsing as some distance as lawful form, and then we make a name what to construct in step with the parse consequence of that expression:

  • If it’s a honest name, conclude attempting to parse for added of this expression-as-assertion.
  • Otherwise, if the following token is a comma or equal, parse an assignment assertion.

What’s lacking above is amazingly glaring. It has no branch for which an identifier will seemingly be led by one more one. All we get to construct then is pattern match on the expression:

  1. Is it an identifier?
  2. Is the title of that identifier equal to “form”?
  3. Is the following token any arbitrary identifier?

Voilà, you fetch backward-treasure minded syntax with a context-sensitive key phrase.

form Foo = quantity — form alias

form(x) — honest name

form = {x = 1} — assignment

form.x = 2 — assignment

As a bonus snippet, this still parses within the trusty same contrivance as Lua 5.1 because we weren’t parsing from the context of an announcement:

native foo = form

bar = 1

Classes realized

The takeaways right here, it looks, is that we’re going to get to originate the syntax for Luau to be ahead treasure minded and with least context-sensitive parse paths. It eliminates the need of 2nd-guessing that requires the parser to again off and try one thing else from that level of failure. Now not handiest does that give us the helpful thing about getting a posthaste parser to lawful chug along to the conclude of the source code, but also it will provide support the AST with out desiring diverse forms of phases to disambiguate.

It also manner that we are able to get to be careful when adding recent syntax in overall, which is now now not necessarily a grisly draw to be. A properly-idea-out language calls for its designers to retract the long look for.

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