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83 lines
2.9 KiB
Markdown
83 lines
2.9 KiB
Markdown
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---
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layout: single
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title: "Luau Recap: September & October 2022"
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---
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Luau is our new language that you can read more about at [https://luau-lang.org](https://luau-lang.org).
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[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-september-october-2022/).]
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## Semantic subtyping
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One of the most important goals for Luau is to avoid *false
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positives*, that is cases where Script Analysis reports a type error,
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but in fact the code is correct. This is very frustrating, especially
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for beginners. Spending time chasing down a gnarly type error only to
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discover that it was the type system that's wrong is nobody's idea of fun!
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We are pleased to announce that a major component of minimizing false
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positives has landed, *semantic subtyping*, which removes a class of false positives caused
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by failures of subtyping. For example, in the program
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```lua
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local x : CFrame = CFrame.new()
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local y : Vector3 | CFrame
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if (math.random()) then
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y = CFrame.new()
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else
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y = Vector3.new()
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end
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local z : Vector3 | CFrame = x * y -- Type Error!
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```
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an error is reported, even though there is no problem at runtime. This
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is because `CFrame`'s multiplication has two overloads:
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```lua
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((CFrame, CFrame) -> CFrame)
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& ((CFrame, Vector3) -> Vector3)
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```
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The current syntax-driven algorithm for subtyping is not sophisticated
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enough to realize that this is a subtype of the desired type:
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```lua
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(CFrame, Vector3 | CFrame) -> (Vector3 | CFrame)
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```
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Our new algorithm is driven by the semantics of subtyping, not the syntax of types,
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and eliminates this class of false positives.
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If you want to know more about semantic subtyping in Luau, check out our
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[technical blog post](https://luau-lang.org/2022/10/31/luau-semantic-subtyping.html)
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on the subject.
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## Other analysis improvements
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* Improve stringification of function types.
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* Improve parse error warnings in the case of missing tokens after a comma.
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* Improve typechecking of expressions involving variadics such as `{ ... }`.
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* Make sure modules don't return unbound generic types.
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* Improve cycle detection in stringifying types.
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* Improve type inference of combinations of intersections and generic functions.
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* Improve typechecking when calling a function which returns a variadic e.g. `() -> (number...)`.
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* Improve typechecking when passing a function expression as a parameter to a function.
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* Improve error reporting locations.
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* Remove some sources of memory corruption and crashes.
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## Other runtime and debugger improvements
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* Improve performance of accessing debug info.
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* Improve performance of `getmetatable` and `setmetatable`.
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* Remove a source of freezes in the debugger.
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* Improve GC accuracy and performance.
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## Thanks
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Thanks for all the contributions!
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* [AllanJeremy](https://github.com/AllanJeremy)
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* [JohnnyMorganz](https://github.com/JohnnyMorganz)
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* [jujhar16](https://github.com/jujhar16)
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* [petrihakkinen](https://github.com/petrihakkinen)
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