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January 2022 recap (#331)
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Andy Friesen
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docs/_posts/2022-01-27-luau-recap-january-2022.md
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---
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layout: single
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title: "Luau Recap: January 2022"
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---
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Luau is our programming language that you can read more about at [https://luau-lang.org](https://luau-lang.org).
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[Find us on GitHub](https://github.com/Roblox/luau)!
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[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-january-2022/).]
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## Performance improvements
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The implementation of `tostring` has been rewritten. This change replaces the default number->string conversion with a
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new algorithm called Schubfach, which allows us to produce the shortest precise round-trippable representation of any
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input number very quickly.
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While performance is not the main driving factor, this also happens to be significantly faster than our old
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implementation (up to 10x depending on the number and the platform).
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---
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Make `tonumber(x)` ~2x faster by avoiding reparsing string arguments.
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---
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The Luau compiler now optimizes table literals where keys are constant variables the same way as if they were constants, eg
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```lua
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local r, g, b = 1, 2, 3
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local col = { [r] = 255, [g] = 0, [b] = 255 }
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```
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## Improvements to type assertions
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The `::` type assertion operator can now be used to coerce a value between any two related types. Previously, it could
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only be used for downcasts or casts to `any`. The following used to be invalid, but is now valid:
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```lua
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local t = {x=0, y=0}
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local a = t :: {x: number}
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```
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## Typechecking improvements
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An issue surrounding table literals and indexers has been fixed:
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```lua
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type RecolorMap = {[string]: RecolorMap | Color3}
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local hatRecolorMap: RecolorMap = {
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Brim = Color3.fromRGB(255, 0, 0), -- We used to report an error here
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Top = Color3.fromRGB(255, 0, 0)
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}
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```
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---
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Accessing a property whose base expression was previously refined will now return the correct result.
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## Linter improvements
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`table.create(N, {})` will now produce a static analysis warning since the element is going to be shared for all table entries.
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## Error reporting improvements
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When a type error involves a union (or an option), we now provide more context in the error message.
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For instance, given the following code:
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```lua
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--!strict
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type T = {x: number}
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local x: T? = {w=4}
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```
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We now report the following:
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```
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Type 'x' could not be converted into 'T?'
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caused by:
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None of the union options are compatible. For example: Table type 'x' not compatible with type 'T' because the former is missing field 'x'
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```
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---
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Luau now gives up and reports an `*unknown*` type in far fewer cases when typechecking programs that have type errors.
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## New APIs
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We have brought in the [`coroutine.close`](https://luau-lang.org/library#coroutine-library) function from Lua 5.4. It accepts a suspended coroutine and marks it as non-runnable. In Roblox, this can be useful in combination with `task.defer` to implement cancellation.
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## REPL improvements
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The `luau` REPL application can be compiled from source or downloaded from [releases page](https://github.com/Roblox/luau/releases). It has grown some new features:
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* Added `--interactive` option to run the REPL after running the last script file.
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* Allowed the compiler optimization level to be specified.
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* Allowed methods to be tab completed
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* Allowed methods on string instances to be completed
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* Improved Luau REPL argument parsing and error reporting
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* Input history is now saved/loaded
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## Thanks
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A special thanks to all the fine folks who contributed PRs over the last few months!
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* [Halalaluyafail3](https://github.com/Halalaluyafail3)
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* [JohnnyMorganz](https://github.com/JohnnyMorganz)
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* [Kampfkarren](https://github.com/Kampfkarren)
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* [kunitoki](https://github.com/kunitoki)
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* [MathematicalDessert](https://github.com/MathematicalDessert)
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* [metatablecat](https://github.com/metatablecat)
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* [petrihakkinen](https://github.com/petrihakkinen)
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* [rafa_br34](https://github.com/rafa_br34)
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* [Rerumu](https://github.com/Rerumu)
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* [Slappy826](https://github.com/Slappy826)
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* [SnowyShiro](https://github.com/SnowyShiro)
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* [vladmarica](https://github.com/vladmarica)
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* [xgladius](https://github.com/xgladius)
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[Contribution guide](https://github.com/Roblox/luau/blob/2f989fc049772f36de1a4281834c375858507bda/CONTRIBUTING.md)
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