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88 lines
3.8 KiB
Markdown
88 lines
3.8 KiB
Markdown
---
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layout: single
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title: "Luau Recap: June 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-june-2022/).]
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# Lower bounds calculation
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A common problem that Luau has is that it primarily works by inspecting expressions in your program and narrowing the _upper bounds_ of the values that can inhabit particular variables. In other words, each time we see a variable used, we eliminate possible sets of values from that variable's domain.
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There are some important cases where this doesn't produce a helpful result. Take this function for instance:
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```lua
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function find_first_if(vec, f)
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for i, e in ipairs(vec) do
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if f(e) then
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return i
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end
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end
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return nil
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end
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```
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Luau scans the function from top to bottom and first sees the line `return i`. It draws from this the inference that `find_first_if` must return the type of `i`, namely `number`.
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This is fine, but things go sour when we see the line `return nil`. Since we are always narrowing, we take from this line the judgement that the return type of the function is `nil`. Since we have already concluded that the function must return `number`, Luau reports an error.
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What we actually want to do in this case is to take these `return` statements as inferences about the _lower_ bound of the function's return type. Instead of saying "this function must return values of type `nil`," we should instead say "this function may _also_ return values of type `nil`."
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Lower bounds calculation does precisely this. Moving forward, Luau will instead infer the type `number?` for the above function.
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This does have one unfortunate consequence: If a function has no return type annotation, we will no longer ever report a type error on a `return` statement. We think this is the right balance but we'll be keeping an eye on things just to be sure.
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Lower-bounds calculation is larger and a little bit riskier than other things we've been working on so we've set up a beta feature in Roblox Studio to enable them. It is called "Experimental Luau language features."
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Please try it out and let us know what you think!
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## Known bug
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We have a known bug with certain kinds of cyclic types when lower-bounds calculation is enabled. The following, for instance, is known to be problematic.
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```lua
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type T = {T?}? -- spuriously reduces to {nil}?
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```
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We hope to have this fixed soon.
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# All table literals now result in unsealed tables
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Previously, the only way to create a sealed table was by with a literal empty table. We have relaxed this somewhat: Any table created by a `{}` expression is considered to be unsealed within the scope where it was created:
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```lua
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local T = {}
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T.x = 5 -- OK
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local V = {x=5}
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V.y = 2 -- previously disallowed. Now OK.
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function mkTable()
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return {x = 5}
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end
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local U = mkTable()
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U.y = 2 -- Still disallowed: U is sealed
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```
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# Other fixes
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* Adjust indentation and whitespace when creating multiline string representations of types, resulting in types that are easier to read.
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* Some small bugfixes to autocomplete
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* Fix a case where accessing a nonexistent property of a table would not result in an error being reported.
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* Improve parser recovery for the incorrect code `function foo() -> ReturnType` (the correct syntax is `function foo(): ReturnType`)
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* Improve the parse error offered for code that improperly uses the `function` keyword to start a type eg `type T = function`
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* Some small crash fixes and performance improvements
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# Thanks!
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A very special thanks to all of our open source contributors:
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* [Allan N Jeremy](https://github.com/AllanJeremy)
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* [Daniel Nachun](https://github.com/danielnachun)
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* [JohnnyMorganz](https://github.com/JohnnyMorganz/)
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* [Petri Häkkinen](https://github.com/petrihakkinen)
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* [Qualadore](https://github.com/Qualadore)
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