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44 lines
1.7 KiB
Markdown
44 lines
1.7 KiB
Markdown
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# Typed variadics
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> Note: this RFC was adapted from an internal proposal that predates RFC process
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## Summary
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Add syntax for ascribing a type to variadic pack (`...`).
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## Motivation
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Luau's type checker internally can represent a typed variadic: any number of values of the same type. Developers should be able to describe this construct in their own code, for cases where they have a function that accepts an arbitrary number of `string`s, for example.
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## Design
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We think that the postfix `...: T` syntax is the best balance of readability and simplicity. In function type annotations, we will use `...T`:
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```
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function math.max(...: number): number
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end
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type fn = (...string) -> string
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type fn2 = () -> ...string
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```
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This doesn't introduce syntactical ambiguity and should cover all cases where we need to represent this construct. Like `...` itself, this syntax is only legal as the last parameter to a function.
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Like all type annotations, the `...: T` syntax has no effect on runtime behavior versus an unannotated `...`.
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There are currently no plans to introduce named variadics, but this proposal leaves room to adopt them with the form `...name: Type` in function declarations in the future.
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## Drawbacks
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The mismatch between the type of `...` in function declaration (`number`) and type declaration (`...number`) is a bit awkward. This also gets more complicated when we introduce generic variadic packs.
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## Alternatives
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We considered several other syntaxes for this construct:
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* `...T`: leaves no room to introduce named variadics
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* `...: T...`: redundant `...`
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* `... : ...T`: feels redundant, same as above
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* `...: T*`: potentially confusing for users with C knowledge, where `T*` is a pointer type
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