Improve TS type narrowing · gleam-lang/gleam#5715

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help wanted

描述

In the following issue, the example are made with a simple Option(a) in Gleam.

When using an Option(a) to TS, we know for sure that the expected type is Option$, so we can type functions signatures as (value: $option.Option$<EL>) => ..... When using the new API, "pattern matching" is defined as $option.Option$isSome(value)). The type narrowing works, but will refine value from Option$<EL> to Option$<unknown>, because isSome is defined as export function Option$isSome<EL>(value: any): value is Option$<unknown>;.

Thanks to TS and function overloading, we could do way better, and keep the generic type, while narrowing to the correct type. We could write an overload export function Option$isSome<EL>(value: Option$<EL>): value is Some<EL>. That way, it would keep the generic.

貢獻者指南

技術棧: typescript
領域: developer experience
議題類型: feature
難度: 2
預計時間: 1-3 hours
活動狀態: fresh
清晰度: clear
前置要求: TypeScript genericsfunction overloading
新手友善度: 70
研究方向: The issue requests an improvement to TypeScript type narrowing for the Option type. Currently, `Option$isSome` narrows `value` to `Option$<unknown>`, losing the generic type. The solution is to add a function overload: `export function Option$isSome<EL>(value: Option$<EL>): value is Some<EL>`. The fix should be applied in the TypeScript definition file generated by the Gleam compiler. Look for where `Option$isSome` is defined, likely in a generated `.d.ts` file or in the compiler's type generation logic.