gleam-lang/gleam

Improve TS type narrowing

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#5715 opened on May 13, 2026

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help wanted

Description

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.

Contributor guide

Tech stack
typescript
Domain
developer experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript genericsfunction overloading
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
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.