Result-Style Unions

A result union returns either a success value or an error message without throwing.

result.ts

type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = ;
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = ;
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = ;
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

result union A result union uses a discriminant such as `ok` to separate success and failure paths.