no-nested-match

Intent: a match handler contains neither another match nor a ternary.

// ❌ no
const run = (argv: ReadonlyArray<string>) =>
    Result.match(parseArgs(argv), {
        onFailure: usageError,
        onSuccess: (command) =>
            Option.match(command.only, { onNone: () => fillAll, onSome: fillOnly }),
    })

// ✅ yes, outside a generator: a named handler
const fill = (command: CliCommand) =>
    Option.match(command.only, { onNone: () => fillAll, onSome: fillOnly })

const run = (argv: ReadonlyArray<string>) =>
    Result.match(parseArgs(argv), { onFailure: usageError, onSuccess: fill })

// ✅ yes, inside a generator: guard clauses
const run = Effect.fn("run")(function* (argv: ReadonlyArray<string>) {
    const parsed = parseArgs(argv)
    if (Result.isFailure(parsed)) return yield* usageError(parsed.failure)

    const { only } = parsed.success
    if (Option.isNone(only)) return yield* fillAll

    return yield* fillOnly(only.value)
})

Trigger: a handler of Option.match, Result.match, Exit.match, Either.match, $match or Match.valueTags that is itself a data-last match (also Match.type().pipe(…) and pipe(Match.type(), …)), or that returns a match, a Match.value(…).pipe(…) or pipe(Match.value(…), …) chain or a ternary.

Exemptions and blind spots: if inside handlers, and whatever they return from inside an if or another nested block; the handlers of an outer Match.value/Match.type chain; matches nested deeper than the returned value, or assigned to a variable before being returned.

Why: two nested levels of choice hide which branch is the normal one.

Options: none

Fix: none