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oxlint-plugin-effect

no-manual-discriminant-comparison

Intent: branching on a union’s discriminant goes through an exhaustive match, never through comparisons or switch.

// ❌ no
switch (event._tag) {
    case "Created":
        return onCreated(event)
    case "Deleted":
        return onDeleted(event)
}


// ✅ yes
Match.value(event).pipe(Match.tagsExhaustive({ Created: onCreated, Deleted: onDeleted }))


// ✅ yes, with `discriminants: ["_tag", "type"]`
Match.value(payment).pipe(
    Match.discriminatorsExhaustive("type")({ card: fromCard, transfer: fromTransfer }),
)

Trigger: an equality comparison between a discriminant (also ["type"]) and a string literal, or a switch on a discriminant. The discriminants are the names in discriminants.

Exemptions and blind spots: comparisons against a variable rather than a literal. On _tag, the handlers covered by anti-slop-no-manual-effect-error-tag: the two never fire on the same node. Guard clauses such as if (x.type === "a") return … fire too; on _tag use Predicate.isTagged. A _tag compared with a variant of Effect’s own types (Success/Failure of Result and Exit, Left/Right, Some/None) is reported with their refinement as the remedy, such as Result.isFailure: those types cannot gain a variant; a switch is reported as usual. An exhaustive ts-pattern match is an equally valid remedy. A name added to discriminants also fires on objects outside closed unions (DOM events, external payloads): the list is a choice of the codebase.

Why: Match.tagsExhaustive, Match.discriminatorsExhaustive and $match fail to compile when a variant is added; a comparison or a switch does not.

Options:

Fix: none