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Severity: error
Category: match
Stage: Stage 0 (prototype)

Description

A match expression over a sum type must have an arm for every variant. If one or more variants are missing and there is no catch-all _ arm, SJS emits SJS-E007.

Exhaustiveness is enforced at compile time so that adding a new variant to a sum type immediately flags every match that does not handle it.

Example

type Shape = Circle(radius: number) | Rect(w: number, h: number) | Point

// ✗ error — Point arm missing
function area(s: Shape): number {
  return match s {
    Circle(r) => Math.PI * r * r,
    Rect(w, h) => w * h,
    // SJS-E007: missing arm for Point
  }
}

Fix

Add the missing arm, or use _ as a catch-all:

// ✓ correct — all variants handled
function area(s: Shape): number {
  return match s {
    Circle(r)  => Math.PI * r * r,
    Rect(w, h) => w * h,
    Point      => 0,
  }
}

// ✓ correct — catch-all arm
function describe(s: Shape): string {
  return match s {
    Circle(r) => `circle r=${r}`,
    _         => "other shape",
  }
}

Related codes

  • SJS-W003 — unreachable match arm (inverse problem: arm that can never be reached)
Documentation