is_a recognizes sum-type variants; match is how you branch on them

let b = Box { w: 3 }         // Box is a `variants` constructor of Shape
is_a(b, "Box")               // true — the variant's name
is_a(b, "Shape")             // true — its sum type's name
type_of(b)                   // "Variant" (a record literal is a "Map")
b.w                          // 3 — a struct variant's field reads with `.`

// branch on the kind with an exhaustive match (a missing variant is a check error;
// a tuple variant's fields, `Pair(a, b)`, are only reachable this way)
on kind(s: Map) {
    return match s {
        Box { w } -> "Box of {w}"
        Dot -> "Dot"
    }
}