Geometric Constraint Matching

Geometric rules are often clearer as named constraints than as inline if statements.

struct Vec3 {
  double x;
  double y;
  double z;
};

Name the Predicate

auto inside_unit_sphere = [](double x, double y, double z) {
  return x * x + y * y + z * z < 1.0;
};

Use It as a Guard

const char *classify(const Vec3 &p) {
  using namespace ptn;

  return match(p) | on(
    $(has<&Vec3::x, &Vec3::y, &Vec3::z>)[inside_unit_sphere] >> "inside",
    _ >> "outside"
  );
}

Refining the Constraint

You can combine a named predicate with a structured-binding member name for simple extra checks:

const char *classify_upper(const Vec3 &p) {
  using namespace ptn;
  constexpr auto xyz = has<&Vec3::x, &Vec3::y, &Vec3::z>;
  auto [x, y, z] = xyz;

  return match(p) | on(
    $(xyz)[z >= 0 && inside_unit_sphere] >> "inside",
    _ >> "outside"
  );
}

For multi-value guards, prefer structured-binding member names or a callable that accepts all bound values. Only z is used here; unused names such as x and y are fine.