Policy Constraint Matching

Policy logic is usually a sequence of overlapping conditions. Patternia turns those rules into an ordered case list.

enum class Role {
  Guest,
  User,
  Admin
};

struct Request {
  Role role;
  int  resource_id;
  bool read_only;
};

Naming Domain Predicates

auto is_public_resource = [](int id) {
  return id >= 0 && id < 1000;
};

Declaring the Rules

bool allowed(const Request &req) {
  using namespace ptn;

  return match(req) | on(
    $(has<&Request::role>)[arg<0> == Role::Admin] >> true,
    $(has<&Request::role, &Request::resource_id, &Request::read_only>)
        [arg<0> == Role::User && arg<1> >= 0 && arg<2> == true] >> true,
    $(has<&Request::role, &Request::resource_id>)
        [arg<0> == Role::Guest && is_public_resource] >> true,
    _ >> false
  );
}

Why This Helps

Each rule is self-contained:

• the binding states which fields matter
• the guard states why the rule applies
• the handler states the outcome

That keeps policy code readable even when ordering matters.