Marker interface extending MessageInterface. Application commands (write operations) implement this.

NamedCommandInterface adds getName(): string to a command. NamedCommandTrait provides a default implementation backed by a protected $name property, falling back to static::class when $name isn't set.

CommandResultInterface extends both CommandInterface and ResultInterface, so a command result can be treated as a message and re-enter the pipeline. CommandResult is the concrete, final readonly implementation, wrapping the original CommandInterface, a MessageStatus, and the raw result value.

Marker interface extending MessageInterface. Application queries (read operations) implement this.

Mirrors the Command result types: QueryResultInterface extends QueryInterface and ResultInterface, and QueryResult is the final readonly implementation wrapping the original QueryInterface, a MessageStatus, and the raw result value.

CommandHandlerInterface/QueryHandlerInterface::handle() is typed to the broad MessageInterface. Parameter types are contravariant, so an implementation can widen but never narrow that type, however declaring it as a union such as MessageInterface|CommandInterface (or MessageInterface|QueryInterface) is still a legal override, since the union is at least as wide as MessageInterface alone. This costs nothing at runtime but gives static analysis (and readers) CommandInterface/QueryInterface typing without an assertion.

To narrow further, down to the concrete command/query class, this library's own tests use Psl\Type::instance_of(), see Usage Examples for a complete example.