埃菲尔铁塔：类型一致性，如何提供类型而不是实例

Question

General

I don't understand why there are like 2 generating_type kinds, the one with exclamation point, and the other not.

My case

I have a function returning a DB_SERVICE[G] from a given instance from a collection of db_services so that from another service I can retrieve it and call it. When I pass an instance generating_type to that function, the conforms_to returns True, as when I give ({ENUMERATE}).generating_type it doesn't.

Why is that so?

-- Humanly unreadable
if attached {ENUMERATE_DB_SERVICE} {SIT_ENVIRONMENT}.app_instance.db_service_from_entity_type (item_prototype.charge_unit_relationship.secondary_prototype.an_index_relationship.secondary_prototype.generating_type) as l_enum_dbs then

-- Humanly readable but not working
if attached {ENUMERATE_DB_SERVICE} {SIT_ENVIRONMENT}.app_instance.db_service_from_entity_type (({ENUMERATE}).generating_type) as l_enum_dbs then

My function

db_service_from_entity_type (an_entity_type: TYPE[detachable DB_ENTITY]): detachable like db_services.item
    do
        across
            db_services as l_dbs
        until
            Result /= Void
        loop
            if l_dbs.item.item_prototype.generating_type.conforms_to (an_entity_type) then
                Result := l_dbs.item
            end
        end
    ensure
        service_found: attached Result
    end

Edit (20190405-11:26 UTC)

As the screenshot shows, giving {ENUMERATE} instead of ({ENUMERATE}).generating_type doesn't work either

Answer 1

generating_type returns a dynamic type of an object. Therefore, ({ENUMERATE}).generating_type produces TYPE [!TYPE [!ENUMERATE]] . But you need just TYPE [ENUMERATE] . This can be achieved by removing the call to generating_type and using a detachable version of the type: {detachable ENUMERATE} .

The corresponding object test would look like

if attached {ENUMERATE_DB_SERVICE} {SIT_ENVIRONMENT}.app_instance.db_service_from_entity_type
    ({detachable ENUMERATE}) as l_enum_dbs then