Eiffel: Invalid constraint for formal generic paramete - generic-programming

Question

Following with strange patterns for some, can't I do that? The compiler says Invalid constraint for formal generic parameter
class PARENT[G -> CHILD[like Current]]
feature -- Access
children: LIST[G]
end
class CHILD[H -> PARENT[like Current]]
feature -- Access
father: H
end
to be able to do something like
class WIDOW_PARENT
inherit
PARENT[BLACK_CHILD]
end
class BLACK_CHILD
inherit
CHILD[WIDOW_PARENT]
end
If I don't do it with genericity, I'd have to redefine the children collection from
children: LIST[CHILD] to children: LIST[BLACK_CHILD] into the WIDOW_PARENT class
father: PARENT to father: WIDOW_PARENT into the BLACK_CHILD class
instead of only specify it in the inherit clause... Hope it makes sense
Update
As I solved it with Alexanders answer, I'm stuck further doing a conformity check. I'm trying to set an HTTP router depending on entities and if its a child entity it should be able to do a http://host:port/entity/child_entity/id to get all child entities from entity. For that I'd like to add to the generic router a check. On something like ANY_PARENT_DB_ENTITY such as
if ({G}).conforms_to ({CHILD_DB_ENTITY[ANY_PARENT_DB_ENTITY]}) then
friend.act_like_a_father
else
friend.act_like_a_mate
end

Answer 1

In contemporary Eiffel, anchored types cannot be used in formal generic constraints, thus the error. It's still possible to have mutual constraints by repeating class types explicitly:
class PARENT [G -> CHILD [PARENT [G]]]
class CHILD [H -> PARENT [CHILD [H]]]
With this change, the example compiles.

Answer 2

Being still the best solution for me, the only solution to my whole pattern is to redefine in my router class the set_handler method with
CHILD_DB_ENTITY
deferred class
CHILD_DB_ENTITY[H -> PARENT_DB_ENTITY[CHILD_DB_ENTITY[H]]]
inherit
DB_ENTITY
feature
parent: H
ENTITY_HANDLER
ENTITY_HANDLER[G -> DB_ENTITY, H -> DB_SERVICE[G] create make end]
feature
item_prototype: detachable G
set_handler
do
setting_url_("http://host:port/" + {like item_prototype}.out)
...
end
end -- Class
CHILD_ENTITY_HANDLER
CHILD_ENTITY_HANDLER[G -> CHILD_DB_ENTITY[PARENT_DB_ENTITY[G]], H -> DB_SERVICE[G]]
inherit
ENTITY_HANDLDER
redefine
set_handler
end
feature
set_handler
do
Precursor
setting_url_("http://host:port/" + ({like item_prototype}).out + "/" + ({like item_prototype.parent}).out + "/{id}")
end
end -- Class
PARENT_ENTITY_HANDLER
PARENT_ENTITY_HANDLER[G -> PARENT_DB_ENTITY[CHILD_DB_ENTITY[G]], H -> DB_SERVICE[G]]
inherit
ENTITY_HANDLDER
redefine
set_handler
end
feature
set_handler
do
Precursor
setting_url_("http://host:port/" + ({like item_prototype}).out + "/" + ({like item_prototype.children.item}).out + "/{id}")
-- children being a LINKED_LIST[PARENT_DB_ENTITY]
end
end -- Class
I hoped there was a way to get it with polymorphism in same class, but it also makes sense to have to redefine it that way...