EDM Trigger Schema

SCHEMA MyExpressSchema;
    ENTITY MyEntity;
        attr1 : STRING;
    END_ENTITY;
END_SCHEMA;
 
TRIGGER_SCHEMA MyTriggerSchema FOR MyExpressSchema;
    DECLARE target_model INSTANCE OF MyExpressSchema;
 
---------------------------------------------------------------------------
-- the system recognized events is proposed to define as INTEGER XPX pseudo constants
-- then we can expand range of trigger events without enhancing grammar
-- declaration can be assumed as something like:
---------------------------------------------------------------------------
trigger_source = ENUMERATION OF(
        XPX_Create_Entity,      -- raised after entity is created
        XPX_Delete_Entity,      -- raised before an entity is deleted
        XPX_Read_Entity,        -- any attribute is accessed
        XPX_Write_Entity,       -- any attribute is modified
 
        XPX_Pre_Set_Attribute,      -- raised before specified attribute is assigned (do we need PRE/POST?)
        XPX_Post_Set_Attribute,     -- raised after specified attribute is assigned (do we need PRE/POST?)
        XPX_Pre_Unset_Attribute,    -- raised before an attribute is set to UNSET
        XPX_Post_Unset_Attribute,   -- raised after an attribute is set to UNSET
        XPX_Read_Attribute,         -- any attribute is accessed
 
        XPX_Create_Model,       -- raised after model is created
        XPX_Delete_Model,       -- raised before model is deleted
        XPX_Read_Model,         -- raised if any data in model accessed (pre)
        XPX_Write_Model,        -- raised if any data in model written (pre)
 
        XPX_Commit_Write_Transaction,       -- raised just before write transaction committed (what about tx levels?)
        XPX_Abort_Write_Transaction,        -- raised just before write transaction aborted(what about tx levels?)
 
    );
 
---------------------------------------------------------------------------
-- An event is handeled as a system entity ala XPXEXCEPTION
---------------------------------------------------------------------------

	ENTITY XPXEVENT;
		xpxEventType 	: INTEGER;
		xpxEventTarget 	: GENERIC_ENTITY;
		xpxEventTime	: _DATETIME;
		xpxEventSource	: GENERIC_ENTITY;	-- the schema/function active when event was raised
		xpxEventUser	: GENERIC_ENTITY;	-- user logged in when event was raised
		<... more ...>
	END_ENTITY;
 
---------------------------------------------------------------------------
-- we use the DECLARE keyword to connect events to handlers
-- Pattern: DECLARE trigger_function TRIGGER_FUNCTION(trigger_source) FOR (target);
---------------------------------------------------------------------------
    DECLARE EntityTrigger       TRIGGER_FUNCTION(XPX_Create_Entity)     FOR MyEntity;
    DECLARE AttributeTrigger    TRIGGER_FUNCTION(XPX_Pre_SetAttribute)  FOR MyEntity.MyAttribute;
    DECLARE ModelTrigger        TRIGGER_FUNCTION(XPX_Create_Model)      FOR target_model;
 
     
    TRIGGER_FUNCTION EntityTrigger(theEntity: GENERIC_ENTITY) : GENERIC;
        RETURN(?);
    END_TRIGGER_FUNCTION;
 
    TRIGGER_FUNCTION AttributeTrigger(theEntity: GENERIC_ENTITY) : GENERIC; -- look in DECLARE TRIGGER declarartion to identify attribute
        RETURN(?);
    END_TRIGGER_FUNCTION;
 
    TRIGGER_FUNCTION ModelTrigger(theModel: GENERIC_ENTITY) : GENERIC; -- theModel will refer to SDAI_MODEL instance
        RETURN(?);
    END_TRIGGER_FUNCTION;
     
     
---------------------------------------------------------------------------
-- so for possibility to user defined events
-- firstly the events need to be enumerated somehow. Not trivial as they are cross-schema and cross-model
-- could be kept in a trigger schema, meaning both raise and catch must be done within that schema.
-- anyway, if identified as integer constants we may declare them as such:
---------------------------------------------------------------------------
    CONSTANT
        somethingCreatedEvent : INTEGER := 1001;
        somethingDeletedEvent : INTEGER := 1002;
    END_CONSTANT;
     
-- to raise events - easy way out is to use same technique as for XPX exception:       
    PROCEDURE ThrowAnEvent(arg : GENERIC);
        xpxRaiseEvent(eventType,eventTarget,.....);
    EBD_PROCEDURE;
     
 
-- to catch events is like catching trigger events, so we use same declaration
-- Pattern: DECLARE eventHandler TRIGGER(eventType) FOR target; -- target is a model I think...
     
    DECLARE EventHandler TRIGGER_FUNCTION(somethingCreatedEvent) FOR target_model;
    DECLARE EventHandler TRIGGER_FUNCTION(somethingDeletedEvent) FOR target_model;
     
    TRIGGER_FUNCTION EventHandler(event : GENERIC_ENTITY) : GENERIC;
    LOCAL
        myEventType : INTEGER;
    END_LOCAL;
        myEventType := event.XPXEventType;
        RETURN(?); 
    END_TRIGGER_FUNCTION;
     
END_TRIGGER_SCHEMA;

Proposed:
    DECLARE EventHandler TRIGGER_FUNCTION(somethingCreatedEvent) FOR target_model;
 
Alt:
    DECLARE EventHandler(somethingDeletedEvent) TRIGGER_FUNCTION FOR target_model;
    DECLARE TRIGGER_FUNCTION EventHandler(somethingCreatedEvent) FOR target_model;