Parent Delegation and Inheritance

One of the most important facilities in an object-oriented programming language is the ability for a child object to make use of a parent's implementation of some operation, even when the child provides its own definition for that operation. The pass() function provides this facility in MOO.

Function: pass

value pass(arg, ...)

calls the verb with the same name as the current verb but as defined on the parent of the object that defines the current verb.

Often, it is useful for a child object to define a verb that augments the behavior of a verb on its parent object. For example, in the LambdaCore database, the root object (which is an ancestor of every other object) defines a verb called description that simply returns the value of this.description; this verb is used by the implementation of the look command. In many cases, a programmer would like the description of some object to include some non-constant part; for example, a sentence about whether or not the object was 'awake' or 'sleeping'. This sentence should be added onto the end of the normal description. The programmer would like to have a means of calling the normal description verb and then appending the sentence onto the end of that description. The function pass() is for exactly such situations.

pass calls the verb with the same name as the current verb but as defined on the parent of the object that defines the current verb. The arguments given to pass are the ones given to the called verb and the returned value of the called verb is returned from the call to pass. The initial value of this in the called verb is the same as in the calling verb.

Thus, in the example above, the child-object's description verb might have the following implementation:

return pass() + "  It is " + (this.awake ? "awake." | "sleeping.");

That is, it calls its parent's description verb and then appends to the result a sentence whose content is computed based on the value of a property on the object.

In almost all cases, you will want to call pass() with the same arguments as were given to the current verb. This is easy to write in MOO; just call pass(@args).