The Method Code

Each method-clause translates to a Prolog clause with two extra arguments: Self (a variable) and Myself. The latter argument is needed to cater for passing object parameters to the method body, which is desribed further in next section.

The method body is translated to a Prolog clause body as follows. The code is traversed, and the goals are transformed according to the following transformation patterns and rules. In the transformation rules, the notation Msg(X,Y) denotes the term produced by augmenting Msg by the two arguments X and Y:

Goal

where Goal is a variable, is translated to
objects:call_from_body(Goal,Self,Myself,Src) where Src is the source module. objects:call_from_body/4 will meta-interpret Goal at runtime.

:: Msg

is translated to Myself:Msg(Myself,Myself) if Msg is a non variable. Otherwise, it is translated to objects:call_object(Myself, Msg, Myself).

<: Msg

is translated to Myself:Msg(Self,Myself) if Msg is a non variable. Otherwise, it is translated to objects:call_object(Myself, Msg, Self).

super :: Msg is translated to

objects:call_super_exp(Myself,Msg(Super,Myself),Super) if Msg is a non variable. call_super_exp/3 searches the supers of Myself. Super is bound to the super object where the method is found. If Msg is a variable, the goal is translated to objects:call_super(Myself,Msg,Super,Super), which expands Msg and performs otherwise the same actions as call_super_exp/3.

super <: Msg

is translated to objects:call_super_exp(Myself,Msg(Self,Myself),Super) if Msg is a non variable. call_super_exp/3 searches the supers of Myself. Super is bound to the super object where the method is found. If Msg is a variable, the goal is translated to objects:call_super(Myself,Msg,Self,Super), which expands Msg and performs otherwise the same actions as call_super_exp/3.

Obj :: Msg

• If Msg is non-variable, this is translated to Obj:Msg(Obj,Obj).
• Otherwise, it is translated to objects:call_object(Obj,Msg,Obj).

Obj <: Msg

• If Msg is non-variable, this is translated to Obj:Msg(Self,Obj).
• Otherwise, if Msg is a non-variable, it is translated to functor(Obj,O,_), O:Msg(Self,Obj).
• Otherwise, it is translated to objects:call_object(Obj,Msg,Self).

self <: Msg

self :: Msg

Msg

are all translated like Self :: Msg.

Module:Goal

is translated to Module:Goal.

:Goal

is translated to Src:Goal where Src is the source module.

To illustrate the expansion, consider the object history_point directives, all executed in the history_point module:

     :-objects:create_object(history_point,
       [point-[]],
       [attributes/3,display/3,move/4,new/4,print_history/3,super/4],
       [],
       [y(0),x(0),history([])],
       tree(history_point,[tree(point,[tree(object,[])])])).
     
     history_point:super(point, [], _, history_point).
     
     history_point:attributes([history([])], _, _).
     
     history_point:display(A, B, _) :-
             objects:call_super_exp(history_point, display(A,B,C), C),
             history_point:print_history(A, B, history_point).
     
     history_point:'$so_type'(history_point, static).
     
     history_point:move(A, B, C, _) :-
             objects:call_super_exp(history_point, move(A,B,C,E), E),
             prolog:'$get_module_data'(C, history, D),
             prolog:'$set_module_data'(C, history, [(A,B)|D]).
     
     history_point:print_history(A, B, _) :-
             prolog:'$get_module_data'(B, history, C),
             A:format('with location history ~w~n', [C], A, A).
     
     history_point:new(A, xy(D,E), B, _) :-
             objects:call_super_exp(history_point, new(A,xy(D,E),B,C), C),
             prolog:'$set_module_data'(A, history, [(D,E)]).
     

The directive create_object/6 creates the object, performs the inheritance by importation, and initializes attributes. The last argument is a tree representing the ancestor hierarchy during compilation. It is used to check that the load time and compile time environments are consistent.