Extensions via Meta-Interpreters

Meta-interpreters of ABCL/R3 are meta-level objects that executes body part of base-level methods and functions. Although these are defined as objects, the definition looks like the traditional meta-circular interpreters of Lisp, and those of sequential reflective languages (e.g., 3-Lisp[15] and Brown[17]). The definition is divided into smaller methods, each of which corresponds to a specific type of an expression. Followings are part of the method definitions in the meta-interpreter:

(eval-entry exp env)⁶:

This method is called at the beginning of each base-level method/function invocation. The argument exp is an unevaluated expression of the method/function, and the argument env is an environment, which binds instance variables of an object , and formal parameters of a method/function. It simply calls the method eval by default.

(eval exp env):

This method serves as a dispatcher--it calls an appropriate sub-method (e.g., eval-var and eval-method-call) according to the expression type.

(eval-var var env):

This method handles variable references--it returns value of the variable in the environment.

(eval-method-call exp env):

This method handles method-invocation forms in base-level programs. By default, (1) it first determines the invocation type (e.g., future type or present type); and (2) it also determines the target object (i.e., the receiver of the message), and the arguments by using the method eval. (3) The ``meta-arguments'' of the invocation, which is explained later, are determined by calling the method meta-args. (4) Finally, it sends a invocation request to the target object using the method do-method-call.

(do-method-call type target selector args meta-args env):

This method actually sends a message to the target object. Firstly, a data structure which has all the information for the invocation (e.g., method name and arguments) is composed by the method make-message; and then the created data is passed by calling the method message of the meta-object of the target.

The extensions are written using delegation, so that meta-level description are re-usable.⁷