Processing method bodies

Before a method can be created, the list of forms comprising the method body must be converted to a method function. This conversion is a two step process.


The body of methods can also appear in the :initial-methods option of defgeneric forms. Initial methods are not considered by any of the protocols specified in this document.

The first step occurs during macro-expansion of the macro form. In this step, the method lambda list, declarations and body are converted to a lambda expression called a method lambda. This conversion is based on information associated with the generic function definition in effect at the time the macro form is expanded.

The generic function definition is obtained by calling ensure-generic-function with a first argument of the generic function name specified in the macro form. The :lambda-list keyword argument is not passed in this call.

Given the generic function, production of the method lambda proceeds by calling make-method-lambda. The first argument in this call is the generic function obtained as described above. The second argument is the result of calling class-prototype on the result of calling generic-function-method-class on the generic function. The third argument is a lambda expression formed from the method lambda list, declarations and body. The fourth argument is the macro-expansion environment of the macro form; this is the value of the &environment argument to the defmethod macro.

The generic function make-method-lambda returns two values. The first is the method lambda itself. The second is a list of initialization arguments and values. These are included in the initialization arguments when the method is created.

In the second step, the method lambda is converted to a function which properly captures the lexical scope of the macro form. This is done by having the method lambda appear in the macro-expansion as the argument of the function special form. During the subsequent evaluation of the macro-expansion, the result of the function special form is the methodfunction.