ANSI Common Lisp   7 Objects   7.6 Generic Functions and Methods

7.6.4 Congruent Lambda-lists for all Methods of a Generic Function

• 1. Each lambda list must have the same number of required parameters.

• 2. Each lambda list must have the same number of optional parameters. Each method can supply its own default for an optional parameter.

• 3. If any lambda list mentions &rest or &key, each lambda list must mention one or both of them.

• 4. If the generic function lambda list mentions &key, each method must accept all of the keyword names mentioned after &key, either by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own. The checking of the validity of keyword names is done in the generic function, not in each method. A method is invoked as if the keyword argument pair whose name is :allow-other-keys and whose value is true were supplied, though no such argument pair will be passed.

• 5. The use of &allow-other-keys need not be consistent across lambda lists. If &allow-other-keys is mentioned in the lambda list of any applicable method or of the generic function, any keyword arguments may be mentioned in the call to the generic function.

• 6. The use of &aux need not be consistent across methods.

  If a method-defining operator that cannot specify generic function options creates a generic function, and if the lambda list for the method mentions keyword arguments, the lambda list of the generic function will mention &key (but no keyword arguments).