defmacro features

[Robert Virding]

Macros are different from functions in that a macro can be called with any number of arguments. Defmacro can handle this and can be used in the same way as defun:

- If there is an explicit list of variables after the macro name then it expects exactly that number of arguments and otherwise generates an error.

(defmacro do3 (a b c) `(progn ,a ,b ,c))

- Defmacro can also take clauses like defun where each clause has one argument which is a list of the arguments to the macro call. This is one way the macro handles any number of arguments.

(defmacro list*
  ((list e) e)                            ;A list of one element (cons e ())
  ((cons e es) `(cons ,e (list* . ,es)))
  (() ()))

- But defmacro can also take just a variable, not a list of variables, in which case the variable will be the of arguments to the macro call. This is another way the macro handles any number of arguments

(defmacro list* args
  (case args
    ((list e) e)                            ;A list of one element (cons e ())
    ((cons e es) `(cons ,e (list* . ,es)))
    (() ())))

The last two are equivalent, you can use the one that feels the best. Normally replacing matching using clauses with a case becomes messier if there are many arguments but here there is always only one.

Robert