Predefining functions that are automatically generated with Common Lisp macros

Question

Macros may create functions in the global scope. For example:

(defmacro test-macro (&body functions)
 `(progn ,@(loop for function in functions
               collect `(defun ,function ()
                           *some-interesting-thing*))))

Another example (albeit with methods) would be the automatically generated accessors for a CLOS class.

The functions are not defined at macro expansion time, but rather when the generated code is compiled/interpreted. This can cause some difficulty. If these functions are being expected, a warning will be thrown. What is the idiomatic way to define these functions before they are correctly defined? One potential solution might be the following:

(defmacro test-macro (&body functions)
  (macrolet ((empty-function (name)
                `(defun ,name ())))
    (dolist (function functions)
       (empty-function function)))
  `(progn ,@(loop for function in functions
                collect `(defun ,function ()
                            *some-interesting-thing*))))

Note that the intermediate function is defined during macro expansion time.

The problem you are describing (as I understand it) should not exist (as per the spec) and does not exist (in any implementation I know of). Could you please provide explicit code samples? — sds
– sds, Commented Mar 17, 2015 at 15:57

Rainer Joswig · Accepted Answer · 2015-03-17 16:33:49Z

4

If you need to define a function at compile time on the toplevel, then use:

(eval-when (:compile-toplevel)
  (defun foo ...))

Note that you can define a macro which generates such code. Such a form can be part of a PROGN.

answered Mar 17, 2015 at 16:33

Rainer Joswig

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2 Comments

Svante Over a year ago

In the majority of cases, if you use eval-when at the toplevel, you want all of (:compile-toplevel :load-toplevel :execute).

Rainer Joswig Over a year ago

@Svante: I wouldn't say that. Common Lisp implementations use :compile-toplevel often for compile-time-only side effects. For example DEFUN notes the function at compile-time, but does not define it. Often that's a useful pattern to follow. Also eval-when makes only sense at the so-called toplevel. Other than that it has no real use. Note that 'toplevel' has a special meaning in the context of compilation and that forms inside a PROGN are still at 'toplevel'.

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