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includelispcommon-lispclisp

#ifndef in Common Lisp

In C, to make sure we don't re-include headers that are included we use the following structure:

#ifndef UTILS
#define UTILS

#include "my_utils.h"

#endif

I've broken my Lisp program into separate files; multiple files sometimes use the same file (e.g., my_utilities is used by multiple files). When I run the program, I get warnings that I am redefining things (calling load of the same file multiple times).

This would be fixed by doing something similar to #ifndef in C. What is the Common Lisp equivalent or standard method of doing this?

I am fairly new to Lisp. Let me know if there are best practices (perhaps, a different method of structuring my programs?) that I am missing.

Solution

  • The question you asked

    The direct analogue of preprocessor conditions like #if in C is the #+ read-time conditionalization facility.

    The question you wanted to ask

    To avoid multiple loading of a file, you can either use the standard (but deprecated) provide/require facility, or an add-on system like ASDF.