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5.12.3 Complex Argument Syntax

Common Lisp has very powerful argument passing mechanisms. Unfortunately, two of the most powerful mechanisms, rest arguments and keyword arguments, have a significant performance penalty:

• With keyword arguments, the called function has to parse the supplied keywords by iterating over them and checking them against the desired keywords.
• With rest arguments, the function must cons a list to hold the arguments. If a function is called many times or with many arguments, large amounts of memory will be allocated.

Although rest argument consing is worse than keyword parsing, neither problem is serious unless thousands of calls are made to such a function. The use of keyword arguments is strongly encouraged in functions with many arguments or with interfaces that are likely to be extended, and rest arguments are often natural in user interface functions.

Optional arguments have some efficiency advantage over keyword arguments, but their syntactic clumsiness and lack of extensibility has caused many Common Lisp programmers to abandon use of optionals except in functions that have obviously simple and immutable interfaces (such as subseq), or in functions that are only called in a few places. When defining an interface function to be used by other programmers or users, use of only required and keyword arguments is recommended.

Parsing of defmacro keyword and rest arguments is done at compile time, so a macro can be used to provide a convenient syntax with an efficient implementation. If the macro-expanded form contains no keyword or rest arguments, then it is perfectly acceptable in inner loops.

Keyword argument parsing overhead can also be avoided by use of inline expansion (see inline-expansion) and block compilation (section block-compilation.)

Note: the compiler open-codes most heavily used system functions which have keyword or rest arguments, so that no run-time overhead is involved.