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Common Lisp Extensions

A.2 Error Checking

Common Lisp compliance has in general not been sacrificed for the sake of efficiency. A few exceptions have been made for cases where substantial gains were possible at the expense of marginal incompatibility.

The Common Lisp standard (as embodied in Steele's book) uses the phrase "it is an error if" to indicate a situation which is not supposed to arise in complying programs; implementations are strongly encouraged but not required to signal an error in these situations. This package sometimes omits such error checking in the interest of compactness and efficiency. For example, do variable specifiers are supposed to be lists of one, two, or three forms; extra forms are ignored by this package rather than signaling a syntax error. The endp function is simply a synonym for null in this package. Functions taking keyword arguments will accept an odd number of arguments, treating the trailing keyword as if it were followed by the value nil.

Argument lists (as processed by defun* and friends) are checked rigorously except for the minor point just mentioned; in particular, keyword arguments are checked for validity, and &allow-other-keys and :allow-other-keys are fully implemented. Keyword validity checking is slightly time consuming (though not too bad in byte-compiled code); you can use &allow-other-keys to omit this check. Functions defined in this package such as find and member* do check their keyword arguments for validity.