Maintaining Portable Lisp Programs

We examine the use of read-time feature conditionals, with particular emphasis on writing portable Common Lisp code which aspires to both forwardsand backwards-compatibility. We examine cases from real libraries which demonstrate the various pitfalls, and propose a scheme for robust treatment of implementation-specific functionality by performing programmatic tests at the appropriate time. We discuss the additional difficulties in dealing with differences in the operating environment, and suggest that agreements between Lisp distributors would alleviate these difficulties.