Subject: predicates and generalized booleans From: Erik Naggum <firstname.lastname@example.org> Date: 1997/04/08 Newsgroups: comp.lang.lisp Message-ID: <email@example.com> I'm sorry to interrupt the Java discussions with a simple Lisp question. I needed a form that is t if a segment of an input string is empty, the expression read from the substring if it is a complete expression, or the bounding positions of the substring if end of file was encountered in reading it. I had this in mind (slightly simplified): (or (= start end) (read-from-string buffer nil nil :start start :end end) (cons start end)) but then I remembered something about generalized booleans, and looked up `=' in ANSI X3.226. it seems that an implementation is free to return any non-nil value as the result of comparing two equal numbers. now, this is probably an academic issue, since no Common Lisp implementation I have ever seen returns anything but t for true in such cases, so my question is not what to do in practice, but why so many predicates were specified to return a "generalized boolean" instead of just a "boolean". the relevant glossary entry for "t" reads: t ... 1. ... b. the canonical generalized boolean representing true. (Although any object other than nil is considered true as a generalized boolean, t is generally used when there is no special reason to prefer one such object over another.) ... clearly, `=' and the like do not have any such special reasons, so what motivated the use of "generalized boolean" instead of "boolean"? #\Erik -- I'm no longer young enough to know everything.