On 2014-11-20, Emmanuel Charpentier <emanuel.charpent...@gmail.com> wrote:
> BTW, Maxima has this kind of logic (for example, "is" can return "true", > "false" or "unknown") and uses it, so it's at least conceptually doable. > And useful ! But with very deep consequences. For the record, Maxima has a global flag prederror which governs the evaluation of predicates. When prederror=false (default), is(p) => unknown when p is not known (to Maxima) to be true or false, and if p then ... evaluates to a partially-evaluated conditional expression. When prederror=true, is(p) and if p then ... trigger an error. In practice, this has worked well enough, i.e. without causing too much confusion -- I don't remember any complaints about programs assuming true/false not working as expected. A more theoretical problem is that Maxima's partial evaluation policy isn't entirely consistent -- when p isn't decidable to Maxima, you can get a partially-evaluated conditional, but not a partially-evaluated loop (triggers an error), and various programming functions (e.g. length, first, integerp) might act in an unexpected way. This, too, hasn't caused trouble, from what I remember. FWIW Robert Dodier -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To post to this group, send email to sage-support@googlegroups.com. Visit this group at http://groups.google.com/group/sage-support. For more options, visit https://groups.google.com/d/optout.
