On Tue, May 24, 2011 at 16:44, John Cowan <[email protected]> wrote: > leppie scripsit: > >> Take for example: >> >> (define (foo bar) >> (unless (list? bar) (error 'foo "not a list" foo)) >> (map values bar)) >> >> Now applying the above with say (foo 1) will raise an error. If I set >> my debugger to break inside ERROR, I get a nice complete stacktrace, >> including the caller, IOW FOO. >> >> Now take what is suggested: >> >> (define (foo bar) >> (if (not (list? bar)) (error 'foo "not a list" foo) >> (map values bar))) >> >> Now inside ERROR, you do not have 'lost' the caller (FOO) as the >> application of ERROR was a tail call. Given the context of FOO's >> application that may also be a tail call, and so on. > > I would certainly want the last form in WHEN/UNTIL to be a tail position. > The obvious implementation ((when x . y) (if x (begin . y))) would certainly > make it so.
The point leppie made is subtler. In the UNLESS case the whole UNLESS form is not in tail position, while in the IF case it is. P. _______________________________________________ Scheme-reports mailing list [email protected] http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports
