On 08/05/2015 18:45, Mittelbach, Frank wrote: > On 08.05.2015 18:48, Ulrike Fischer wrote: >> Is this a bug? Or simply something one must remember when using >> c-type arguments? > > > interesting question. On the whole we have not burdened expl3 with a lot > of checking to avoid it getting too slow, i.e., in case with very well > defined rules it is up to the programmer not to violate them > > finite recursion (especially those that end up in quarks executing are > something we made usually an exception as they are pretty nasty. > > as for the rule: "c" should not be misused to do an implicit "new" even > though that obviously works for some data types (if you know the > underlying coding). For the "prop" type that doesn't work as > \c_empty_prop" is quite different from \relax > > so perhaps it is worth thinking of adding to functions for types like > that always a quick initial check to see that the variable is a prop or > rather at least not simply generated as a name from "c" > > of course it would be a test happening each and every time > > thoughts? > > frank Probably we should add something: as Ulrike says, at the very least \prop_show should complain. Joseph