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