[Ur] Catching errors

[Adam Chlipala]

Edward Z. Yang wrote:
> Is there a way to catch an 'error<xml>asdf</xml>'?  For example
> the json module uses error to indicate parse failure, and instead
> I'd like to catch it and try something else.

There's currently no way to catch errors with nestable constructs (e.g., 
exception handling as in ML), but there is [onError], as you mentioned.  
The nice approach to your concrete problem would be to build alternate 
parsing machinery that returns a type indicating errors. ;)

> The onError function seems like a partial mechanism for this, but
> I don't understand how scoping on onError works; if I register
> something as onError, is it permanently called on error? Is it
> displaced by the next onError I call?

Within a single client-side page execution, registered event handlers 
stay registered forever.  When multiple handlers are registered, they 
will all run in reverse registration order.  (I've added a bit about 
this to the manual.)