[Ur] Exceptions handling

[Adam Chlipala]

Vladimir Shabanov wrote:
> Do you mean that things like "set my_source new_value" will also roll
> back on transaction error?
>    

In server-side code, yes.  Client-side code isn't really transactional, 
despite the misleading use of the [transaction] monad.  On the server 
side, examples of FFI state to roll back include e-mails queued to be 
sent and manual filesystem changes.