[Ur] Exceptions handling

[Adam Chlipala]

Vladimir Shabanov wrote:
> 2010/9/5 Adam Chlipala<adamc at impredicative.com>:
>    
>> I've started implementing this.  Everything works, save that the semantics
>> differs between Postgres and MySQL/SQLite.  Postgres won't let you continue
>> using a transaction after one command has failed, whereas MySQL and SQLite
>> allow many errors within one transaction.  (Remember that every Ur/Web page
>> execution is implicitly inside a single transaction.)  What does everyone
>> think about whether that semantics is acceptable for Ur/Web?  The more
>> liberal semantics can be implemented for Postgres using savepoints, but it
>> would decrease performance.  If no one would want to keep running SQL after
>> an error, then there doesn't seem to be a reason to do more work.
>>
>> Personally, I'm in favor of never using this new feature, anyway, but I'm
>> interested in what y'all who argued for it think. :)
>>      
> I think it would be nice to have possibility to manually restart
> transaction so DB still can be accessed after error.
>    

After I sent the above message, I realized that some kind of 
compensating action was necessary to make this feature at all useful for 
Postgres.  Otherwise, a transaction wouldn't be able to commit after an 
error, so any actions an application had taken in response to the error 
would be rolled back.  Because of this, I'm planning to add a savepoint 
before every piece of DML executed by the new function.