Re: On exceptions, errors, and contract violations

[bearophile via Digitalmars-d]

Sean Kelly:

Another issue is what the error tells us about the locality of 
the failure.  A precondition indicates that the failure simply 
occurred sometime before the precondition was called, while a 
postcondition indicates that the failure occurred within the 
processing of the function.

In presence of high-order functions the assignment of contract 
failure blame is less simple.


Invariant failures might indicate either, which leads me to 
think that they are too coarse-grained to be of much use.  In 
general, I would rather know whether the data integrity problem 
was preexisting or whether it occurred as the result of my 
function.  Simply knowing that it exists at all is better than 
nothing, but since we already have the facility for a more 
detailed diagnosis, why use invariants?

Invariants are more DRY. There are plenty of state parts that can 
be changed by a function, if you want to test the invariant each 
time you risk duplicating that testing code in every 
post-condition. The invariant helps you avoid that. It's a safety 
net that helps against problems that you can forget.

Bye,
bearophile