enforce()?

[Walter Bright]

Ary Borenszweig wrote:
> On 06/16/2010 04:15 PM, Walter Bright wrote:
>> The difference is not based on those 3 points, but on what Andrei wrote
>> here. Contracts and error checking are completely distinct activities
>> and should not be conflated.
> 
> Could you please explain them? There are many people here that don't 
> understand the difference between these two concepts (including me). So 
> maybe we are too dumb, maybe those concepts are not generally known or 
> maybe the explanation is not very well clear in the documentation.

It has nothing to do with being dumb, as it is not obvious.

Contracts are for verifying that your program is in a state that it is designed 
to be in. A contract failure is defined as a program bug.

Errors, on the other hand, are things that can go wrong at run time, like your 
disk is full when trying to write a file. These are NOT program bugs.

Another way to look at it is your program should continue to operate correctly 
if all the contracts are removed. This is not true of removing all error 
checking and handling.

Furthermore, errors are something a program can recover from and continue 
operating. Contract failures are ALWAYS fatal. A common newbie (and some expert) 
misconception is that contract failures can or even must be recovered. This 
comes from a misunderstanding of the basic principles of engineering a safe and 
reliable system.