Assert and the optional Message

[Jonathan M Davis]

On Friday, March 09, 2012 11:29:43 H. S. Teoh wrote:
> Hmph. Well, then that defeats the purpose of checking contracts, because
> checking contracts is only justifiable if it's complex enough, which
> means that it's liable to involve things like dtors and scope
> statements. It's silly to want to check a trivial contract like
> assert(x>0);, because if something *that* simple can go wrong, then so
> can the unittest, so you're not proving anything at all.
> 
> But this isn't that big a deal. One could argue that if a contract is
> convoluted enough to warrant a unit test, then perhaps most (or all) of
> its complexity should be factored out into a separate, unit tested
> function, which is then just invoked from the contract.
> 
> (I find this a bit ironic, since TDPL states that the reason contracts
> allow statements is so that complicated conditions can be tested for,
> rather than being limited to just a single expression, as is the case in
> most other languages that support DbC. Now it seems that simple
> contracts are the way to go.)

It's only an issue if you're trying to test your contracts, and I don't think 
that that's a very typical thing to do. That's kind of like writing tests to 
test your unit tests.

And the ability to have complicated conditions in contracts is still better 
than having a single expression even if you do want to test your more 
complicated contracts, because it allows you to have multiple assertions 
rather than just one expression, and then you know _which_ condition was false 
and can provide a decent error message (e.g. one which included the values of 
some of the arguments) instead of having one big condition that either 
succeeds or fails and really doesn't give you any useful information beyond 
that (like you get with template constraints).

- Jonathan M Davis