Good Contract programming idiom?

[Lutger]

Theoretically exceptions are situations you expect can happen, even though 
the code is bug free, while assertions always indicate a bug. 

I think the main implication of this is that assertions should behave 
differently. They should halt the program at once so the developer's 
attention is forced upon the bug, even though the bug may be recoverable.  
Furthermore, your development version of your software can run very slow, 
it's usually no problem. With easy way to compile out assertions, you are 
more inclined to use them liberally. Or to put it the other way: you are 
likely to implement less checking if it comes at a cost for your release 
build. 

Then there is the case where you want to leave some 'assertion checks' in 
release mode too, like you mention. Ideally you may want something that 
throws AssertError when compiled in debug mode and InvalidArgumentException 
in release mode. I think that whenever code is important enough to have such 
extensive checking, it is a good idea to do something like this.