A problem with D contracts

[Jonathan M Davis]

On Saturday 31 July 2010 19:47:10 bearophile wrote:
> Automatic verification of contracts is an important feature, that can be
> made much harder (or impossible) to implement if contract contents aren't
> clean and simple. Putting print statements inside contracts is just wrong.
> As in future probably a second documentation system will be added/used
> to/in D, a second unit-testing system too, probably eventually a second
> contract system too will be added/used, because the built-in ones are not
> designed in a serious way, all three of them are just toys :-) "keeping
> their usage simple" doesn't hold when they lack essential features (I have
> discussed about missing parts in unittest system recently).


Actually, truth be told,  I wouldn't consider automatic verification to be all 
that important at all - particularly because it would be so hard to do ands so 
few people would likely use it. I totally agree that it would be a good feature, 
but I doubt that it's one that will ever materialize - or if it does it won't be 
any time soon. Besides, if you assume that contracts are pure or have something 
that verifies that they are, then you could still do those checks. Enforcing 
purity would be a serious impairment. Putting print statements in contracts for 
debugging purposes is a major and positive feature. Now, I agree that outside of 
debugging a problem print statements have no business in contracts - they 
certainly shouldn't stay there longterm - but it would impair debugging to 
disallow them.

I really don't understand what your problem with contracts or unit testing is. 
What we have works quite well. I'm sure that it could be improved, but it works 
well. And honestly, I don't see automatic verification as being particularly 
practical or useful any time soon - not to mention that it wouldn't be all that 
hard for you to have assertions in there that couldn't be verified statically 
anyway. What we currently have is extremely practical and does its job.

What we have in D is a major step forward, and with D2 stabilizing, you're not 
going to get it changed in any big way. And in this particular case, I really 
don't think that you're going to win any arguments - certainly not with folks 
like Walter or Andrei. Purity in contracts is definitely not one of the goals, 
and IIRC it has been explicitly stated by Walter and/or Andrei that enforcing 
purity in contracts would be bad for D.

- Jonathan M Davis