Assert

[David B. Held]

BCS wrote:
> Reply to David,
> 
>> I think assert() will get fixed when we get AST macros.  It won't get
>> fixed by Walter, of course, but rather, by us.  Everyone will invent
>> their favorite assert(), and the builtin will only get called by
>> Phobos.
>> And then, 10 years down the road, Walter will be writing "The Design
>> and Evolution of D", where he will have to explain why assert(o)
>> doesn't
>> do the obvious thing, even though everyone else's does. ;>
> 
> what is there to fix? (that is not a rhetorical question)

Well, for one, the fact that "assert(obj)" doesn't do what 99% of noobs 
(including me!) expect it to do.  Two, you can't write a custom assert() 
that has non-standard messaging.  For instance, there is an extremely 
common idiom in all languages (that support exceptions) that basically 
looks like this:

if (!everything_ok)
{
     throw SomeException("Helpful message");
}

Andrei happens to call this micro-pattern "enforce()", which I think is 
a perfectly reasonable name for it.  What we would like to do is 
something like so:

#define enforce(cond, message) \
     if (!cond) throw SomeException(#cond + ": " + message);

Bar* ptr = foo();
enforce(ptr != null, "foo() failed");

Obviously there's a thousand ways to riff on that basic idea, but try 
writing this function in D.  As you can see, it's essentially a form of 
assert().

Dave