[phobos] Pureness of enforce()

[Jonathan M Davis]

On Friday, November 12, 2010 09:17:15 Andrei Alexandrescu wrote:
> 3. I'm not sure there will more pure functions than impure out there. If
> there are more impure functions, using enforce() becomes a net
> pessimization

Well, since few of them are likely to call globals or C functions, most of them 
should theoretically be pure. The main problem we're likely to run into (I would 
think) would be cases where functions are called which could be pure depending 
on the implementation. For instance, front could be pure in many (probably most) 
cases, but in others it can't be. It would be _huge_ to be able to reliably make 
functions that use such functions pure in cases where they can be and non-pure 
where they can't. The same would go for nothrow. Unfortunately, I think that 
that that would currently mean having multiple versions of functions where they 
use template constraints to determine whether the pure, nothrow, pure nothrow, 
or non-pure non-nothrow version should be used (bleh). I wonder if we need some 
kind of inout for pure and nothrow.

Ideally, we would make absolutely everything that can be pure (and/or nothrow) 
pure (and/or nothrow). The lack of it makes using pure and nothrow in user code 
a pain. At least with nothrow, you can catch all exceptions and then assert 0 
that there weren't any (in cases where you're sure that there won't be any) - 
though having to do so _is_ annoying - but you can't do that with pure.

- Jonathan M Davis