assume, assert, enforce, @safe

[Walter Bright via Digitalmars-d]

On 7/31/2014 12:40 AM, David Bregman wrote:
> On Wednesday, 30 July 2014 at 22:01:23 UTC, Walter Bright wrote:
>> I'd like to sum up my position and intent on all this.
>>
>> 1. I can discern no useful, practical difference between the notions of assume
>> and assert.
>
> People have explained the difference repeatedly, a ridiculous number of times
> now. Could you please take a minute to understand it this time instead of
> flippantly dismissing it again?
>
> assert does a runtime check, assume does not
> assume affects code generation/optimization, assert does not
> assert is for debugging, assume is not
> assume is for optimization, assert is not
>
> In terms of what they practically do, they have *nothing* in common, their
> functions are entirely orthogonal.

They are inextricably entangled. Consider:

    if (x == 0) abort();   // essentially what assert(x) does
    ... at this point, the optimizer knows, beyond doubt, that x!=0 ...
    if (x)  // optimizer can remove this check
       ...

which has the behavior of assume as you listed above, yet it is assert. We can 
pretend assert doesn't affect code, like we can pretend to have massless points 
in physics class, but in reality points have a mass and assert most definitely 
affects code generation, and does in every compiler I've checked, and it affects 
it in just the way the assume does.


> Still think there is no practical difference?

Yes.


>> 2. The compiler can make use of assert expressions to improve optimization,
>> even in -release mode.
>
> This will introduce a lot of undefined behavior, including making @safe code
> with asserts unsafe. I really think this needs to be acknowledged. As far as I
> can tell from the other thread, it still hasn't been.

I did acknowledge it for the array bounds case.

Note that your assume() will have the same effect, and worse, there will be no 
option to have the compiler insert a check, because then it would be an assert() 
and you might as well just use assert().

This is why I see the distinction as being pointless.