Improving assert-printing in DMD

[Andrei Alexandrescu via Digitalmars-d]

On 10/02/2015 02:31 AM, Per Nordlöw wrote:
> On Thursday, 1 October 2015 at 19:04:51 UTC, Andrei Alexandrescu wrote:
>> * I don't think we need a new flag, just make the new behavior work.
>
> So you mean that extra diagnostics should kick in when extra overloads
> are made visible via import of, for instance, `core.assert`?
>
>> * Should the lowering happen only on the function called if the
>> assertion actually fails? Then no more need for laziness and other
>> complications.
>
> Could explain what you mean by *lowering*, please?

Rewrite proposed constructs into simpler constructs that already exist. 
That way we don't need to invent semantics, just "see lowering".

> I'm currently unsure whether
>      in L lhs
> or
>      lazy L lhs

That's why I think it's better to go with calling the function only 
after the assertion has failed. E.g.,

assert(e1 == e2)

could be lowered into:

{
   auto a = e1, b = e2;
   if (a == b) return;
   onAssertFailed!"=="(a, b, __FILE__, __LINE__, __FUNCTION__, __MODULE__);
}()

or something similar (I'm glossing over the details). Point is the 
expressions are only evaluated once and then passed into onAssertFailed.

There'd be a default definition of onAssertFailed in object.d.

> should be used and whether or not we should use
>      version(assert)

Yah, all expansions related to assert() are only in effect if assertions 
are used. Otherwise, assert disappears and that's not configurable.

> See the added example at
> http://wiki.dlang.org/DIP83
>
>> * Extend to other expressions (!=, ordering etc).
>
> How should we categorize expressions? Like this
> - Unary: assert(x UNOP y)
> - Binary: assert(x BINOP y)
> - Function Calls: assert(f(x,y))
> - Other: assert(x)
>
> Does it suffice to just mention these or should I be explicit exactly
> about which operators for each category that should be included?

I think you should use the names of the grammatical constructs, e.g. 
http://dlang.org/expression.html#EqualExpression.


Andrei