No need for version expression is a lie

[Don]

On Monday, 21 August 2023 at 19:27:57 UTC, Walter Bright wrote:
> On 8/21/2023 11:46 AM, Basile B. wrote:
>> However that approach was used. It really looks like an 
>> alternative to a version OrOr. There are plenty of other 
>> examples in drutime sources.
>
> Druntime was written by many people, and does not exhibit 
> consistent best practices.
>
>> Sure at some point there is also the problem of the unix 
>> standards...Every distribution can choose its own value for 
>> the flag of this func or that one...which creates complexity 
>> in C `define`s, but should not in D. There is a problem that 
>> is not solved.
>
> I've dealt with enough professional C .h files to conclude that 
> #ifdef algebra is the worst solution. This includes the ones I 
> have written.
>
> The objectives of minimizing keystrokes and maximizing clarity 
> are only rarely aligned.
>
> I understand that many of D's design decisions are out of step 
> with conventional wisdom. They aren't the result of reading 
> about best practices, but instead decades of living with those 
> practices.

There are multiple problems with the #ifdef algebra.
The prohibition against boolean expressions in version 
conditionals is a good one and fixes some of the problems.
But #ifdef also allows horribly nested conditionals, and D 
doesn't stop that.
Also D doesn't deal with versions defined on the command line.

The whole algebra of "defined" vs "undefined" is a sloppy 
error-prone concept that is out of place in a statically typed 
language. It's like PHP or something.
In that original example, a typical typo would be MICOBLAZE. That 
could go undetected for years. A statically typed language should 
catch that.

Years ago I proposed that versions should be considered to be 
bools in a kind of compile-time only 'version' scope and obey 
normal bool rules. You could then allow boolean expressions in 
version _declarations_ (not in conditionals).

In the original example you would define something like:

version DEFAULT_MS_ENUMS = MICROBLAZE || SH || ARM_Any || 
IBMZ_Any || IA_64 || M68K || TILE || X86_Any || RISCV_Any || 
SPARC_Any || PPC_Any;

The syntax I suggested was something like:
version <version_identifier> = <version_expression>;
version <version_identifier> = true;  // same as the existing 
version = <version_identifier>; but this would only be used in 
testing I think
version <version_identifier> = extern;  // for something defined 
on the command line
version <version_identifier> = default; // for a compiler built-in

where <version_expression> is a boolean expression of 
<version_identifier> using || and &&.

If additionally you disallowed version declarations and version 
statements inside version conditionals, you'd have a 
one-definition rule and it would be impossible to create a rat's 
nest. (You could still create a rat's nest with static if, but 
not with version).

- Don.ghostof
/disappears again into the mist