Vision for the D language - stabilizing complexity?

[Ola Fosheim Grøstad via Digitalmars-d]

On Wednesday, 13 July 2016 at 10:25:55 UTC, sarn wrote:
> On Wednesday, 13 July 2016 at 10:02:58 UTC, Ola Fosheim Grøstad 
> wrote:
>> «Undefined» simply means that such code is not part of the 
>> specified language, as in, it is no longer the language 
>> covered. The optimizer is an implementation detail, the 
>> optimizer is not allowed to change the semantics of the 
>> language.
>>
>> If casting away immutable is claimed to be undefined behaviour 
>> it simply means that code that does this is not in the 
>> language and the compiler could refuse to compile such code if 
>> it was capable of detecting it. Or it _could_ specify it to 
>> have a specific type of semantics, but that would be a new 
>> language.
>
> You're confusing "undefined" with "implementation defined".

I am not confusing anything. A superset of a language is still 
covering the language, but it is also a new language. I think you 
are confusing "language" with "parsing".

> Implementation-defined stuff is something that's not specified, 
> but can be presumed to do *something*.  Undefined stuff is 
> something that's officially considered to not even make sense, 
> so a compiler can assume it never happens (even though a 
> programmer can make the mistake of letting it happen).  This is 
> sometimes controversial, but does let optimisers do some extra 
> tricks with sane code.

No. «Undefined» means exactly that, not defined by the language 
specification, not part of the language.  It does not say 
anything about what should or should not happen. It is simply not 
covered by the spec and a compiler could be compliant even if it 
turned out rubbish for such code if the spec does not require the 
compiler to detect all valid programs in the language. It has 
nothing to do with optimisers, that's just an implementation 
detail.

«Implementation defined» means that the implemented 
compiler/interpreter _must_ define it in a sensible manner, 
depending on the context, in order to comply with the spec.