[OT] Re: DIP 1027---String Interpolation---Format Assessment

[Steven Schveighoffer]

On 2/27/20 9:32 AM, Petar Kirov [ZombineDev] wrote:
>> An example of this is the built-in associative array, which has a 
>> series of
>> fairly intractable problems as a result. Another example is the built-in
>> complex type in D, which turned out to be a bad idea - a much better 
>> one is
>> building it as a library type.
> 
> AFAIR, most of the problems with D's built-in AAs are that they have an 
> extern (C) interface that relies on typeinfo. If they are fully 
> converted to templated library types, the situation would be much 
> better. IIRC, one of the blocking issues was that D didn't have 
> autovivification [1] operators, so a library type wouldn't be a complete 
> replacement without additional help from the compiler.

We're going very off topic here, but I wanted to address this.

Large hidden invisible types are not the problem (look at normal dynamic 
arrays, the large hidden type built into the runtime is a huge success I 
think). The problem is that the compiler gives special features to such 
types.

In the case of AA, the ONLY missing piece that cannot be implemented by 
user types is this:

int[string][string] aalist;

aalist["hello"]["world"] = 5;

In essence, the compiler knows how to make this work. The operators 
available do not allow this expression to be captured properly by user 
code (i.e. we don't have opIndexIndexAssign or opIndexIndexIndexAssign, 
nor does that scale).

I believe the last person to try and implement a full template type that 
could replace AAs is H. S. Teoh. He would have a better explanation (and 
possibly contradict me, I don't know).

Other than that, we've ripped out all other "magic" into templates in 
the language. If we could get that one feature (not sure how to do this 
in a scalable way), I think we have a full library type that can be 
further factored out of the compiler. We might even be able to avoid 
using TypeInfo, and make CTFE AAs compatible with runtime ones.

-Steve