Interface Limitations in D

[Elmar]

On Monday, 20 September 2021 at 18:08:03 UTC, Alexandru Ermicioi 
wrote:
> On Monday, 20 September 2021 at 13:24:50 UTC, Elmar wrote:
>> Providing default implementation support seems not more 
>> difficult:
>>
>> ```D
>> mixin Interface!("MyInterface",
>> q{
>>     static immutable PI = 3.141592f;
>>     default static immutable CONSTANT = 0.0f;
>>
>>     float toImplement(string s);
>>
>>     default float isDefaulted()
>>     {
>>         return (CONSTANT ^^ PI) % toImplement(r"tau"));
>>     }
>> });
>> ```
>>
>> Providing default method implementations without code 
>> duplication and too much verbosity could work by defining the 
>> default method implementations as **compile-time token 
>> strings**. The above would generate
>
> You can actually try use mixin templates, not just mixin 
> strings. See https://dlang.org/spec/template-mixin.html .
>
> It has nicer syntax although not as powerful as mixin strings. 
> You can also mix in the template into a dedicated scope inside 
> implementor of interface:
>
> ```
> mixin MyDefaultImpl!() myScope;
> ```
>

Yeah, using `mixin templates` is exactly the point of the code 
snippet you have quoted, but together with string `mixin`s it 
allows one to mimic syntax features. I do collect some language 
features I'd like to see in D and if I ever should have enough 
time, I'd prototype them with string mixins for others to try 
them.

> I didn't try it, but you may be able to mimic default methods 
> using this feature, in conjunction with alias this expression, 
> although I'm pretty skeptic about this due to mangled name 
> generation (i.e haven't tried it).
> 
> So in your class you may try and explore whether this works:
>
> ```
> mixin MyDefaultImpl!() def;
>
> alias this def;
> ```

I've been using `alias this` in `mixin templates` recently. 
According to Online D Editor, it works.