Multiple delegates, contextual delegates

[XavierAP]

On Wednesday, 26 June 2019 at 12:16:19 UTC, Aphex wrote:
> On Wednesday, 26 June 2019 at 09:48:35 UTC, angel wrote:
>>
>> Indeed it looks like a library solution.
>> Its alleged usefulness has nothing to do with being a library 
>> as opposed to being a part of the core language. Normally, the 
>> core language implements stuff that is very hard or very ugly 
>> to implement in a library, and your proposal will look perfect 
>> in a library implementation.
>
> 1. The problem with a library solution is that it requires it 
> part of the library. This is not alway effective. It would need 
> to be part of phobos so at least it is used every. Not 
> everything is a candidate for libraries even if they work in 
> libraries. Virtually everything can, in theory, be a library 
> solution. One could have make delegates a library solution, it 
> is possible and functional... but it is too common and it loses 
> functionality.
>
>
> 2. It is a direct extension of something that is already a core 
> semantic in the compiler and since it can be directly extend 
> the keyword `delegate` with being 100% backwards compatible, 
> splitting it in to a library prevents such a useful extension.

The problem is that "delegates" are not so core to D (unlike C# 
for example). D has delegates proper, function pointers, anything 
that implements opCall, etc. Under the hood they are completely 
different. The most common solution in current D is duck-typed 
templates and alias parameters; I think  very few people use 
delegates proper in D in practice. And the direction is not clear 
even in the spec:

https://dlang.org/spec/function.html#closures
«Future directions: Function pointers and delegates may merge 
into a common syntax and be interchangeable with each other.»

Another secondary point is why do you need this into an 
overloaded operator. Giving new meanings to old operators makes 
the code a little bit shorter but possibly less clear. I believe 
Walter picked ~ instead of + for string concatenation for this 
reason, so even in the std library the operator overloading is 
going to be a tough sell.