Named constructors

[Atila Neves]

On Thursday, 10 January 2019 at 14:35:33 UTC, JN wrote:
> On Thursday, 10 January 2019 at 13:53:40 UTC, Atila Neves wrote:
>>
>> What's the _real_ difference between 
>> `someFunc(firstName="Alice")` and 
>> `someFunc(FirstName("Alice"))`? The former saves one 
>> character? Is that really worth the cost of language support? 
>> I have a feeling a lot of feature requests neglect that each 
>> and every one of them has a cost, and there's only enough 
>> budget for so many.
>>
> Isn't there a performance cost added when using structs like 
> that?

No.

> And the real cost is the fact that you have to create all those 
> structs meticulously

Meticulously? It's a name and the type you were going to write 
anyway. You can always call the field `_` to avoid even more 
typing.

> and you can only use that with functions that expect such usage.

Yes. But I'm writing the API.

> With language support, you could have named arguments for every 
> code there exists.

I'd love to see a DIP that did that and dealt with all the corner 
cases. I suspect that it's not as easy as implied here.

> Anyway, the fact that arguments are named are only a bonus, the 
> real strength of named arguments comes with optional arguments.

Again, I want a library for this. I know how to write it, I just 
haven't yet.

> Look at the Builder pattern, it's pretty much a workaround for 
> lack of named/keyword arguments.

Java doesn't have variadic templates, D does. No builder pattern 
needed.

> Also, named/keyword arguments shouldn't be an issue in regards 
> to overloading and argument order. Every language I know that 
> has named/kw arguments allows either only keyword arguments or 
> keywords arguments at the end of the argument list. You can do:
>
> foo(x=10)
> foo(10, 20, z=30)
>
> but you can't do:
>
> foo(x=10, 20)
>
> because it'd be ambiguous.

I don't know of any language that does that and has overloading. 
Walter knows way more about this than I do.