Operator overloading through UFCS doesn't work

[Maxim Fomin]

On Saturday, 13 October 2012 at 15:06:14 UTC, Tommi wrote:
> On Saturday, 13 October 2012 at 11:50:40 UTC, Maxim Fomin wrote:
>> I think implementing UFCS operator overloading is problematic. 
>> Firstly, you want to put this language addition too far.
>
> I don't see this as taking UFCS functionality "further". 
> Rather, I think it's simply more logical that with UFCS you 
> could provide extra operator methods just like you can provide 
> extra "regular" methods. I assumed that UFCS operator 
> overloading would work for sure, and it seems arbitrary to me 
> that it doesn't.

I don't consider request to put the language in consistency with 
superficial logic (UFCS allows to use free functions as methods 
=> allow operator overloading hijacking too) as a rational one.

>> Secondly, compiler needs to know whether operator was 
>> overloaded or not. If it knows, it generates code to call 
>> "opSomething", if nor - it just doesn't generate anything. 
>> Now, imagine what would happen if you write in some module 
>> "free" function, supposed to hijack operator overloading 
>> method of class or struct in another module. If you compile 
>> them together, operator would be overloaded, if separately - 
>> nothing would happen. This means that operator overloading 
>> would depend on with what you compile your module - sometimes 
>> nothing would be overloaded, sometimes it would be with one 
>> function, sometimes with another.
>
> You use the word "hijack", but free functions can't hijack 
> anything.

Their inability to hijack actual methods is not an issue.

> They can only provide new functionality. The situation you 
> describe is exactly parallel to using UFCS (with regular 
> functions) like this:
>
> //File: mystruct.d
> module mystruct;
>
> struct MyStruct
> {
>     int _value;
> }
>
> //File: incr1.d
> module incr1;
>
> import mystruct;
>
> void incr(ref MyStruct ms)
> {
>     ms._value += 1;
> }
>
> //File: incr2.d
> module incr2;
>
> import mystruct;
>
> void incr(ref MyStruct ms)
> {
>     ms._value += 2;
> }
>
> //File: main.d
> module main;
>
> import std.stdio;
> import mystruct;
>
> static if (true) // change to false to print "2"
>     import incr1;
> else
>     import incr2;
>
> void main()
> {
>     MyStruct ms;
>     ms.incr();
>     writeln(ms._value); // prints "1"
> }
>
>

Yes. Do you want to have this with operator overloading too? UFCS 
is useful if you import some library and want to add extra 
functionality to particular needs of a module. So, if you do this 
multiple times and merge modules you may not get into trouble 
with high probability of conflicting names. Even if they 
conflict, you can slightly rename them. Can you do this if you 
define in several modules different operator overloading methods? 
Guess not.

>> Thirdly, I see no reason in allowing it - for what purpose 
>> does you proposal service for?
>
> The main reason to me is that it would make more sense. It'd 
> seem more logical that way. I can't think of any use cases.

Different groups of people have different mind and same things 
produce different sense on them. From my point of view operator 
overloading methods are special functions and not treating them 
as candidates for UFCS does make more sense. Even if you convince 
in your opinion, language addition without applied purposes makes 
no benefit.