equivariant functions

[Denis Koroskin]

On Wed, 15 Oct 2008 05:30:52 +0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Denis Koroskin wrote:
>> On Tue, 14 Oct 2008 23:45:32 +0400, Andrei Alexandrescu  
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> Simen Kjaeraas wrote:
>>>> I don't really see this as a problem. Returning mutable or invariant  
>>>> would be worse. Anyways, if you need the two arguments to be of the  
>>>> same type, I'd prefer this syntax:
>>>>    inout min(inout(A) a1, typeof(a1) a2){}
>>>
>>> IMHO we could simplify by discounting min. The major need is to pass  
>>> the type of one argument only. Min and max are templates anyway, and  
>>> for templates we have other ways to make things work.
>>>
>>> Andrei
>>>
>>  IMHO, the solution should be consistent and general enough to cover  
>> templates, too, as well as zero, single and multiple input arguments.  
>> Templates would benefit from it, too, reducing the generated file size  
>> (there are many complains about this issue).
>
> If you can find a solution that is simple and general enough, my hat is  
> off to you.
>
> Andrei

We are here to discuss it. I made many suggestions, but I don't know  
whether they fine, too complex or just lame untill someone comments it  
(interesting enough, some reply without reading). What's wrong with  
typeof(this) (I just generalized your clone() idea combined with implicit  
upcasting suggested by Steven)? The "I don't like it" comment would be  
useful, too.

What's wrong with inout/whatever? So far you brought just one  
'problematic' example:

inout(C) foo(inout(B) function(inout(A)) fn);

I believe this is as meaningless as
inout(A) foo(); // what does it return? A, const(A) or invariant(A)?

and thus should be statically disallowed.

I mean, inout(return) doesn't have any sense unless a function accepts  
some inout(parameter):

inout(B) foo(inout(A) a); // ok

// your example with an added inout(in) parameter. Now it is fine
inout(C) foo(inout(C) c, inout(B) function(inout(A) a) fn);

In the last example, constancy of return value matches constancy of input  
parameter.