equivariant functions

[Steven Schveighoffer]

"Andrei Alexandrescu" wrote
> KennyTM~ wrote:
>> Andrei Alexandrescu wrote:
>>> Steven Schveighoffer wrote:
>>>> "Andrei Alexandrescu" wrote
>>>>> I discussed with Walter a variant that implements equivariant 
>>>>> functions without actually adding an explicit feature to the language. 
>>>>> Consider:
>>>>>
>>>>> typeof(s) stripl(const(char)[] s);
>>>>
>>>> As another point on this, I think someone else mentioned it, but I 
>>>> can't find the post.
>>>>
>>>> I don't like the way this looks.  The way it reads is 'stripl returns 
>>>> the same type as s', but really, the typeof(s) is actually modifying 
>>>> the type of the argument also.  This seems very unintuitive.
>>>
>>> I agree. We need to look for a better notation.
>>>
>>>> I understand the need to not change the language, but I think most 
>>>> would prefer a syntax where the type modifier is specified on at least 
>>>> the argument.  People are going to be extremely confused when they 
>>>> can't treat 's' like a normal const(char)[].
>>>>
>>>> If the ultimate result is that no intuitive syntax can be made without 
>>>> changing the language, then I think it is more important to have this 
>>>> feature than to not change the language.
>>>>
>>>> One other syntax that Janice proposed (and I later put into a 
>>>> bugzilla), is to use the dead keyword inout.  Meaning, what you send in 
>>>> is what you get out.  ref already completely replaces inout, so there 
>>>> is no need to keep it under its current meaning:
>>>>
>>>> inout(char)[] stripl(inout(char)[] s);
>>>>
>>>> I'm not in love with this completely, but it has the benefit of not 
>>>> requiring a new keyword.
>>>
>>> Also I guess:
>>>
>>> class C
>>> {
>>>     Range!(inout(C)) foo() inout;
>>> }
>>>
>>> And also:
>>>
>>> class Base {}
>>> class Derived : Base {}
>>> inout foo(inout Base b);
>>>
>>> I think this could work and doesn't look half bad. Of course, you'll be 
>>> tasked with addressing protests about yet another D1/D2 incompatibility. 
>>> :o)
>>>
>>>
>>> Andrei
>>
>> I'm afraid the meaning of inout here is very unclear without explanation.
>
> I think more explanation is needed for the other case:
>
> inout(char)[] stripl(inout(char)[] s);
>
> What if I pass a const char[]? Is that going to work? In contrast, this is 
> simple:
>
> inout stripl(inout const(char)[] s);
>
> Accept and return any subtype of const(char)[].

I like this a lot better.

What about returning a member?  i.e.:

inout(typeof(s.ptr)) getptr(inout const(char)[] s) { return s.ptr;}

is that what you had in mind?

this syntax is going to be used often, since it's what you would use for an 
accessor.  So it should be simple to understand if possible.

-Steve