equivariant functions

[Steven Schveighoffer]

"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.

You are correct.  It means 'what you send in as input gets returned as 
output.'

In reality, it's not the best name for this type of thing, but it has the 
benefit of using a defunct keyword -- no new keywords necessary.

I can't think of a really good keyword that is short and would not require 
explanation.  Perhaps you can?  Then we have to weigh the benefits of having 
a new keyword vs. having to post an explanation as to what it means.  My 
guess is we'll have to post an explanation no matter what.

-Steve