equivariant functions

[Denis Koroskin]

On Sun, 12 Oct 2008 23:34:05 +0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Many functions return one of their parameters regardless of the way it  
> was qualified.
>
> char[] stripl(char[] s);
> const(char)[] stripl(const(char)[] s);
> invariant(char)[] stripl(invariant(char)[] s);
>
> Stripl is not a particularly good example because it needs to work on  
> wchar and dchar too, but let's ignore that aspect for now.
>
> There's been several proposals in this group on tackling that problem.
>
> In unrelated proposals and discussions, people mentioned the need for  
> functions that return the exact type of this:
>
> class A { A clone(); }
> class B : A { B clone(); }
>
> How can we declare A.clone such that all of its derived classes have it  
> return their own type?
>
> It took me a while to realize they are really very related. This is easy  
> to figure out if you think that invariant(char)[] and char[] are  
> subtypes of const(char)[]!
>
> 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);
>
> This signature states that it returns the same type as an argument. I  
> propose that that pattern means stripl can accept _any_ subtype of  
> const(char)[] and return that exact type. Inside the function, however,  
> the type of s is the type declared, thus restricting its use.
>
> I need to convince myself that function bodies of this type can be  
> reliably typechecked, but first I wanted to run it by everyone to get a  
> feel of it.
>
> Equivariant functions are not (necessarily) templates and can be used as  
> virtual functions. Only one body is generated for one equivariant  
> function, unless other template mechanisms are in vigor.
>
> Here are some examples:
>
> a) Simple equivariance
>
> typeof(s) stripl(const(char)[] s);
>
> b) Parameterized equivariance
>
> typeof(s) stripl(S)(S s) if (isSomeString!S);
>
> c) Equivariance of field:
>
> typeof(s.ptr) getpointer(const(char)[] s);
>
> d) Equivariance inside a class/struct declaration:
>
> class S
> {
>      typeof(this) clone();
>      typeof(this.field) getfield();
>      int field;
> }
>
> What do you think? I'm almost afraid to post this.
>
>
> Andrei


Sorry to hit a dead horse but...

auto dg = &stripl; // what is the type of dg?
string z = dg("foo");
char[] x = dg("foo".dup);

It is great that you brought the issue to discussion, but I think this  
solution is a miss.