equivariant functions

[Sergey Gromov]

Sun, 12 Oct 2008 14:34:05 -0500,
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);
> 
> 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.

I'd use it.