equivariant functions

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
>>  const?(X) f(const?(Y) y, const?(Z) z);
> 
> All these sound like valid alternatives, I'd be fine with any of them.

The const? has been on the table. It is a fave of mine because it offers 
a consistent solution for a related issue - detecting lvalueness:

void foo(ref? Widget w);

This means foo should bind to a ref Widget if you pass it an lvalue, and 
to a Widget if offered an rvalue. There is precedent on using "x?" to 
mean an optional x in regexes, so I wouldn't be surprised if many 
figured what const? does when first seeing it.

One issue  have with const? is that it binds the behavior to const, 
thereby eliminating the chance of making things more general. Also the 
const? will have to do something NOT suggested by the notation, namely 
pass the invariant, if present, along.

Anyhow, I wanted to share one more thought. On the face of it, as 
someone already said, we DO have a solution to avoid code duplication: 
templates. Maybe a fertile direction to take would be to use regular 
template syntax and just let the compiler figure out that it can 
actually define only one function instead of three.

What I'm saying is that we're really trying to beat the compiler in the 
head until it understands that:

S stripl(S s) if (is(S : const(char)[]) { stmts }

really means to us:

char[] stripl(char[] s) { stmts }
const(char)[] stripl(const(char)[] s) { stmts }
invariant(char)[] stripl(invariant(char) s) if (is(S : const(char)[]) { 
stmts }

What if, instead of finding yet another notation for that simple fact, 
we actually used the "right" notation (it is right because it's already 
there!) and figure out whether the compiler can understand it?

I was about to post when I realized that that idea won't work prettily 
at all with member functions... sigh :o/


Andrei