Template constraints in D

[Walter Bright]

Bill Baxter wrote:
> A more natural way to declare required functions would be nice.
> 
> First of all, about the isAddable example, can't you just use T.init 
> instead of writing an anonymous delegate?  Like
>    __traits(compiles, T.init + T.init)

I think you're right.

> Anyway, for a less trivial example I think that becomes pretty 
> cumbersome.  E.g.:
> 
> concept Stack<typename X> {
>   typename value_type;
>   void push(X&, const value_type&);
>   void pop(X&);
>   value_type top(const X&);
>   bool empty(const X&);
> };
> 
> is going to translate to something like this?:
> 
> template Stack(T)
> {
>     const Stack =
>         is(T.value_type) &&
>         __traits(compiles, (T t, T.value_type v) { push(t, v); }) &&
>         __traits(compiles, (T t) { pop(t); }) &&
>         __traits(compiles, (T t) { top(t); }) &&
>         is(typeof(top(T.init))==T.value_type) &&
>         __traits(compiles, (T t) { empty(t) }) &&
>         is(typeof(empty(T.init))==bool);
> }
> 
> I hope you can work out a way to give this a more natural syntax, like 
> Concepts will have in C++0x.

You're the first I've heard say that Concepts have a natural syntax! But 
let me try:

template Stack(T)
{
     const Stack =
         __traits(compiles,
	    (T t)
	    {	T.value_type v = top(t);
		push(t, v);
		pop(t);
		if (empty(t)){}
	    });
}

The idea is that the desired operations are expressed as operations, 
rather than as function signatures.

> An unrelated comment is that I don't think it is relevant or respectful 
> to compare the lengths of the specs.  The spec for Concepts that you 
> link to includes many many more in depth examples than the D spec does. 
>  It's an apples vs oranges comparison.  So 5 pages vs 49 pages really 
> doesn't have much meaning at all.

I wished to make the point that D constraints build on what one already 
knows - how to write a D expression. Concepts, on the other hand, have a 
major new syntax and semantic thing that must be learned. Perhaps I 
expressed the point badly, do you have a suggestion?