Template constraints in D

[Bill Baxter]

Walter Bright wrote:
> Bill Baxter wrote:
>> Maybe you should provide an example somewhere of how to translate this 
>> basic C++0x example into D:
>>
>> ----
>> auto concept LessThanComparable<typename T> {
>>     bool operator<(T, T);
>> };
>>
>> template<LessThanComparable T>
>> const T& min(const T& x, const T& y) {
>>     return x < y? x : y;
>> }
>> ----
> 
> 
> See the isAddable example.

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)


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.
----

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.

--bb