Another day in the ordeal of cartesianProduct

[xenon325]

On Saturday, 27 October 2012 at 16:19:43 UTC, H. S. Teoh wrote:
> [snip]
> I think there is some merit to being able to declare concepts; 
> for
> example:
>
> 	// This concept matches any type that has fields that satisfy
> 	// what's specified inside the body.
> 	concept InputRange(T) {
> 		bool empty;	// this matches @property bool empty() too
> 		T front;
> 		void popFront();
> 	}
>
> 	auto myRangeBasedFunc(InputRange r) {
> 		r.popBack();	// compile-time error: InputRange does
> 				// not define .popBack
> 	}
>
> This way, *both* the user of the template and the template 
> writer are
> kept "honest" (i.e., they both have to conform to the 
> requirements of an
> input range). The compiler can thus statically check the 
> template for
> correctness *before* it ever gets instantiated.
>
> Not only so, the compiler will be able to generate a meaningful 
> error
> message when the templates fail to match -- it can tell the 
> user "the
> template didn't match 'cos struct X that you tried to pass to 
> it isn't
> an InputRange, nor a ForwardRange, ... etc.".

We've had a short discussion with Jacob Carlborg about almost 
exactly this syntax 
(http://forum.dlang.org/post/jukabm$1btd$1@digitalmars.com) in 
the context of better compiler messages.

I will quote my concerns:

=====

> void foo (InputRange range);

1. How would you signify `foo` is generic ?
    For complier it's probably possible - by type of `InputRange`,
         but that will be one more special case
    What about user ?

2. How would you put 2 constraints on `range` ?

3. Also you forgot to address:

>> template isInputRange(R)
>> {
>>      enum bool isInputRange = is(typeof(
>>      (inout int _dummy=0)
>>      {
>>          R r = void;  /// how would you express this
>>                       /// b.t.w. ?

    range.d comment on this line is "can define a range object".

===== end quote

However I do agree it would be nice if compiler verifies template 
body against constraints. IMNA compiler-writer, but I wonder if 
it's possible with current syntax.