Metaprogramming in D : Some Real-world Examples

[grauzone]

Andrei Alexandrescu wrote:
> grauzone wrote:
>> Don wrote:
>>> Christopher Wright wrote:
>>>> grauzone wrote:
>>>>> You're not testing for types, you're testing if it compiles. Inside 
>>>>> the tested block of code, all sorts of things could go wrong. You 
>>>>> can't know if is(typeof(...)) really did what you wanted, or if 
>>>>> something broke.
>>>
>>> You're testing, "is everything inside that OK?". If you want to know 
>>> WHY it's wrong, you'd better make sure you're testing something simple.
>>
>> Andrei's range lib uses it more in a way "does this type support this 
>> and that range interface?". Example: 
>> http://dsource.org/projects/phobos/browser/trunk/phobos/std/range.d#L58
> 
> Then different isXxxRange are used by higher-order ranges in defining 
> refined interfaces depending on the interfaces offered by their inputs. 

That means if one isXxxRange fails because the user maybe made a typo in 
the needed range function, the code will silently do something else.

But my main problem is that the user just gets a "does not match 
template declaration" compiler error when he messes up his range 
interface. He's neither told that e.g. his range-related function 
returns the wrong type, nor is there any other refined error message.

Now what if we'd introduce some sort of interfaces for type checking at 
compile time?

interface InputRange(T) {
	void popFront();
	bool empty();
	T front();
}

struct MyRange : InputRange!(int) {
	void popFront() { ... }

	//compiler error goes here...
	void empty() { ... }

	int front() { ... }
}

(or something like this)

PS: there are two aspects to the problem: 1. even compile time duck 
typing shares some of the problems of runtime duck typing, and 2. 
utterly unhelpful error messages. If you wouldn't explicitly check the 
interface with is(typeof()), the compiler's error messages would be even 
worse because of 1.

> I fail to see how that's terrible. I am very happy D has that feature - 
> no other statically-typed language has it, and it can be used to great 
> effect. Look e.g. at Chain:
> 
> http://dsource.org/projects/phobos/browser/trunk/phobos/std/range.d#L799
> 
> There, the uses of static if (is(...)) allow Chain to define as capable 
> an interface as its inputs allow.
> 
> 
> Andrei