Metaprogramming in D : Some Real-world Examples

[Jeremie Pelletier]

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. 
> 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

I really like Andrei's range library, I use it all the time when I need 
to pass slices of generic types around, it really is more convenient 
than a pair of iterators.

The way I see ranges is as a form of interface without being bound to 
classes; its the only way to make structs and D arrays pass the 
isXxxRange traits.

The prototypes for using ranges then just go from:

void dosomething(T)(IForwardRange!T range) {}

to

void dosomething(Range)(Range range) if(isForwardRange!Range) {}

and it means the same thing, but you can't send a string to IForwardRange.