Ranges

[Tim Matthews]

dsimcha wrote:
> == Quote from Yigal Chripun (yigal100 at gmail.com)'s article
>> Steve Teale wrote:
>>> Robert Fraser Wrote:
>>>
>>>> Steve Teale wrote:
>>>>> template isInputRange(R)
>>>>> {
>>>>>     enum bool isInputRange = is(typeof(
>>>>>     {
>>>>>         R r;             // can define a range object
>>>>>         if (r.empty) {}  // can test for empty
>>>>>         r.popFront;          // can invoke next
>>>>>         auto h = r.front; // can get the front of the range
>>>>>     }()));
>>>>> }
>>>>>
>>>>> I can not possibly be the only D enthusiast who finds this completely
> incomprehensible.
>>>> Yeah, that one is a bit tricky, and what makes it worse is that it seems
>>>> officially sanctioned by Walter/Andrei as the "right way" to check if a
>>>> type supports some operations. Basically, if you have:
>>>>
>>>> is(typeof({ @@@ }()));
>>>>
>>>> this means "if I made a function containing @@@, would that function
>>>> compile?". It's a hack which stems from the way the is expression works.
>>>>
>>>>> What is a range?
>>>> As others have mentioned, it's just a struct (or other type) that
>>>> happens to support certain operations.
>>> So does this mean that interfaces are just a tragic mistake. I'd always
> thought that what you said was a pretty good description of what an interface is!
>> IMHO, duck-typing in D is a tragic mistake...  This should have been
>> implemented with compile time interfaces.
> 
> Why?  Duck typing is incredibly flexible and simple, but the downside is that, in
> its traditional implementation it's inefficient and only checkable at runtime.
> The whole beauty of D's template system is that it allows something similar to
> duck typing that is checked at compile time and has usually negligible (I won't
> say zero since object file bloat can be practically significant in a few corner
> cases) overhead.

It sometimes makes up for a lack of an actual type system but it is not 
a true duck type system built into the language anyway as you have to go 
through the manual process of asking whether it is of a certain type 
through templates.