Why are there Properties in D?

[Marc Schütz" <schuetzm at gmx.net>]

On Friday, 14 February 2014 at 16:16:28 UTC, Robin wrote:
> On Friday, 14 February 2014 at 15:21:04 UTC, Jesse Phillips 
> wrote:
>> FYI an infinite range is defined to have
>>
>>    struct Infinite {
>>        enum empty = false;
>>    }
>>
>> You know it will never be empty at compile time.
>
> Hiho,
>
> thank you for this interesting input. =)
>
> Couldn't this be equally possible (also at runtime) with the 
> following:
>
> struct Infinite {
>     enum empty_ = false;
>     bool empty() pure { return empty_; }
> }

Yes, but only at runtime. For some things to work, you need to be 
able to know it at compile time. `std.range` defines a template 
`isInfinite` that checks for this.

This information can be used for some optimizations: 
`std.algorithm.count` and `std.range.walkLength` are only defined 
for non-infinite ranges (to avoid creating an infinite loop), 
`std.algorithm.cartesianProduct` can work with two infinite 
ranges using a special enumeration strategy if it can detect 
them, `std.range.chain` can optimize index access by stopping at 
the first infinite range, `std.range.take` can always define a 
length for infinite ranges (even if they don't have a length 
property themselves), ...

Another example: The `length` property of ranges. It is possible 
to turn builtin slices (dynamic arrays) into ranges by importing 
`std.range` or `std.array`. Slices already have a member field 
`length` by default. Here you have an example where it's 
impossible to define a method `length()`. With properties, 
nothing special needs to be done: You can always use `length` 
without parens, even if it happens to be a method.