protocol for using InputRanges

[Joseph Rushton Wakeling]

On 23/03/14 08:53, Jonathan M Davis wrote:
> But again, front and empty should normally function as if they were variables.
> They should be property functions and should be pure (or at least act like
> they're pure). I'm sure that a _lot_ of code will break if that isn't
> followed.

There are some not-very-nice exceptions to that in std.random, where in many of 
the range types you have stuff along the following lines:

     auto front() @property
     {
         if (notInitialized)
         {
             init();
         }
         return _value;
     }

see e.g. MersenneTwister, RandomSample, and others.  It's all a nasty 
consequence of that RNGs-as-value-types problem we've discussed previously.

However, the class-based std.random2 that is being discussed on the announce 
list fixes that:
http://forum.dlang.org/thread/cyytvhixkqlbwkmiugex@forum.dlang.org

> In general though, I think that most of us would agree that front and empty
> should be treated as properties - i.e. as if they were variables - and that
> they should have try to stick to those semantics as closely as possible.
> Ranges that stray from that seriously risk not working with a lot of range-
> based code.

Yes, absolutely.