Truly lazy ranges, transient .front, and std.range.Generator

[HaraldZealot via Digitalmars-d]

On Saturday, 15 August 2015 at 10:06:13 UTC, Joseph Rushton 
Wakeling wrote:
> ...
>
> In some cases we're going to want true laziness of evaluation, 
> but _not_ transience of .front.  In these cases, the _first_ 
> time .front is called, its value should be freshly evaluated; 
> but thereafter, the value is cached, until .popFront() is 
> called, at which point .front will be re-evaluated lazily the 
> next time it's called.  Something like std.algorithm.cache is 
> inappropriate here, precisely because it's eager in its 
> calculation of the cached values.
>
> ...
>
>     -- Joe

Do you mean something like that:

```d
struct Range
{
public:

     enum empty = false;

     auto ref front() @property
     {
         if(mustBeEvaluated)
         {
             cache = fun();
             mustBeEvaluated = false;
         }

         return cache;
     }

     void popFront()
     {
         mustBeEvaluated = true;
     }

private:
     ReturnType!fun cache;
     bool mustBeEvaluated = true;
}
```
?