On 16/01/15 07:38, H. S. Teoh via Digitalmars-d wrote:
I've been wondering about that. Must ranges have an "underlying" container? Or are they allowed to be more abstract entities that basically function as generators, producing data on demand? (This distinction is also somewhat related to transient ranges, in that the difference between providing a "view" of some static underlying data vs. computing something on-the-fly in a buffer that gets reused, could serve as a deciding factor on how to deal with transient ranges.)
This is almost a surprising question for me, because I've taken it for granted for so long that one of the most powerful tools they offer is lazily-generated data that is not based on underlying storage. The examples you cite (iota, recurrence) are good examples, RNGs are another.
In this sense, an RNG could be thought of as a complex variant of recurrence(), except with a far more complicated generating expression than what one would normally use with recurrence(). If recurrence() qualifies as a range (and a forward range, no less), then an RNG ought to qualify too.
Yes, theoretically you can envision a pseudo-RNG as being defined by a state variable, and a pure function that transforms this into a new state variable. And then those individual states can be mapped to the individual variates in different ways, depending on the type of variate you want to generate.
The practical range implementation obfuscates this a bit (helpfully) by encapsulating the state (which in practice we update via mutation), and implementing a particular state-to-random-variate method in .front, resulting in the desired effect of a range whose elements are the individual random variates you care about.
