[Issue 5249] Strongly pure random generator

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http://d.puremagic.com/issues/show_bug.cgi?id=5249


Joseph Rushton Wakeling <joseph.wakeling at webdrake.net> changed:

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--- Comment #5 from Joseph Rushton Wakeling <joseph.wakeling at webdrake.net> 2013-08-30 04:33:02 PDT ---
(In reply to comment #0)
> As pure functions become more and more common in D2 programs, I'd like to
> generate some random values inside them too. So I suggest to add to the
> std.random module a strongly pure function that keeps no state and generates
> random values.

It's theoretically possible for quite a few existing RNGs to have a much better
degree of purity.

If we consider the range interface, then we'd like to have:

    enum bool empty = false;

    auto front() @property @safe const pure nothrow {}

    void popFront() @safe pure nothrow {}

Among the current challenges are that the existing design/use of RNGs means
that initialization cannot be assumed, so many (e.g. Mersenne Twister) have
conditions inside front, popFront, etc. which amount to

    if (/* is not initialized */)
    {
        seed();
    }

... which kills const (where desirable) and may have an impact on purity as
well.

Some RNGs also have internals which mitigate against @safe.

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