[Issue 7067] std.random.RandomSample and RandomCover are poorly designed

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=7067

--- Comment #20 from Joseph Rushton Wakeling <joseph.wakeling at webdrake.net> ---
(In reply to Martin Nowak from comment #19)
> We had a talk about this during the 2nd D meetup in Berlin.
> One remaining question was how to deal with memory management when using a
> reference type RNG.
> 
> I came up with a different solution and less invasive solution.
> 
> We simply disable implicit copying of RNGs (@disable this(this)) and ask
> people to use explicitly use .save when copying is indeed intended.
> 
> We can even deprecate postblit like so.
> 
>     deprecated("Please use .save to explicitly copy RNGs.")
>     this(this) {}
> 
> Internally we'd need to change all copying to use std.algorithm.move.

Interesting thought.  We'd have to take care that phobos-side things don't get
immediately "fixed" by people just using .save inside RandomCover and
RandomSample constructors.


> If reference semantic is wanted people can use either
>     new Random(unpredictableSeed)
> or
>     refCounted(rnd)
>     refCounted(Random(unpredictableSeed))
> or (unsafe)
>     refRange(&rnd)
> .
> 
> This should also work for combined random generators like.
> 
> new Exponential(Random(unpredictableSeed))
> 
> refCounted(Exponential(Random(unpredictableSeed)))
> 
> auto rng = Exponential(Random(unpredictableSeed));
> refRange(&rng);

I guess what I don't like about this solution is that it requires the user to
take responsibility for generating the RNG (or wrapper-of-RNG) as a reference
type, when that ought to be the default behaviour.  But OTOH we could take
advantage of the fact that RNG structs are templated, and rework their
convenience aliases, e.g.:

    alias Mt19937 = RefCounted!(MersenneTwisterEngine!(uint, 32LU, 624LU,
397LU, 31LU, 2567483615u, 11LU, 7LU, 2636928640u, 15LU, 4022730752u, 18LU))

... which would still leave the underlying struct available for use by someone
who really wants to use it directly in that form.

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