Discussion: Forcing parallel reproducibility via the default RNG kind?

(This issue should serve as a public place for the discussion @HenrikBengtsson and I had via mail recently)

I wonder if it makes sense to support reproducible parallel streams via the default RNG kind "Mersenne-Twister" within a package to help users who are not aware that this RNG kind does not provide reproducible streams in parallel.

(I am talking about the standard parallel backends in R and not specifically about the way one can do this via the {future} package.)

# Multicore backend

```r
old.seed = .Random.seed
seed = sample(1:100000, 1)
# we need to reset the seed first in case the user supplied a seed,
# otherwise "L'Ecuyer-CMRG" won't be used
rm(.Random.seed, envir = globalenv())
set.seed(seed, "L'Ecuyer-CMRG")
```

If the user uses `set.seed()` and goes parallel via the `multicore` backend, the code above will ensure parallel RNG streams.

If you want to see in action, https://github.com/mlr-org/parallelMap/pull/80 has some tests to ensure the correct functioning.

# Socket backend

Here, one can do

```r
clusterSetRNGStream(cl, iseed = sample(1:100000, 1))
```

to support the default RNG kind in parallel scenarios.

# General

Whenever doing this, I wonder if one should at least tell the user that this was done behind the scenes to make them aware of whats happening (including eventual decreases in speed).