# Best way to alternate RNG in inner functions?

**URL:** <https://discourse.julialang.org/t/best-way-to-alternate-rng-in-inner-functions/97487>\
**Category:** General Usage\
**Created:** [April 14, 2023, 6:33pm UTC](https://discourse.julialang.org/t/best-way-to-alternate-rng-in-inner-functions/97487 "2023-04-14T18:33:29Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 14, 2023, 6:33pm UTC](https://discourse.julialang.org/t/best-way-to-alternate-rng-in-inner-functions/97487/1 "2023-04-14T18:33:29Z")

</div>

Given a code in which `rand` is called in a inner function, as in this simplified example:

```julia
module Test

import Random
export outer

RNG = Random.Xoshiro()

function inner(x)
     x += rand(RNG)
end

function outer(x)
    x += inner(x)
end

end

```

Which do you think is the most idiomatic way to allow `RNG` choice without causing type instabilities?

One possibility is to pass `RNG` as a parameter on every interface, of course. Do you see a better alternative? (without the `const RNG` the call to `outer` is obviously type-unstable).

The purpose of this is be able to use `StableRNGs` to test some functions with reproducible random states.

---

<div class="post-metadata">

**Author:** ![goerz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerz/32/3269_2.png) [@goerz](https://discourse.julialang.org/u/goerz)\
**Post date:** [April 14, 2023, 6:56pm UTC](https://discourse.julialang.org/t/best-way-to-alternate-rng-in-inner-functions/97487/2 "2023-04-14T18:56:05Z")

</div>

[I have used an `rng` keyword argument](https://github.com/JuliaQuantumControl/QuantumControlTestUtils.jl/blob/master/src/random.jl) for exactly this purpose,

```julia
module Test

import Random
export outer

RNG = Random.Xoshiro()

function inner(x; rng=RNG)
     x = rand(rng)
end

function outer(x; rng=RNG)
    inner(x; rng)
end

end

```

which I guess is what you mean by “pass `RNG` as a parameter on every interface”?

I did this more as a matter of avoiding globaI state than a worry about type stability. I’m surprised though that having a non-const `RNG` would cause type instability. Doesn’t `rand` have a known output type? What if you wrote your own `rand` wrapper; something like

```julia
RNG = Random.Xoshiro()

function set_rng(rng)
    global RNG
    RNG = rng
end

function myrand()::Float64
    global RNG
    return rand(RNG)
end

```

---

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 14, 2023, 7:14pm UTC](https://discourse.julialang.org/t/best-way-to-alternate-rng-in-inner-functions/97487/3 "2023-04-14T19:14:51Z")

</div>

> [@goerz](#):
>
> which I guess is what you mean by “pass `RNG` as a parameter on every interface”?

indeed

> [@goerz](#):
>
> Doesn’t `rand` have a known output type?

It does, and you can even specify it:

```julia
julia> rand(Random.Xoshiro(), Int)
5009175639301388561

```

but using that or annotating the output is not solving the problem (the annotation prevents the instability from propagating only):

```julia
julia> RNG = Random.Xoshiro()
       function test(x::T) where {T}
           x + rand(RNG, T)::T
       end
test (generic function with 1 method)

julia> @btime test(0.0)
  116.872 ns (1 allocation: 16 bytes)
0.2989532433595429

julia> function test2(x::T, RNG) where {T}
           x = rand(RNG, T)
       end
test2 (generic function with 1 method)

julia> @btime test2(0.0, $RNG)
  3.010 ns (0 allocations: 0 bytes)
0.6171378061283105

```

I can live with passing `RNG` on every interface, but this is a case where I would be fine with a global state.