# Can rand generate random vectors with custom type entries?
**URL:** https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028
**Category:** General Usage
**Tags:** random
**Created:** [December 2, 2023, 1:18am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028 "2023-12-02T01:18:02Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [December 2, 2023, 1:18am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/1 "2023-12-02T01:18:02Z")
I’m trying to understand how to implement `rand` for custom types. After looking at the documentation, I’ve come up with the following:
```julia
using Random
struct A{T}
x::T
end
Random.rand(rng::AbstractRNG, ::Type{A{T}}) where T = A(rand(rng, T))
```
This generates single elements as expected:
```julia
julia> rand(A{Bool})
A{Bool}(false)
```
I thought that generating random vectors would work automatically, but it doesn’t:
```julia
julia> rand(A{Bool}, 3)
ERROR: MethodError: no method matching Random.Sampler(::Type{TaskLocalRNG}, ::Random.SamplerType{A{Bool}}, ::Val{1})
Closest candidates are:
Random.Sampler(::Type{<:AbstractRNG}, ::Random.Sampler, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:147
Random.Sampler(::Type{<:AbstractRNG}, ::Any, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:183
Random.Sampler(::Type{<:AbstractRNG}, ::BitSet, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/generation.jl:450
...
Stacktrace:
[1] Random.Sampler(T::Type{TaskLocalRNG}, sp::Random.SamplerType{A{Bool}}, r::Val{1})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:147
[2] Random.Sampler(rng::TaskLocalRNG, x::Random.SamplerType{A{Bool}}, r::Val{1})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:139
[3] rand(rng::TaskLocalRNG, X::Random.SamplerType{A{Bool}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:255
[4] rand!
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:273 [inlined]
[5] rand!
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:269 [inlined]
[6] rand
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:290 [inlined]
[7] rand
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:291 [inlined]
[8] rand(::Type{A{Bool}}, ::Int64)
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:294
[9] top-level scope
@ REPL[5]:1
```
I find this particularly surprising because the first closest candidate displayed above should match in my opinion because
```julia
julia> TaskLocalRNG <: AbstractRNG, Random.SamplerType{A{Bool}} <: Random.Sampler
(true, true)
```
(The second should also match, but it’s less specific.) Am I missing anything?
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [December 2, 2023, 1:24am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/2 "2023-12-02T01:24:29Z")
(Note that you can always just do `[rand(T) for _=1:n]`.)
---
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [December 2, 2023, 1:26am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/3 "2023-12-02T01:26:27Z")
Sure. But should `rand(A{Bool}, 3)` work? (So my question is rather about the API of `Random`. I’ve clarified the topic title.)
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [December 2, 2023, 2:18am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/4 "2023-12-02T02:18:17Z")
[https://docs.julialang.org/en/v1/stdlib/Random/#Hooking-into-the-Random-API](https://docs.julialang.org/en/v1/stdlib/Random/#Hooking-into-the-Random-API)
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [December 2, 2023, 3:47am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/5 "2023-12-02T03:47:03Z")
> [@matthias314](#):
>
> `ERROR: MethodError: no method matching Random.Sampler(::Type{TaskLocalRNG}, ::Random.SamplerType{A{Bool}}, ::Val{1})`
It does seem like there should be a default `Sampler` for `SamplerType{T}` that simply calls `rand(rng, T)` …
cc @rfourquet
---
### Author:  [@danielwe](https://discourse.julialang.org/u/danielwe)
#### Post date: [December 2, 2023, 4:41am UTC](https://discourse.julialang.org/t/can-rand-generate-random-vectors-with-custom-type-entries/107028/6 "2023-12-02T04:41:09Z")
Just to make sure the solution is spelled out here: the one change you need is to replace `Type` with `Random.SamplerType` in your method definition.
```julia
using Random
struct A{T}
x::T
end
Random.rand(rng::AbstractRNG, ::Random.SamplerType{A{T}}) where {T} = A(rand(rng, T))
```
```julia-repl
julia> rand(A{Bool})
A{Bool}(false)
julia> rand(A{Bool}, 3)
3-element Vector{A{Bool}}:
A{Bool}(true)
A{Bool}(false)
A{Bool}(false)
```