# How to define a Sampler for an object?
**URL:** https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965
**Category:** General Usage
**Tags:** question
**Created:** [April 21, 2020, 2:01pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965 "2020-04-21T14:01:46Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@logankilpatrick](https://discourse.julialang.org/u/logankilpatrick)
#### Post date: [April 21, 2020, 2:01pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/1 "2020-04-21T14:01:46Z")
Hey all,
I am getting an error when trying to use the `Rand` function for my custom type. The error is:
> ERROR: LoadError: ArgumentError: Sampler for this object is not defined
I read the docs [here](https://docs.julialang.org/en/v1/stdlib/Random/#Random.Sampler) and [here](https://docs.julialang.org/en/v1/stdlib/Random/#Custom-sampler-types-1) but it is not clear to me where I need to define the Sampler or what the syntax I should use to do so would be.
I cannot share a MWE as the code is private, unfortunately. Any suggestions on how set up the Sampler would be much appreciated!
Here is the `Rand` function:
```julia
function rand(rng::AbstractRNG, p::VDPTagPOMDP{CardinalBarriers})
# p is just some arbitrary type used to dispatch correctly.
return TagState([0.0, 0.0], 8.0*rand(rng, 2) .- 4.0)
end
struct TagState
agent::Vec2
target::Vec2
end
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [April 21, 2020, 2:16pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/2 "2020-04-21T14:16:14Z")
> [@logankilpatrick](#):
>
> I read the docs [here](https://docs.julialang.org/en/v1/stdlib/Random/#Random.Sampler) and [here](https://docs.julialang.org/en/v1/stdlib/Random/#Custom-sampler-types-1) but it is not clear to me where I need to define the Sampler or what the syntax I should use to do so would be.
Without an MWE, it would be great if you could explain _what_ is not clear in the docs. All examples there are meant to be adapted for practical use easily.
Generally, if you need precalculated data, go with a `SamplerSimple`, otherwise a `SamplerTrivial` will do.
---
### Author:  [@rfourquet](https://discourse.julialang.org/u/rfourquet)
#### Post date: [April 21, 2020, 3:00pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/3 "2020-04-21T15:00:50Z")
I second @Tamas_Papp’s answer, but in your example of `rand` method, the second argument is of type `VDPTagPOMDP`, is it a subtype of `Random.Sampler`? The API requires that.
---
### Author:  [@logankilpatrick](https://discourse.julialang.org/u/logankilpatrick)
#### Post date: [April 21, 2020, 3:21pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/4 "2020-04-21T15:21:18Z")
I previously had:
```julia
function rand(rng::AbstractRNG, p::VDPTagPOMDP{CardinalBarriers})
# return TagState([0.0, 0.0], 8.0*rand(rng, 2)-4.0)
sp = Random.Sampler(AbstractRNG, VDPTagPOMDP{CardinalBarriers}, Val(Inf))
return TagState([0.0, 0.0], 8.0*rand(rng, sp) .- 4.0) # sp used to be 2
end
```
but I ended up getting the same error as before.
---
### Author:  [@rfourquet](https://discourse.julialang.org/u/rfourquet)
#### Post date: [April 21, 2020, 4:01pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/5 "2020-04-21T16:01:45Z")
This other version doesn’t change the property of `VPDTagPOMDP` being a subtype of `Sampler`. It’s the second argument of the `rand` definition which must be a subtype of it.
I don’t know what is `VPDTagPOMDP`, but if it’s not already a `Sampler`, it looks like your signature should be `rand(rng::AbstractRNG, p::Random.SamplerTrivial{VDPTagPOMDP{CardinalBarriers}})`.
---
### Author:  [@logankilpatrick](https://discourse.julialang.org/u/logankilpatrick)
#### Post date: [April 21, 2020, 5:04pm UTC](https://discourse.julialang.org/t/how-to-define-a-sampler-for-an-object/37965/6 "2020-04-21T17:04:32Z")
Thanks @rfourquet, I was no aware I had to do `rand(rng::AbstractRNG, p::Random.SamplerTrivial{VDPTagPOMDP{CardinalBarriers}})` but this seems to have solved my issue. Thanks again!