# How does instance of a type works/interact with type parameters?

**URL:** <https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475>\
**Category:** New to Julia\
**Tags:** question, parametric-types, parametric-methods\
**Created:** [October 1, 2023, 7:14pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475 "2023-10-01T19:14:42Z")\
**Posts on this page:** 12\
**Page:** 1

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**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 1, 2023, 7:14pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/1 "2023-10-01T19:14:42Z")

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Hi everyone,

I am studying the code from repo [ReinforcementLearningAnIntroduction.jl](https://github.com/JuliaReinforcementLearning/ReinforcementLearningAnIntroduction.jl).  
In notebook [Chapter01\_Tic\_Tac\_Toe.jl](https://github.com/JuliaReinforcementLearning/ReinforcementLearningAnIntroduction.jl/blob/master/notebooks/Chapter01_Tic_Tac_Toe.jl), there is this line that confuses me

```julia
E = DefaultStateStyleEnv{Observation{Int}()}(env)

```

Why can we pass `Observation{Int}()` in to the brace of `DefaultStateStyleEnv{}`?

From my understanding, the stuff inside the brace `{}` should be some kind of type (e.g. ` DataType`) or literal values, but `Observation{Int}()` is an instance of `Observation{Int64}`

```julia
typeof(Observation{Int}()) # Observation{Int64}
typeof(typeof(Observation{Int}())) # DataType

```

For example, these will not work

```julia
# they will produce
# ERROR: TypeError: in Array, in element type, expected Type, got a value of type Float64
Array{1.0, 1}();
x=()->1.0; Array{x(), 1}()

```

So what is happening there?  
I am guessing it is something to do with “UnionAll Types” and “Value types” but I couldn’t figure it out.

Thank you in advance.

(There is a 2 links max limit for new users, so I will paste the link to def of DefaultStateStyleEnv as raw text here `https://github.com/JuliaReinforcementLearning/ReinforcementLearning.jl/blob/2e1de3e5b6b8224f50b3d11bba7e1d2d72c6ef7c/src/ReinforcementLearningEnvironments/src/environments/wrappers/DefaultStateStyle.jl#L7-L11`)

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<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [October 2, 2023, 12:56am UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/2 "2023-10-02T00:56:10Z")

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> [@EricH](#):
>
> the stuff inside the brace `{}` should be `DataType` or literal values, but `Observation{Int}()` is an instance of `Observation{Int64}`

`Int` aliases `Int64` on 64-bit systems and aliases `Int32` on 32-bit systems.

```julia
julia> Int
Int64
julia> typeof(Int64)
DataType
julia> typeof(Int)
DataType

```

Any type can be a parameter in the brace, it does not have to be an instance of `DataType`, e.g. `Vector{Union{Int, Nothing}}`.  
Or are you referring to something else?

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<div class="post-metadata">

**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 2, 2023, 11:37am UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/3 "2023-10-02T11:37:40Z")

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Hi, yes I was referring to something else, specifically:  
Why can we pass `Observation{Int}()` in to the brace of `DefaultStateStyleEnv{}`

I have edited my question to make it clearer.  
And thanks for the help!

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**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [October 2, 2023, 11:47am UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/4 "2023-10-02T11:47:53Z")

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From here: [Types · The Julia Language](https://docs.julialang.org/en/v1/manual/types/#man-types)

> Both abstract and concrete types can be parameterized by other types. They can also be parameterized by symbols, by values of any type for which isbits returns true (essentially, things like numbers and bools that are stored like C types or structs with no pointers to other objects), and also by tuples thereof.

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<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [October 2, 2023, 11:49am UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/5 "2023-10-02T11:49:45Z")

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Check `isbitstype(Observation{Int})`, is it `true`? Then instances of it can be a type parameter.

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [October 2, 2023, 11:51am UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/6 "2023-10-02T11:51:45Z")

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Note that in your own example you are parameterizing `Array` with the value `1` to get a 1-dimensional array.

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**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 2, 2023, 12:31pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/7 "2023-10-02T12:31:19Z")

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> [@DNF](#):
>
> Note that in your own example you are parameterizing `Array` with the value `1` to get a 1-dimensional array.

Thank you for the help and yes, I notice that, but that is a literal value, which very often treated differently than runtime values in programming languages.  
But yes, I don’t fully understand what are the allowed values inside `{}` in Julia and would be great if you can provide some examples to clarify it. I have read the document you quoted but don’t fully understand it.

I thought only types and literal values are allowed and if you want a instace as type, you need to convert it using the value type `Val{x}` (provided that it can be converted).

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<div class="post-metadata">

**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 2, 2023, 12:35pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/8 "2023-10-02T12:35:00Z")

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> [@Benny](#):
>
> Check `isbitstype(Observation{Int})`, is it `true`? Then instances of it can be a type parameter.

Yes, indeed it is, so that qualify it as a type parameter.

What about `Val{x}` and `Val{x}()` I think they are kinda related, do you mind explaining their relation and/or differences with some examples?

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [October 2, 2023, 3:52pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/9 "2023-10-02T15:52:02Z")

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> [@EricH](#):
>
> I notice that, but that is a literal value, which very often treated differently than runtime values in programming languages.

Value-parameters do not need to be literal, but in order to have type stable code, the parameters must be compile-time constants, and of course literal numbers are constant at compile time.

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**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 5, 2023, 6:39pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/10 "2023-10-05T18:39:00Z")

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I think I now know how to check for valid type parameter (by `isbitstype()`), but the relation of all these with `Val{x}` and `Val{x}()` still confuses me.

Thanks for all the help anyways!

---

<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [October 5, 2023, 9:46pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/11 "2023-10-05T21:46:55Z")

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> [@EricH](#):
>
> I now know how to check for valid type parameter (by `isbitstype()`)

Well, `isbits` doesn’t cover it all, types, Symbols, and tuples of Symbols and `isbits` values also work. Actually, the easiest way to check if an instance `X` is a valid parameter is running `Val{X}` in the REPL and see if it throws an `TypeError`.

> [@EricH](#):
>
> the relation of all these with `Val{x}` and `Val{x}()`

There’s no special relation, `Val{x}` is just one of many types with a parameter. Type parameter values, unlike argument values, are known at a method’s compile-time. Since you can put _some_ non-type values in the parameters, that allows for some compile-time calculations in the method.

You can do that with any parametric type, but in cases where you just need to move a value into compile-time and don’t need to create a new type with new features, `Val{x}` exists for consistency. Otherwise, people would just be creating their own `struct MyVal{T} end` versions that are incompatible with each other. `Val{x}()` is just the type’s constructor call that makes its instance, in other words `Val{x}() isa Val{x}` the same way `1 isa Int`.

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<div class="post-metadata">

**Author:** ![EricH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erich/32/202185_2.png) [@EricH](https://discourse.julialang.org/u/EricH)\
**Post date:** [October 15, 2023, 11:14pm UTC](https://discourse.julialang.org/t/how-does-instance-of-a-type-works-interact-with-type-parameters/104475/12 "2023-10-15T23:14:48Z")

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Thank you for clarification, I think I have some rough ideas on the relation/differences now. 😀