# Why Unions dont inheret its union?
**URL:** https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166
**Category:** New to Julia
**Created:** [October 11, 2020, 3:14pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166 "2020-10-11T15:14:55Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@systems](https://discourse.julialang.org/u/systems)
#### Post date: [October 11, 2020, 3:14pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/1 "2020-10-11T15:14:55Z")
Why does this return false
```julia
IntOrString = Union{Int,AbstractString}
(IntOrString <: String, IntOrString <: Int)
```
I am not questioning the decision, just curious to know the reason behind it
Also while we are at it why are unions not a DataType
```julia
Union <: DataType
DataType <: Union
```
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 11, 2020, 3:25pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/2 "2020-10-11T15:25:31Z")
> [@systems](#):
>
> I am not questioning the decision, just curious to know the reason behind it
Because, in your example, an `Int` is not a subtype of `String`. Think about it like sets. You have the set of all `Int` unioned with all `AbstractString`. This is clearly bigger than the set of `String`. In fact you have:
```julia
julia> String <: IntOrString
true
```
---
### Author:  [@systems](https://discourse.julialang.org/u/systems)
#### Post date: [October 11, 2020, 3:41pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/3 "2020-10-11T15:41:14Z")
I think I might have figured it out
Union are not treated as a DataType, because typeof(), can never return a Union
The type of a value can never be a Union
But now this make me think that
```julia
IntOrString <: String
```
should instead raise an error, not just return `false`
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [October 11, 2020, 3:44pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/4 "2020-10-11T15:44:47Z")
No. It is critical for Julia that `<:` return either `true` or `false` for every combination of types.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 11, 2020, 3:46pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/5 "2020-10-11T15:46:19Z")
> [@systems](#):
>
> Union are not treated as a DataType, because typeof(), can never return a Union
I’m not sure what you mean but note:
```julia
julia> abstract type Foo end
julia> Foo isa DataType
true
```
but `typeof` can never return `Foo`.
---
### Author:  [@systems](https://discourse.julialang.org/u/systems)
#### Post date: [October 11, 2020, 3:58pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/6 "2020-10-11T15:58:23Z")
> [@kristoffer.carlsson](#):
>
> but `typeof` can never return `Foo`.
It can never return Foo, because Foo is an abstract type, that cannot be instantiated
Only concrete type can be instantiated
---
### Author:  [@systems](https://discourse.julialang.org/u/systems)
#### Post date: [October 11, 2020, 4:00pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/7 "2020-10-11T16:00:20Z")
Exactly, IntOrString is not a type or a DataType , its more like a function on Types, but is not itself a type , hence the raise an error make sense to me
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 11, 2020, 4:03pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/8 "2020-10-11T16:03:30Z")
> [@systems](#):
>
> It can never return Foo, because Foo is an abstract type, that cannot be instantiated
> Only concrete type can be instantiated
Yes, I know, my point was that you said
> [@systems](#):
>
> Union are not treated as a DataType, because typeof(), can never return a Union
and I gave an example of where something is treated as a `DataType` even though `typeof` cannot return it.
---
### Author:  [@Vasily\_Pisarev](https://discourse.julialang.org/u/Vasily_Pisarev)
#### Post date: [October 11, 2020, 4:46pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/9 "2020-10-11T16:46:33Z")
Unions not being of type `DataType` has nothing to do with the fact that `IntOrString <: String == false`.
Let’s say you have a cage whose dweller has the type `const HamsterOrBird = Union{Hamster, AbstractBird}`. When you ask `HamsterOrBird <: Canary`, it means “if I know that there is a `HamsterOrBird` in the cage, can I be sure that the caged creature is a `Canary`?”
Maybe you meant the subtyping in the opposite direction? `IntOrString >: String` is true, meaning that if `x isa String` then `x isa IntOrString`.
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [October 11, 2020, 6:01pm UTC](https://discourse.julialang.org/t/why-unions-dont-inheret-its-union/48166/10 "2020-10-11T18:01:27Z")
> [@Vasily\_Pisarev](#):
>
> Unions not being of type `DataType` has nothing to do with the fact that `IntOrString <: String == false` .
Also note that
```julia
julia> Union{Int,UInt} <: Integer
true
julia> Union{Int,UInt}
Union{Int64, UInt64}
julia> typeof(Union{Int,UInt})
Union
julia> typeof(Integer)
DataType
```
The `typeof` of a `Type` has little to do with the subtype relation.