# How to make type parameter for \`Tuple\` represent both abstract type and concrete type?

**URL:** <https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420>\
**Category:** General Usage\
**Tags:** type, tuple\
**Created:** [April 13, 2022, 9:19am UTC](https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420 "2022-04-13T09:19:07Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [April 13, 2022, 9:19am UTC](https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420/1 "2022-04-13T09:19:07Z")

</div>

```julia
julia> t1 = NTuple{2, Real}
Tuple{Real, Real}

julia> t2 = NTuple{2, T} where {T<:Real}
Tuple{T, T} where T<:Real

julia> t1 <: t2
false

julia> t3 = NTuple{2, <:T} where {T<:Real}
Tuple{var"#s1", var"#s1"} where {T<:Real, var"#s1"<:T}

julia> t1 <: t3
false

```

It seems that the type parameter `T` cannot be inferred as an abstract type unless I constrain it to be a specific one:

```julia
julia> t4 = NTuple{2, T} where {Real<:T<:Real}
Tuple{T, T} where Real<:T<:Real

julia> t1 <: t4
true

```

Is there a way to let `T` can be both a concrete type that `<: Real` and `Real`? Currently, my workaround is

```julia
julia> t5 = Union{t2, NTuple{2, Real}}
Tuple{Real, Real}

julia> t1 <: t5
true

julia> NTuple{2, Float64} <: t5
true

```

However, it’d be better if I don’t have to use `Union`. Thanks!!

---

<div class="post-metadata">

**Author:** ![goerch](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerch/32/29122_2.png) [@goerch](https://discourse.julialang.org/u/goerch)\
**Post date:** [April 13, 2022, 9:42am UTC](https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420/2 "2022-04-13T09:42:02Z")

</div>

Hm. This

```julia
t1 = NTuple{2, Real}
t2 = Tuple{T1, T2} where {T1<:Real, T2<:Real}
@show t1 <: t2

```

fulfills the requirement of showing `true`,

```julia
t1 <: t2 = true

```

but I’m not sure if it is what you are looking for?

---

<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [April 13, 2022, 12:53pm UTC](https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420/3 "2022-04-13T12:53:30Z")

</div>

Thanks! However, what I want is the common type of all the elements in the `Tuple` as `T` so that I’ll be able to go something like `NTuple{N, T} where {N, T}` where `T` can be both a concrete type and an abstract type.

---

<div class="post-metadata">

**Author:** ![goerch](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerch/32/29122_2.png) [@goerch](https://discourse.julialang.org/u/goerch)\
**Post date:** [April 13, 2022, 3:23pm UTC](https://discourse.julialang.org/t/how-to-make-type-parameter-for-tuple-represent-both-abstract-type-and-concrete-type/79420/4 "2022-04-13T15:23:24Z")

</div>

> [@frankwswang](#):
>
> However, what I want is the common type of all the elements in the `Tuple` as `T` so that I’ll be able to go something like `NTuple{N, T} where {N, T}` where `T` can be both a concrete type and an abstract type

Funnily enough in the above context we even have

```julia
@show t2 <: t1

```

yielding

```julia
t2 <: t1 = true

```

so I don’t believe it gets much better than that.

Edit: however, look at this

```julia
t1 = StaticVector{2, Real}

t2 = StaticVector{2, T} where {T <: Real}

@show t1 <: t2
@show t2 <: t1

```

yielding

```julia
t1 <: t2 = true
t2 <: t1 = false

```