# Incorrect concrete type for Vector{Pair{Any, Any}}?
**URL:** https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885
**Category:** General Usage
**Tags:** type, potential-bug
**Created:** [December 10, 2021, 10:37am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885 "2021-12-10T10:37:40Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@anicusan](https://discourse.julialang.org/u/anicusan)
#### Post date: [December 10, 2021, 10:37am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/1 "2021-12-10T10:37:40Z")
Hello,
My question’s rather simple:
```julia
julia> [1=>2]
1-element Vector{Pair{Int64, Int64}}:
1 => 2
julia> [1=>2] isa Vector{Pair{Any, Any}}
false
```
Why is `Vector{Pair{Int64, Int64}}` not a subtype of `Vector{Pair{Any, Any}}`?
And if useful, here is my Julia version:
```julia
julia> versioninfo()
Julia Version 1.6.4
Commit 35f0c911f4 (2021-11-19 03:54 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
JULIA_DEPOT_PATH = /home/andreinicusan/Prog/Julia/builds/julia-1.6.4/packages:
JULIA_NUM_THREADS = 16
```
Thanks,
Leonard
---
### Author:  [@lawless-m](https://discourse.julialang.org/u/lawless-m)
#### Post date: [December 10, 2021, 10:43am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/2 "2021-12-10T10:43:14Z")
I don’t know the _why_ but
julia\> [1=\>2] isa Vector{Pair{T,T}} where T \<: Any
true
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [December 10, 2021, 10:48am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/3 "2021-12-10T10:48:05Z")
Yes, this is correct and expected behaviour. `Vector{Pair{Any, Any}}` is a concrete type that can be instantiated, it would be a vector of pairs of different types. Concrete types cannot have subtypes, therefore `Vector{Pair{Int, Int}}` cannot be its subtype.
This has to do with invariant and contravariant types, you can read more about this here: [Types · The Julia Language](https://docs.julialang.org/en/v1/manual/types/#man-parametric-composite-types)
In particular, you can find this warning in the docs:
> This last point is _very_ important: even though `Float64 <: Real` we **DO NOT** have `Point{Float64} <: Point{Real}` .
An alternative convenient test could be
```julia
1.7.0> [1=>2] isa Vector{<:Pair}
true
```
---
### Author:  [@anicusan](https://discourse.julialang.org/u/anicusan)
#### Post date: [December 10, 2021, 11:04am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/4 "2021-12-10T11:04:58Z")
Thank you for the very quick replies. It seems odd that `Tuple` is covariant, while `Pair` is invariant; I understand that covariance is special-cased, but why isn’t a type as fundamental as `Pair` - not unlike `Tuple` - made covariant?
I am currently working with a library that asserts types with `constraints::Vector{Pair{Any,Any}}`, so I need to enforce the `Any` quite awkwardly:
```julia
julia> [1=>2, ""=>""][1:end-1]
1-element Vector{Pair{Any, Any}}:
1 => 2
```
---
### Author:  [@aplavin](https://discourse.julialang.org/u/aplavin)
#### Post date: [December 10, 2021, 11:19am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/5 "2021-12-10T11:19:02Z")
A type like `Vector` cannot really be covariant: one cannot pass e.g. `Vector{Int}` everywhere where `Vector{Any}` is expected. This function would fail if you could pass `Vector{Int}` to it:
```julia
f(x::Vector{Any}) = push!(x, "abc")
```
but it works for `Vector{Any}`.
If you do need to create a `Vector{Pair{Any, Any}}` containing pairs of a single type (do you **really** need it?), it’s possible much less awkwardly: `Pair{Any, Any}[1 => 2]`.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [December 10, 2021, 11:58am UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/6 "2021-12-10T11:58:40Z")
> [@anicusan](#):
>
> `Tuple` is covariant
Apparently, `Tuple` is abstract unless its parameters are concrete, so you cannot instantiate it. I am not sure if that makes it contravariant or not, but at least there is nothing preventing it to have subtypes.
> [@aplavin](#):
>
> `Pair{Any, Any}[1 => 2]`
Yes, or, alternatively:
```julia-auto
[Pair{Any, Any}(1,2)]
```
This is a common idiom in Julia, btw, for any type, `T`, you can create a `Vector{T}` by writing `T[...]`
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [December 10, 2021, 12:03pm UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/7 "2021-12-10T12:03:53Z")
> [@anicusan](#):
>
> I am currently working with a library that asserts types with `constraints::Vector{Pair{Any,Any}}`
That is terrible, BTW. Maybe you can open an issue/PR to have it changed. This is really not idiomatic Julia.
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [December 10, 2021, 1:24pm UTC](https://discourse.julialang.org/t/incorrect-concrete-type-for-vector-pair-any-any/72885/8 "2021-12-10T13:24:03Z")
it might not be bad. sometimes this is what you need to prevent ridiculous amounts of compiler specialization. that said, 90% of the time, this is bad.