# Vector of vectors supertype
**URL:** https://discourse.julialang.org/t/vector-of-vectors-supertype/88666
**Category:** General Usage
**Created:** [October 13, 2022, 1:59pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666 "2022-10-13T13:59:30Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@qassiov](https://discourse.julialang.org/u/qassiov)
#### Post date: [October 13, 2022, 1:59pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666/1 "2022-10-13T13:59:30Z")
I would like to define a function that takes arguments of the type `Vector{Vector{Float64}}` but is also not restricted to just Float64s (e.g. it should also be able take arguments of type `Vector{Vector{Int64}}`).
However, if I try and define the function with a more generic type, like here:
```julia
julia> x = [[1.0, 2.0],[0.1, 0.2]]
2-element Vector{Vector{Float64}}:
[1.0, 2.0]
[0.1, 0.2]
julia> foo(v::Vector{Vector}) = return v
foo (generic function with 1 method)
julia> foo(x)
ERROR: MethodError: no method matching foo(::Vector{Vector{Float64}})
Closest candidates are:
foo(::Vector{Vector}) at REPL[116]:1
```
This is because Vector{Vector} is not a supertype of Vector{Vector{Float64}}, despite Vector being a supertype of Vector{Float64}.
```julia
julia> Vector{Float64} <: Vector
true
julia> Vector{Vector{Float64}} <: Vector{Vector}
false
```
Is there a solution to this, or should I just stick to writing multiple functions for the ‘subtypes’ of Vector{Vector} that are relevant for my program?
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [October 13, 2022, 2:01pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666/2 "2022-10-13T14:01:37Z")
> [@qassiov](#):
>
> ```julia
> julia> foo(v::Vector{Vector}) = return v
> foo (generic function with 1 method)
>
> ```
Either
```julia
foo(v::Vector{<:Vector}) = return v
```
or
```julia
foo(v::Vector{Vector{T}}) where T = return v
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [October 13, 2022, 2:01pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666/3 "2022-10-13T14:01:59Z")
> [@qassiov](#):
>
> `julia> Vector{Vector{Float64}} <: Vector{Vector}`
julia types are invariant (except Tuple):
[https://docs.julialang.org/en/v1/manual/types/#Parametric-Abstract-Types](https://docs.julialang.org/en/v1/manual/types/#Parametric-Abstract-Types)
```julia
julia> Vector{Vector{Float64}} <: Vector{<:AbstractVector}
true
```
btw, idk what field of research/computing you’re in, but if this gets large, you might want to consider:
> **[GitHub - JuliaArrays/ArraysOfArrays.jl: Efficient storage and handling of...](https://github.com/JuliaArrays/ArraysOfArrays.jl)**
>
> Efficient storage and handling of nested arrays in Julia - GitHub - JuliaArrays/ArraysOfArrays.jl: Efficient storage and handling of nested arrays in Julia
---
### Author:  [@qassiov](https://discourse.julialang.org/u/qassiov)
#### Post date: [October 13, 2022, 2:04pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666/4 "2022-10-13T14:04:13Z")
Thank you so much! I needed to be more familiar with the syntax for type selectors for this one…
---
### Author:  [@qassiov](https://discourse.julialang.org/u/qassiov)
#### Post date: [October 13, 2022, 2:05pm UTC](https://discourse.julialang.org/t/vector-of-vectors-supertype/88666/5 "2022-10-13T14:05:24Z")
That is very useful to know about, thank you.