# Why \[1, 2, 3\] is not a Vector{Number}?

**URL:** <https://discourse.julialang.org/t/why-1-2-3-is-not-a-vector-number/52645>\
**Category:** New to Julia\
**Tags:** question, parametric-types\
**Created:** [December 31, 2020, 8:26am UTC](https://discourse.julialang.org/t/why-1-2-3-is-not-a-vector-number/52645 "2020-12-31T08:26:16Z")\
**Posts on this page:** 1\
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**Author:** ![jules](https://avatars.discourse-cdn.com/v4/letter/j/41988e/32.png) [@jules](https://discourse.julialang.org/u/jules)\
**Post date:** [December 31, 2020, 8:56am UTC](https://discourse.julialang.org/t/why-1-2-3-is-not-a-vector-number/52645/3 "2020-12-31T08:56:09Z")

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Just to extract some relevant points because the manual is quite dense:

`Vector{Number}` is a concrete type, and `[1, 2, 3]` has type `Vector{Int}`, which is also a concrete type. One concrete type is never a subtype of another concrete type, they are the leaves of the type tree.

`Vector{Number}` is concrete, even though `Number` is not a concrete type. That’s because it has a concrete implementation which can store all types that are subtypes of `Number`, it has a specific memory layout etc. On the other hand, `AbstractVector{Int}` is the other way around and not a concrete type, because the container is abstract even though the element is concrete.

What you can do instead is `[1, 2, 3] isa Vector{<:Number}` which is `true`. That’s because `<:Number` is a sort of placeholder which means “any type which is a subtype of Number”. This is often needed for dispatching on containers where you want to allow set of element types. `f(x::Vector{Number})` can take only arguments of type `Vector{Number}`, whereas `g(x::Vector{<:Number})` can take e.g. `Vector{Int}`, `Vector{Float64}`, `Vector{Real}`, `Vector{Number}`, etc.

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