# 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\
**Showing post:** 16

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [January 2, 2021, 10:55pm UTC](https://discourse.julialang.org/t/why-1-2-3-is-not-a-vector-number/52645/16 "2021-01-02T22:55:06Z")

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> [@CameronBieganek](#):
>
> that means that the main reason that parametric types are invariant in Julia is due to performance considerations

Yes, it’s critical for performance. See my explanation here: [Problem with Complex{Rationals} - #7 by stevengj](https://discourse.julialang.org/t/problem-with-complex-rationals/9474/7)

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