# Julia's type system : what am I missing?

**URL:** https://discourse.julialang.org/t/julias-type-system-what-am-i-missing/130618
**Category:** General Usage
**Tags:** question
**Created:** [July 10, 2025, 8:00pm UTC](https://discourse.julialang.org/t/julias-type-system-what-am-i-missing/130618 "2025-07-10T20:00:41Z")
**Posts on this page:** 1
**Showing post:** 12

<div class="post-metadata">

### Author: ![juliohm](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/juliohm/32/215266_2.png) [@juliohm](https://discourse.julialang.org/u/juliohm)
#### Post date: [July 11, 2025, 3:41am UTC](https://discourse.julialang.org/t/julias-type-system-what-am-i-missing/130618/12 "2025-07-11T03:41:44Z")

</div>

Adding to @Daniel_Berge 's answer, you can easily dispatch on more specific aliases:

```julia
const Len{T} = Quantity{T,u"𝐋"}
const Met{T} = Quantity{T,u"𝐋",typeof(m)}
const Deg{T} = Quantity{T,NoDims,typeof(°)}
const Rad{T} = Quantity{T,NoDims,typeof(rad)}

```

You don’t need to construct concrete Unitful.jl types for dispatch. And you can let Julia figure out the exact type in a field using a type parameter:

```julia
struct Foo{L<:Met}
  len::L
end

```

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