Consider this MWE: using Unitful # 1. Get the type of a quantity constructed from the Unitful macro t1 = typeof(1u"m") # 2. Manually construct a type with the same structure t2 = Unitful.Quantity{Int64, Unitful.𝐋, Unitful.FreeUnits{(Unitful.m,), Unitful.𝐋, nothing}} # 3. Compare println("t1 == t…

This has come up before, though I can’t find the other thread at the moment. I think the type isn’t being printed faithfully. You can do: t3 = Quantity{Int64, dimension(u"m"), typeof(u"m")} @show t1 == t3 # true @show t1 === t3 # also true

Thanks for the response; this is a very subtle issue that took a long time to track down. Since dispatch depends on the faithful comparison of types, what is a good strategy to prevent this problem?

Wow, that’s very cursed. It’s the difference between: julia> Unitful.FreeUnits{(Unitful.Unit{:Meter, Unitful.𝐋}(0, 1//1),), Unitful.𝐋, nothing}() m julia> Unitful.Unit{:Meter, Unitful.𝐋}(0, 1//1) m Both of those things print as m, but only one of them is the same as Unitful.m. julia> typeof(Unit…

In the end I wanted to do something like this: const DeviceUnits{T} = Union{ Quantity{T, dimension(u"m"), typeof(u"m")}, Quantity{T, dimension(u"cm"), typeof(u"cm")}} where T <: Number and there was no easy way to construct the types. This works but the prior construction which used Quant…

I’m not adept enough to understand the mechanism that enables this.

It’s not about Julia’s type system; it’s that Unitful is choosing to print two subtly different values as the same thing. And then it also defines a variable with the same name as what prints out. It just becomes particularly confusing when you put these values into a type parameter. Here’s what’…

aha, this makes dispatch with Unitful types particularly painful. I read in the docs that they recommend avoiding it. Thanks for the response!

I don’t know if this was missed in the discussion, but I frequently use dispatch on the supertypes like Unitful.Length based on the SI dimensions. foo(x::Unitful.Length) = "Is a Unit of Length" foo(x::Unitful.AbstractQuantity) = "Is NOT a Unit of Length" julia> foo(1u"m") "Is a Unit of Length" ju…

Adding to @Daniel_Berge 's answer, you can easily dispatch on more specific aliases: 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 t…

Julia's type system : what am I missing?

General Usage

danielmatz July 10, 2025, 8:18pm 4

Here’s a related thread: How To Properly Use Unitful - #12 by ajkeller34

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Julia's type system : what am I missing?

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