I want to declare a struct with: struct MyStruct{L,P} a::SVector{L,Other1} b::SVector{P,Other2} c::SVector{L+P,Float64} end The third field gives error: MethodError: no method matching +(::TypeVar, ::TypeVar) How do I do arithmetic in the type system?

In addition to what others have said, you can mostly achieve what you’re looking for with an extra type parameter, but it is inconvenient: struct MyStruct{L,P, N} a::SVector{L,Other1} b::SVector{P,Other2} c::SVector{N,Float64} function MyStruct{L, P, N}(...) # Check that N …

you can’t, Julia does not have dependent type.

https://github.com/vtjnash/ComputedFieldTypes.jl

Instead of copying the answers from the previous discussion, I’ll just link it: [image] Parametric type with numbers General Usage Hello! I am having difficulties trying to model the adequate code for what I want to achieve. Imagine I have the following struct: struct …

[image] rdeits: # Convenient outer constructor that computes N for you function MyStruct{L, P}(...) MyStruct{L, P, L + P}(...) end Can you define an outer constructor with partial parametric types? Does this have well-defined semantics/support?

[image] Henrique_Becker: Can you define an outer constructor with partial parametric types? Does this have well-defined semantics/support? I hope it is, I have been just implementing a bunch of stuff using that, without even thinking about it :grimacing: But what I wanted to add is that the …

I am not entirely sure because of the extensive use of generators in that piece of code. But these partial constructors are known to have some bugs, the two of us already found some strange behavior on then in the past, see Using parametric type inside outer parametric constructor - #28 by Henrique…

[image] Henrique_Becker: Can you define an outer constructor with partial parametric types? Does this have well-defined semantics/support? Yeah, absolutely. The parameters in the outer constructor just have to be explicitly supplied by the user at the call site: julia> struct Foo{X, Y} end …

[image] rdeits: Yeah, absolutely. I hope so, but I couldn’t find anything in the manual about that. This is probably very common (and very useful), maybe it should be better documented?

I agree with Leandro, the implementation has some subtle bugs, there is no mention in the documentation, and very simple code that I would guess on working simply does not work. EDIT: forgot where in the code.

You need the where A there: julia> struct S{A, B}; a :: A; b :: B; end julia> S{A}(a) where A = S{A,Int}(a,0) julia> S{Int}(1) S{Int64, Int64}(1, 0)

How to do arithmetic in the type system?

General Usage

nilshg February 16, 2022, 3:17pm 4

Instead of copying the answers from the previous discussion, I’ll just link it:

Topic		Replies	Views
Addition to parameter of parametric type General Usage parametric-types	15	1915	January 25, 2019
Operation with struct parameter within struct definition General Usage question	3	96	June 27, 2025
Parametric type with numbers General Usage parametric-types	3	843	January 13, 2022
Arithmetic for parametric type values in structs e.g., N+1 General Usage question	5	251	February 29, 2024
Operations on numerical type parameters General Usage	4	576	January 21, 2019

How to do arithmetic in the type system?

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