Restrict type of value type parameter

General Usage
1

I am declaring a parametric type struct A{L} end, where L should be an integer. For example, in NTuple{N,T}, the N parameter is restricted to be an integer. What is the recommended way to achieve this behavior on my custom time? I could not find the source code of NTuple.

1 Like
2

I think that NTuple per se does not check that N is an integer, Vararg (which it is aliased to) does.

For structs, the recommended solution is to check in the inner constructor, eg

struct A{L}
    function A{L}() where L
        @assert L isa Integer
        new{L}()
    end
end

A(L::Integer) = A{L}()          # convenience constructor

A(1)                            # OK
A("a fish")                     # error
4 Likes
3

PS: the source of NTuple is in boot.jl. It is nothing more than

const NTuple{N,T} = Tuple{Vararg{T,N}}

There is no magic. Being able to write such core constructs in Julia lends a certain elegance to the language.

2 Likes
5

Thanks. If Vararg is also in Julia, do you know where is the source?

6

I think that Vararg is implemented in C.

7

Hi, I want to achieve the same goal except that L must limited to three integer values 1,2, 3. Is there a way to that using Union{Val{1},Val{2},Val{3}}. In fact, after create the type I want implement dispatch
on these values like in the constructor of Array{T,N}

Array{T,1}(::UndefInitializer, m::Int) where {T} =
ccall(:jl_alloc_array_1d, Array{T,1}, (Any, Int), Array{T,1}, m)
Array{T,2}(::UndefInitializer, m::Int, n::Int) where {T} =
ccall(:jl_alloc_array_2d, Array{T,2}, (Any, Int, Int), Array{T,2}, m, n)

But Sadly I do not find the definition of Array{T,N} using @edit

Bests

8

It is not clear to me what you are doing (please open a separate topic with an MWE if you need further help). In any case, validating in the constructor is still the recommended method.

1 Like
9

Hi,
Effectively, I enter too abruptly into the discussion. Next time, I will follow the rules for discourse :smile: . In addition, my question was not clear. I mixed up two points : restrict the parameter values and enable dispatch respective to the different values of the parameter. Anyway, I found a convienent solution for me and I use your trick for restricting values. Thanks