julia get value of type parameters

Question

I have a parametric type. For example:

> Array([1 2;3 4])

Its type is

> typeof(Array([1 2;3 4]))

Array{Int64,2}

I can get the first type parameter using eltype:

> eltype(typeof(Array([1 2;3 4])))

Int64

How can I access second and possibly other type parameters?

Answer 1

If you're speaking specifically of (Abstract)Array types, then the dimension can be retrieved using ndims:

julia> ndims(Array{Int64, 2})
2

If, on the other hand, you want to write a custom function that extracts a given parameter from a parametric type, you can use build one like this:

julia> second_param(::Type{Array{T, N}}) where {T, N} = N
second_param (generic function with 1 method)

julia> second_param(Array{Int64, 2})
2

(I'm using Array here for the sake of the example, but the same kind of construct could be used to extract parameters from any other parametric type)