Simple parametric type question

General Usage
21

As @JeffreySarnoff says, I don’t know that it is the best approach for your particular situation to have “unassigned” values, however, I think what you are looking for is nothing or missing. You could (but maybe shouldn’t) define animal like so:

mutable struct animal{T}
    x::Union{Nothing,Int}
    y::Union{Nothing,T}
    animal{T}() where{T} = new{T}(nothing,nothing)
end

Then these values would be set to the value nothing until you assigned them. That said, it might make more sense to write different data structures for the different scenarios you describe, rather than using one large data structure with unassigned values. For example:

struct Kangaroo
    x::Int
end

struct Wallaby{T}
    y::T 
end

You could then write specific methods that do something different depending on the type. For example

jump(k::Kangaroo) = Kangaroo(k.x+10)
jump(w::Wallaby) = Wallaby(w.y+1)

Without knowing more details about your exact situation, it is hard to know what to do for your specific use case.

22

You almost certainly want a Union{Missing, T} field then, possibly with a keyword constructor with nothing or missing as the default (read the manual about the difference and pick the one that suits your semantics better).

Eg

Base.@kwdef mutable struct Crocodile # note: type name capitalized
    teeth::Union{Int,Missing} = missing
    legs::Union{Float64,Missing} = missing
end

then

julia> c = Crocodile(; teeth = 42)
Crocodile(42, missing)

julia> c.teeth
42

julia> c.legs
missing

It is unclear why you need an inner constructor (I may have missed something).

23

Thanks to you all for the comments, the proposed solution seems to work well :slight_smile:
In any case and in reference to the different solutions including missing and undef terms, I just want to avoid them because the proposed structure I’m using is still under development, and somehow I feel quite free to add more fields as required to the main type. I’m of all the warnings provided so far and was considering to use them once the codes are in its final form, but right now it is easier to simply add fields as required and do nothing else… which, by the way, has been working great so far, no bugs or whatsoever.
Still I’ll consider these possibilities in the future versions, thanks.