Parametric storage and flexible interfaces

The idea

A parametric type records a relationship between its fields and a type parameter. Box{Int} and Box{Float64} are different concrete types, allowing each value’s representation to remain specific. A field declared with an unconstrained abstract type has a different storage contract from a field declared as T in a type parameterized by T. Prefer a representation that states the relationship your operations actually require.

Julia’s ordinary parametric types are invariant: Vector{Int} is not a subtype of Vector{Real}, even though Int is a subtype of Real. A method intended to accept a vector of any real element type can use AbstractVector{<:Real}. This describes a family of vector types without demanding that every caller construct a Vector{Real}. Keep accepted interfaces broad while keeping stored data appropriately concrete.

struct Box{T}
    value::T
end
println(Box(3).value)
println(Box(3.0) isa Box{Float64})
println(Vector{Int} <: Vector{Real})

3
true
false

Constructor inference produces concrete parameterizations. Subtyping of element types does not make the corresponding invariant vector types subtypes.