Why Array{Float64, N} cannot be a parameter of a function whose parameter is Array{Number, N}?

Question

I found that Array in Julia is not covariant and subtypes of Number are not auto-converted to supertypes.

What I mean is, for example,

head(a::Vector{Number}) = a[1] or head(a::Vector{Real}) = a[1]

cannot execute head([1, 2, 3]),

whereas head(a::Vector{T}) where {T <: Number} = a[1] or head(a::Vector{T}) where {T <: Real} = a[1] can.

Is there a reason for this behavior in Julia?

Answer 1

See this section in the manual: https://docs.julialang.org/en/stable/manual/types/#Parametric-Composite-Types-1 which explains this. Note that there is a short form for head(a::Vector{T}) where {T <: Number} =... (which you can use unless you use T in the function body):

head(a::Vector{<:Number}) =...