I am having some issues understanding the concept of parametric constructors in Julia. I am looking at the standard example in the Julia docs:
struct Point{T<:Real}
x::T
y::T
end
To my understanding, this means I can generate a Point-datatype with an input that is subtype of Real, i.e., AbstractFloat, AbstractIrrational, …, Integer, Rational, …, StatsBase.TestStat.
However, both of the examples below result in errors:
Point(Integer(12))
Point(Rational(12))
Why does the above fail given that both integer and rational are subtypes of real?
>Solution :
The type parameter goes inside the curly braces in the constructor call, just like it does in the struct definition:
julia> Point{Integer}(12, 12)
Point{Integer}(12, 12)
julia> Point{Rational}(12, 10//3)
Point{Rational}(12//1, 10//3)
The arguments supplied are the values for the fields of the struct i.e. x and y. If the arguments are already of the type you want, you can leave out explicitly specifying the type parameter:
julia> Point(12, 6)
Point{Int64}(12, 6)
julia> Point(12.0, 6.0)
Point{Float64}(12.0, 6.0)
julia> Point(12, 6.0) #no automatic type promotion happens though
ERROR: MethodError: no method matching Point(::Int64, ::Float64)
Closest candidates are:
Point(::T, ::T) where T<:Real at REPL[1]:2
Stacktrace:
[1] top-level scope
@ REPL[9]:1
julia> Point{Float64}(12, 6.0) #unless you explicitly specify the type
Point{Float64}(12.0, 6.0)