Parametric constructors

Question

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?

did you read the part about parametric types being invariant? — user19242326
– user19242326, Commented Jul 7, 2022 at 7:25
@Empress.Svetlana Yes. When I add the subtype operator <:, to my understanding I am making sure that the input can be any subtype of Real (?) — TylerD
– TylerD, Commented Jul 7, 2022 at 7:27
Right, but Point{Real} isn't a Subtype of Real. When you declare, you have to declare it like this: p = Point{Integer}(12,12) — user19242326
– user19242326, Commented Jul 7, 2022 at 7:30
@Empress.Svetlana Thanks. but where in my example above am I assuming that Point{Real} is a subtype of Real? I am only assuming that Integer and Rational are subtypes of Real - or am I misunderstanding something here? — TylerD
– TylerD, Commented Jul 7, 2022 at 7:32

Sundar R · Accepted Answer · 2022-07-07 07:41:22Z

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)

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