In geometry, a two-dimensional point and a vector are not the same, even if they both have an x and y component. In Julia, we can also define them as different types, as follows:
# see the code in Chapter 6\unions.jl
mutable struct Point
x::Float64
y::Float64
end
mutable struct Vector2D
x::Float64
y::Float64
endHere are the two objects:
p = Point(2, 5)that returnsPoint(2.0, 5.0)v = Vector2D(3, 2)that returnsVector2D(3.0, 2.0)
Suppose we want to define the sum for these types as a point which has coordinates as the sum of the corresponding coordinates:
+(p, v)
This results in an ERROR: MethodError: `+` has no method matching +(::Point, ::Vector2D) error message.
To define a + method here, first do an import Base.+
Even after defining the following, +(p, v) still returns the same error because of multiple dispatch. Julia has no way of knowing that +(p,v) should be the same as +(v,p):
+(p::Point, q::Point) = Point(p.x + q.x, p.y + q.y) +(u::Vector2D, v::Vector2D)...