抽象类型 #

abstract type Animal end
abstract type Mammal <: Animal end
abstract type Bird <: Animal end

Mammal <: Animal
Bird <: Animal
Animal <: Any

Animal()

abstract type Number end
abstract type Real <: Number end
abstract type AbstractFloat <: Real end
abstract type Integer <: Real end
abstract type Signed <: Integer end
abstract type Unsigned <: Integer end

abstract type AbstractCollection end
abstract type AbstractSet <: AbstractCollection end
abstract type AbstractDict <: AbstractCollection end
abstract type AbstractVector <: AbstractCollection end

abstract type Shape end
abstract type Polygon <: Shape end
abstract type Circle <: Shape end
abstract type Triangle <: Polygon end
abstract type Rectangle <: Polygon end

function area(s::Shape)
    error("area not implemented for $(typeof(s))")
end

struct Circle
    radius::Float64
end

area(c::Circle) = π * c.radius^2

struct Rectangle
    width::Float64
    height::Float64
end

area(r::Rectangle) = r.width * r.height

area(Circle(1.0))
area(Rectangle(3.0, 4.0))

function describe(x)
    if x isa Integer
        "Integer: $x"
    elseif x isa AbstractFloat
        "Float: $x"
    else
        "Other: $x"
    end
end

describe(1)
describe(1.0)
describe("hello")

function process(x::Real)
    x * 2
end

process(1)
process(1.0)
process(1 // 2)
process("hello")

isabstracttype(Real)
isabstracttype(Int)
isconcretetype(Int)
isconcretetype(Real)

1 isa Real
1.0 isa Real
1 isa Integer
1.0 isa Integer

Int <: Integer
Int <: Real
Int <: Number
Float64 <: AbstractFloat
Float64 <: Real

abstract type Shape end
abstract type Polygon <: Shape end

struct Circle <: Shape
    radius::Float64
end

struct Triangle <: Polygon
    a::Float64
    b::Float64
    c::Float64
end

struct Rectangle <: Polygon
    width::Float64
    height::Float64
end

area(c::Circle) = π * c.radius^2
area(r::Rectangle) = r.width * r.height
area(t::Triangle) = begin
    s = (t.a + t.b + t.c) / 2
    sqrt(s * (s - t.a) * (s - t.b) * (s - t.c))
end

perimeter(c::Circle) = 2π * c.radius
perimeter(r::Rectangle) = 2 * (r.width + r.height)
perimeter(t::Triangle) = t.a + t.b + t.c

shapes = [Circle(1), Rectangle(3, 4), Triangle(3, 4, 5)]
[area(s) for s in shapes]

abstract type Animal end
abstract type Mammal <: Animal end
abstract type Bird <: Animal end

struct Dog <: Mammal
    name::String
end

struct Cat <: Mammal
    name::String
end

struct Eagle <: Bird
    name::String
end

speak(d::Dog) = "$(d.name) says Woof!"
speak(c::Cat) = "$(c.name) says Meow!"
speak(e::Eagle) = "$(e.name) says Screech!"

animals = [Dog("Buddy"), Cat("Whiskers"), Eagle("Sky")]
[speak(a) for a in animals]

function process_number(x::Number)
    if x isa Integer
        x^2
    elseif x isa AbstractFloat
        round(x, digits=2)
    elseif x isa Rational
        float(x)
    elseif x isa Complex
        abs(x)
    else
        x
    end
end

process_number(5)
process_number(3.14159)
process_number(1 // 3)
process_number(3 + 4im)