Composition and inheritance: the Julian way

General Usage
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60

I think all that can be simplified a bit, if every type that uses Person and wishes to use it’s functions, simply had a person function.

Try the following:

abstract type AbstractPerson end
# basic methods to define: name,age,set_age

#basic AbstractPerson
mutable struct Person <: AbstractPerson
    name::String
    age::Int 
end

person(a::Person) = a

# CONSTRUCTOR
Person(name) = Person(name, 0)

#basic methods
name(a::AbstractPerson) = person(a).name
age(a::AbstractPerson)  = person(a).age
set_age(a::AbstractPerson,x::Integer) = (person(a).age = x; x)

# TYPE METHODS: always use `AbstractPerson` as input type...
Base.display(p::AbstractPerson) = println("Person: ", name(p), " (age: ", age(p), ")")

function happybirthday(p::AbstractPerson)
    set_age(p, age(p) + 1)
    println(name(p), " is now ", age(p), " year(s) old")
end

call(p::AbstractPerson) = (print_with_color(:red, uppercase(name(p)), "!"); println())

#---------------------------------------------------------------------
# DERIVED TYPE : Citizen

# Use abstract type for the interface name, by convention prepend
# `Abstract` to the type name.
abstract type AbstractCitizen <: AbstractPerson end

# here you should think of basic methods for a AbstractCitizen,
# such as nationality(c::AbstractCitizen).

# TYPE MEMBERS (composition of `Person` fields and new ones)
mutable struct Citizen <: AbstractCitizen
    person::Person
    nationality::String # new field (not present in Person)
end

person(c::Citizen) = c.person

#basic abstractcitizen method
nationality(c::Citizen) = c.nationality

#Now everything defined for AbstractPerson should work for Citizen

#And you are not tied to field names anymore:

struct EternalBeing <: AbstractPerson end

name(e::EternalBeing) = "The One who Is"
age(e::EternalBeing) = typemax(Int)
set_age(e::EternalBeing, x::Integer) = nothing

const eternal = EternalBeing()

# All just work
display(eternal)
happybirthday(eternal)
call(eternal)

println()

zulima = Citizen(Person("Zulima Martín García", 44), "Spain")
display(zulima)
happybirthday(zulima)
call(zulima)
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