Paradigm shift - from behaviours inside objects in OOAD to Julia's multiple dispatch...

#julia #designpattern

OOAD Thought Processing

Concept → Owner

Behavior → Object
Polymorphism → Virtual methods
Extension → Subclassing
Encapsulation → Methods inside class

Julia Thought Processing

Concept → Owner

Behavior → Generic function
Polymorphism → Multiple dispatch
Extension → Add new methods
Encapsulation → Types + modules

OO Design Thinking

Identify entities (classes)
Attach behavior to them
Use inheritance for variation

Julia Design Thinking

Identify operations (functions)
Define data types
Implement methods for combinations of types

Worldview Comparison

OOP worldview

The world is made of interacting objects.

Julia worldview

The world is made of operations applied to data.

The State Pattern in Pure Julia Style

Enjoy the State pattern written in Julia.

Note that this works without the context class of the original State Pattern, which is typically implemented using object-oriented languages like C++, Java, or Python.

# 1. Define the State Hierarchy
abstract type ChickenState end

struct Uncleaned  <: ChickenState end
struct Washed     <: ChickenState end
struct Marinated  <: ChickenState end
struct Cooked     <: ChickenState end

# 2. Define State Transitions (Multiple Dispatch)

# --- Washing ---
function wash(::Uncleaned)
    println("Chicken is getting cleaned → Washed")
    sleep(2)
    return Washed()
end

wash(s::ChickenState) = s  # Default: no change if already washed/cooked

# --- Marinating ---
function marinate(::Washed)
    println("Chicken is being marinated → Marinated")
    sleep(2)
    return Marinated()
end

marinate(s::ChickenState) = s

# --- Cooking ---
function cook(::Marinated)
    println("Chicken is being cooked → Cooked")
    sleep(4)
    return Cooked()
end

cook(s::ChickenState) = s

# --- Serving ---
function serve(::Cooked)
    println("Chicken is being served. Final state.")
    return Cooked()
end

serve(s::ChickenState) = (println("Can't serve yet, chicken is $(typeof(s))"); s)

# 3. Execution Flow
state = Uncleaned()

state = wash(state)
state = marinate(state)
state = cook(state)
state = serve(state)