thanglequoc / io-programming-language

A first touch with Io, the prototype programming language, with base guideline from the book "Seven languages in seven weeks"

The Io Programming Language

Introduction

The Io language’s syntax is simple and low-level, like the name. Io syntax simply chains messages together, with each message returning an object and each message taking optional parameters in parentheses. In Io, everything is a message that returns another receiver. With Io, you won't worry about both classes and objects. You'll deal exclusively in objects, cloning them as needed. These clones are called prototypes. Io is the prototype based programming language. In a prototype language , every objects is a clone of an existing object rather than a class.

The Playground

Hello, world with Io

Io> "Hello, world" print
Hello, world==> Hello, world

We're sending the print message to the string "Hello, world". Receivers is on the left, and messages go on the right. This method say: send print command to object Hello, world string.

Object Creation:

Io> Vehicle := Object clone
==>  Vehicle_0x4ebc058:
  type             = "Vehicle"

Object is the root-level object. We send the clone message, which returns a new object. We assign that object to Vehicle. Here, Vehicle is not a class. It’s not a template used to create objects. It is an object, based on the Object prototype.

Object has slots. Think of the collection of slots as a hash. You’ll refer to each slot with a key.

Objects is a collection of slots

Create new slot in object by using <key_slot> := <data> operator:

Io> Vehicle description := "Something takes you to places"
==> Something takes you to places

Reassign slot using = , io will throw exception if slot is not exist:

Io> Vehicle description = "It can moves !"
==> It can moves !

Io> Vehicle description1 = "It can moves !"
  Exception: Slot description1 not found. Must define slot using := operator before updating.
  ---------
  message 'updateSlot' in 'Command Line' on line 1
Io>

Get slot from object:

Io> Vehicle description
==> It can moves !

List slots of object with method slotNames:

Io> Vehicle slotNames
==> list(description, type)

Object has a slot to store the type of object

Io> Vehicle type
==> Vehicle
Io> Object type
==> Object

Prototype and Inheritance

Create object Car from Vehicle object. We created a new object called Car by sending the clone message to the Vehicle prototype

Io> Car := Vehicle clone
==>  Car_0x4fd3570:
  type             = "Car"

Io> Car slotNames
==> list(type)
Io> Car type
==> Car

Create another car ferrari, notice that this object start with lowercase character,

Idiomatically, an object that begins with an uppercase name is a type, so Io sets the type slot. Any clones of that type starting with lowercase letters will simply invoke their parents’ type slot.

Io> ferrari := Car clone
==>  Car_0x4fccd60:
Io> ferrari slotNames
==> list()
Io> ferrari description
==> A vehicle with 4 wheels, run by gas
Io> ferrari type
==> Car

Methods

Method is also an object with type block, syntax to create an assign a method to object slot is familiar

Io> Car horn := method("Piff Piff" println)
==> method(
    "Piff Piff" println
)
Io> Car horn
Piff Piff
==> Piff Piff

The subclass can execute the method of parent object

Io> ferrari horn
Piff Piff
==> Piff Piff

• Every thing is an object. • Every interaction with an object is a message. • You don’t instantiate classes; you clone other objects called prototypes. • Objects remember their prototypes. • Objects have slots. • Slots contain objects, including method objects. • A message returns the value in a slot or invokes the method in a slot. • If an object can’t respond to a message, it sends that message to its prototype.

List and Map

Create an empty list: todos := List clone

Create list using method: todos := (33, "jump")

Get list size: todos size

Append an item: todos append("kick")

Get item at index: todos at (1)

Pop item: todos pop

Prepend item:

Io> Action = list("Hit", "Kick", "Dash")
==> list(Hit, Kick, Dash)
Io> Action size
==> 3
Io> Action append("Punch")
==> list(Hit, Kick, Dash, Punch)
Io> Action at (2)
==> Dash
Io> Action pop
==> Punch

Commonly used messages

Message	Description	Example
clone	Create object	Vehicle := Object clone
slotNames	List all slots	Car slotNames
getSlot	List all slots	Car slotNames
proto	List all slots	Car slotNames
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