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Io
Paradigms	object-oriented prototype-based
Designed by	Steve Dekorte
Developers	Steve Dekorte, Jonathan Wright, Jeremy Tregunna
First appeared	2002; 22 years ago
Stable release	20170906 / September 6, 2017; 7 years ago
Preview release	2019.05.22-alpha / May 22, 2019; 5 years ago
Typing discipline	dynamic, strong
Platform	IA-32, x86-64; ARM; .NET CLR
OS	Windows, macOS
License	BSD 3-clause
Website	iolanguage.org
Major implementations
	Io Io.NET
Influenced by
	Smalltalk, NewtonScript, Self, Lua, Lisp, Python, Act1
Influenced
	Ioke, Potion

Io is a pure object-oriented programming language inspired by Smalltalk, Self, Lua, Lisp, Act1, and NewtonScript.^[2] Io has a prototype-based object model similar to those in Self and NewtonScript, eliminating the distinction between instance and class. Like Smalltalk, everything is an object and it uses dynamic typing. Like Lisp, programs are just data trees. Io uses actors for concurrency.

Remarkable features of Io are its minimal size and openness to using external code resources.^[3] Io is executed by a small, portable virtual machine.

History

The language was created by Steve Dekorte in 2002, after trying to help a friend, Dru Nelson, with his language, Cel. He learned that he really didn't know much about how languages worked, and set out to write a tiny language to understand the problems better.^[4]

Philosophy

Io's goal is to explore conceptual unification and dynamic languages, so the tradeoffs tend to favor simplicity and flexibility over performance.

Features

Pure object-oriented based on prototypes
Code-as-data, homoiconic
Lazy evaluation of function parameters
Higher-order functions
Introspection, reflection and metaprogramming
Actor-based concurrency
Coroutines
Exception handling
Incremental garbage collecting supporting weak links
Highly portable
Shared library, dynamic-link library (DLL), dynamic loading on most platforms
Small virtual machine

Syntax

In its simplest form, Io syntax is composed of one identifier:^[5]

 doStuff

Assuming the above doStuff is a method, it is being called with zero arguments and as a result, explicit parentheses are not required.

If doStuff had arguments, it would look like this:

 doStuff(42)

Io is a message passing language, and since everything in Io is a message (excluding comments), each message is sent to a receiver. The above example demonstrates this well, but not fully. To describe this point better, let's look at the next example:

 System version

The above example demonstrates message passing in Io; the "version" message is sent to the "System" object.

Operators are a special case where the syntax is not as cut-and-dried as the above examples. The Io parser intercepts a set of operators defined by the interpreter, and translates them to method calls. For example, the following:

 1 + 5 * 8 + 1

translates to:

 1 +(5 *(8)) +(1)

All operators in Io are methods; the fact that they do not require explicit parentheses is a convenience. As you can see, there is also a little bit of operator precedence happening here, and the precedence levels are the same as with the C precedence levels.

Methods and blocks

In Io there are two ways of creating anonymous functions: methods and blocks. Between them, they are almost identical except for scope. While blocks have lexical scope, methods have dynamic scope.

Both method and block are higher-order functions.

Examples

The ubiquitous Hello world program:

 "Hello, world!" println

New objects are created by cloning objects. In Io specifically, a new, empty object is created and only the differences between it and its parent are stored within the new object; this behavior is known as differential inheritance. An example of this behavior is shown:

 A := Object clone         // creates a new, empty object named "A"

A simple non-recursive factorial function, in Io:

factorial := method(n,
    if(n == 0, return 1)
    res := 1
    Range 1 to(n) foreach(i, res = res * i)
)

Because assignment of res * i to res is the last action taken, the function implicitly returns the result and so an explicit return expression is not needed. The above demonstrates the usage of ranges, and doesn't use a for() loop, which would be faster.

References

^ "Io Releases". GitHub. Retrieved 2020-02-06.
^ Io Programming Guide
^ "Io Programming: Writing Addons". Wikibooks.org: open books for an open world. Retrieved 2023-06-22.
^ Tate, Bruce (2010). "Chapter 3: Io". Seven Languages in Seven Weeks: A Pragmatic Guide to Learning Programming Languages (1st ed.). Raleigh, North Carolina: Pragmatic Bookshelf. p. 60, 72. ISBN 978-1934356593.
^ "io guide". iolanguage.org. Retrieved 2023-06-22.