Editing Io (programming language)

{{Short description|Prototype-based programming language}}
{{Multiple issues|
{{More citations needed|date=August 2014}}
{{Primary sources|date=August 2014}}
}}

{{Infobox programming language
| name = Io
| logo = Io-logo.svg
| logo_size = 150px
| paradigms = [[Object-oriented programming|object-oriented]] [[Prototype-based programming|prototype-based]]
| designer = Steve Dekorte
| developers = Steve Dekorte, Jonathan Wright, Jeremy Tregunna
| released = {{Start date and age|2002}}
| latest release version = 20170906
| latest release date = {{Start date and age|2017|09|06}}<ref name="Releases">{{cite web |url=https://github.com/IoLanguage/io/releases |title=Io Releases |website=[[GitHub]] |access-date=2020-02-06}}</ref>
| latest preview version = 2019.05.22-alpha
| latest preview date = {{Start date and age|2019|05|22}}
| typing = [[Type system|dynamic]], [[Strong and weak typing|strong]]
| platform = [[IA-32]], [[x86-64]]; [[ARM architecture family|ARM]]; [[.NET]] [[Common Language Runtime|CLR]]
| operating system = [[Microsoft Windows|Windows]], [[macOS]]
| license = [[BSD licenses|BSD 3-clause]]
| implementations = [http://iolanguage.org/ Io] [https://web.archive.org/web/20110716175739/http://synrc.com/io/index.htm Io.NET]
| influenced by = [[Smalltalk]], [[NewtonScript]], [[Self (programming language)|Self]], [[Lua (programming language)|Lua]], [[Lisp (programming language)|Lisp]], [[Python (programming language)|Python]], [[Actor model middle history#Actor programming languages|Act1]]
| influenced = Ioke, Potion
}}

'''Io''' is a pure [[object-oriented programming]] [[Programming language|language]] inspired by [[Smalltalk]], [[Self (programming language)|Self]], [[Lua (programming language)|Lua]], [[Lisp (programming language)|Lisp]], [[Actor model middle history#Actor programming languages|Act1]], and [[NewtonScript]].<ref>[http://iolanguage.org/guide/guide.html#Introduction-Overview Io Programming Guide]</ref> Io has a [[Prototype-based programming|prototype-based]] object model similar to those in Self and NewtonScript, eliminating the distinction between [[Object (computer science)|instance]] and [[Class (computer programming)|class]]. Like Smalltalk, everything is an object and it uses [[Type system|dynamic typing]]. Like Lisp, programs are just data trees. Io uses [[actor model|actor]]s for concurrency.

Remarkable features of Io are its minimal size and openness to using external code resources.<ref>{{Cite web |title=Io Programming: Writing Addons |url=https://en.wikibooks.org/wiki/Io_Programming/Writing_Addons |access-date=2023-06-22 |website=Wikibooks.org: open books for an open world |language=en}}</ref> 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.<ref name="Tate_2010"/>

== Philosophy ==
Io's goal is to explore conceptual unification and [[Dynamic programming language|dynamic languages]], so the tradeoffs tend to favor simplicity and flexibility over performance.

== Features ==
* Pure [[Object-oriented programming|object-oriented]] based on [[Prototype-based programming|prototypes]]
* Code-as-data, [[Homoiconicity|homoiconic]] 
* [[Lazy evaluation]] of function parameters
* [[Higher-order function]]s
* [[Type introspection|Introspection]], [[Reflective programming|reflection]] and [[metaprogramming]]
* [[Actor model|Actor-based]] [[Concurrency (computer science)|concurrency]]
* [[Coroutine]]s
* [[Exception handling]]
* Incremental [[Garbage collection (computer science)|garbage collecting]] supporting [[Weak reference|weak links]]
* Highly [[Porting|portable]]
* Shared [[Library (computer science)|library]], [[dynamic-link library]] (DLL), dynamic loading on most platforms
* Small [[virtual machine]]

== Syntax ==
{{Unreferenced section|date=June 2013}}
In its simplest form, Io [[Syntax (programming languages)|syntax]] is composed of one identifier:<ref>{{Cite web |title=io guide |url=https://iolanguage.org/guide/guide.html#Syntax |access-date=2023-06-22 |website=iolanguage.org}}</ref>

<syntaxhighlight lang="io">
 doStuff
</syntaxhighlight>

Assuming the above doStuff is a [[Method (computer programming)|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:

<syntaxhighlight lang="io">
 doStuff(42)
</syntaxhighlight>

Io is a [[message passing]] language, and since everything in Io is a message (excluding [[Comment (computer programming)|comment]]s), 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:

<syntaxhighlight lang="io">
 System version
</syntaxhighlight>

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

[[Operator (computer programming)|Operators]] are a special case where the syntax is not as cut-and-dried as the above examples. The Io [[Syntax analysis|parser]] intercepts a set of operators defined by the interpreter, and translates them to method calls. For example, the following:

<syntaxhighlight lang="io">
 1 + 5 * 8 + 1
</syntaxhighlight>

translates to:

<syntaxhighlight lang="io">
 1 +(5 *(8)) +(1)
</syntaxhighlight>

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 [[Operator precedence in C|C precedence levels]].

=== Methods and blocks ===
In Io there are two ways of creating [[anonymous function]]s: methods and blocks. Between them, they are almost identical except for [[Scope (programming)|scope]]. While blocks have [[lexical scope]], methods have [[dynamic scope]].

Both ''method'' and ''block'' are [[higher-order function]]s.

=== Examples ===
The ubiquitous [[Hello world program]]:

<syntaxhighlight lang="io">
 "Hello, world!" println
</syntaxhighlight>

New objects are created by [[Cloning (programming)|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:

<syntaxhighlight lang="io">
 A := Object clone         // creates a new, empty object named "A"
</syntaxhighlight>

A simple non-recursive factorial function, in Io:

<syntaxhighlight lang="io">
factorial := method(n,
    if(n == 0, return 1)
    res := 1
    Range 1 to(n) foreach(i, res = res * i)
)
</syntaxhighlight>

Because assignment of <code>res * i</code> to <code>res</code> 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 [[Range (computer programming)|ranges]], and doesn't use a <code>for()</code> loop, which would be faster.

== References ==
{{Reflist|refs=
<ref name="Tate_2010">{{cite book |last=Tate |first=Bruce |year=2010 |title=Seven Languages in Seven Weeks: A Pragmatic Guide to Learning Programming Languages |location=Raleigh, North Carolina |chapter=Chapter 3: Io |publisher=[[Pragmatic Bookshelf]] |page=[https://archive.org/details/sevenlanguagesin00tate/page/60 60, 72] |isbn=978-1934356593 |edition=1st |chapter-url-access=registration |chapter-url=https://archive.org/details/sevenlanguagesin00tate/page/60}}</ref>
}}

== External links ==
* {{Official website|iolanguage.org}}
* {{GitHub|IoLanguage|Io}}
* [https://web.archive.org/web/20121012010044/http://synrc.com/research/io/index.htm Io at Synrc Research Center]
* {{GitHub|bekkopen/jasmineio|Jasmine.Io}}, Behavior Driven Development (BDD) testing framework for Io

{{DEFAULTSORT:Io (Programming Language)}}
[[Category:Programming languages]]
[[Category:Dynamic programming languages]]
[[Category:Dynamically typed programming languages]]
[[Category:Prototype-based programming languages]]
[[Category:Object-oriented programming languages]]
[[Category:Programming languages created in 2002]]
[[Category:Scripting languages]]
[[Category:2002 software]]
[[Category:Homoiconic programming languages]]