Ruby (programming language)

Ruby

Paradigm	multi-paradigm
Appeared in	1995
Designed by	Yukihiro Matsumoto
Developer	Yukihiro Matsumoto, et al.
Stable release	1.9.2-p0 (August 18, 2010; 5 months ago (2010-08-18))
Typing discipline	duck, dynamic, strong
Major implementations	Ruby MRI, YARV, JRuby, Rubinius, IronRuby, MacRuby, HotRuby
Influenced by	Smalltalk, Perl, Lisp, Scheme, Python, CLU, Eiffel, Ada, Dylan
Influenced	Groovy, Nu, Falcon, Ioke, Mirah
OS	Cross-platform
License	Ruby License GNU General Public License
Usual file extensions	.rb, .rbw
Website	www.ruby-lang.org
Ruby Programming at Wikibooks

Ruby is a dynamic, reflective, general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto. It was influenced primarily by Perl, Smalltalk, Eiffel, and Lisp.

Ruby supports multiple programming paradigms, including functional, object oriented, imperative and reflective. It also has a dynamic type system and automatic memory management; it is therefore similar in varying respects to Python, Perl, Lisp, Dylan, Pike, and CLU.

The standard 1.8.7 implementation is written in C, as a single-pass interpreted language. There is currently no specification of the Ruby language, so the original implementation is considered to be the de facto reference. As of 2010^[update], there are a number of complete or upcoming alternative implementations of the Ruby language, including YARV, JRuby, Rubinius, IronRuby, MacRuby, and HotRuby, each of which takes a different approach, with IronRuby, JRuby and MacRuby providing just-in-time compilation and MacRuby also providing ahead-of-time compilation. The official 1.9 branch uses YARV, as will 2.0 (development), and will eventually supersede the slower Ruby MRI.

History

Yukihiro Matsumoto, the creator of Ruby.

Ruby was conceived on February 24, 1993 by Yukihiro Matsumoto who wished to create a new language that balanced functional programming with imperative programming.^[1] Matsumoto has stated, "I wanted a scripting language that was more powerful than Perl, and more object-oriented than Python. That's why I decided to design my own language".^[2]

Etymology of the name "Ruby"

The name "Ruby" was decided on during an online chat session between Matsumoto and Keiju Ishitsuka on February 24, 1993, before any code had been written for the language.^[3] Initially two names were proposed: "Coral" and "Ruby", with the latter being chosen by Matsumoto in a later email to Ishitsuka.^[4] Matsumoto has later stated that a factor in choosing the name "Ruby" was because it was the birthstone of one of his colleagues.^[5]

First publication

The first public release of Ruby 0.95 was announced on Japanese domestic newsgroups on December 21, 1995.^[6][7] Subsequently three more versions of Ruby were released in two days.^[3] The release coincided with the launch of the Japanese language ruby-list mailing list which was the first mailing list for the new language.

Already present at this stage of development were many of the features familiar in later releases of Ruby, including object-oriented design, classes with inheritance, mixins, iterators, closures, exception handling, and garbage collection.^[8]

Ruby 1.0

Ruby reached version 1.0 on December 25, 1996.^[3]

Following the release of Ruby 1.3 in 1999 the first English language mailing list ruby-talk began,^[2] which signalled a growing interest in the language outside of Japan. In September 2000, the first English language book Programming Ruby was printed, which was later freely released to the public further widening the adoption of Ruby amongst English speakers.

Ruby on Rails

Around 2005, interest in the Ruby language surged in tandem with Ruby on Rails, a popular web application framework written in Ruby. Rails is frequently credited with making Ruby "famous" and the association is so strong that the two are sometimes conflated by programmers who are new to Ruby.^[9]

Ruby 1.9.2

The latest stable version of the reference implementation is 1.9.2.

Ruby 1.9.2 introduces many significant changes over version 1.8.6. Some examples are:

• Block local variables (variables that are local to the block in which they are declared)
• An additional lambda syntax (fun = ->(a,b) { puts a + b })
• Per-string character encodings are supported
• New socket API (IPv6 support)

Philosophy

Matsumoto has said that Ruby is designed for programmer productivity and fun, following the principles of good user interface design.^[10] He stresses that systems design needs to emphasize human, rather than computer, needs:^[11]

Ruby is said to follow the principle of least astonishment (POLA), meaning that the language should behave in such a way as to minimize confusion for experienced users. Matsumoto has said his primary design goal was to make a language which he himself enjoyed using, by minimizing programmer work and possible confusion. He has said that he had not applied the principle of least surprise to the design of Ruby,^[11] but nevertheless the phrase has come to be closely associated with the Ruby programming language. The phrase has itself been a source of surprise, as novice users may take it to mean that Ruby's behaviors try to closely match behaviors familiar from other languages. In a May 2005 discussion on the comp.lang.ruby newsgroup, Matsumoto attempted to distance Ruby from POLA, explaining that because any design choice will be surprising to someone, he uses a personal standard in evaluating surprise. If that personal standard remains consistent there will be few surprises for those familiar with the standard.^[12]

Matsumoto defined it this way in an interview:^[11]

Features

• Thoroughly object-oriented with inheritance, mixins and metaclasses ^[13]
• Dynamic typing and Duck typing
• Everything is an expression (even statements) and everything is executed imperatively (even declarations)
• Succinct and flexible syntax ^[14] that minimizes syntactic noise and serves as a foundation for domain specific languages ^[15]
• Dynamic reflection and alteration of objects to facilitate metaprogramming^[16]
• Lexical closures, Iterators and generators, with a unique block syntax ^[17]
• Literal notation for arrays, hashes, regular expressions and symbols
• Embedding code in strings (interpolation)
• Default arguments
• Four levels of variable scope (global, class, instance, and local) denoted by sigils and capitalization
• Automatic garbage collection
• First-class continuations
• Strict boolean coercion rules (everything is true except false and nil)
• Exception handling
• Operator overloading
• Built-in support for rational numbers, complex numbers and arbitrary-precision arithmetic
• Custom dispatch behavior (through method_missing and const_missing)
• Native threads and cooperative fibers
• Initial support for Unicode and multiple character encodings (still buggy as of version 1.9)^[18]
• Native plug-in API in C
• Interactive Ruby Shell (a REPL)
• Centralized package management through RubyGems
• Implemented on all major platforms
• Large standard library

Semantics

Ruby is object-oriented: every data type is an object, including classes and types that many other languages designate as primitives (such as integers, booleans, and "nil"). Every function is a method. Named values (variables) always designate references to objects, not the objects themselves. Ruby supports inheritance with dynamic dispatch, mixins and singleton methods (belonging to, and defined for, a single instance rather than being defined on the class). Though Ruby does not support multiple inheritance, classes can import modules as mixins. Procedural syntax is supported, but all methods defined outside of the scope of a particular object are actually methods of the Object class. Since this class is parent to every other class, the changes become visible to all classes and objects.

Ruby has been described as a multi-paradigm programming language: it allows procedural programming (defining functions/variables outside classes makes them part of the root, 'self' Object), with object orientation (everything is an object) or functional programming (it has anonymous functions, closures, and continuations; statements all have values, and functions return the last evaluation). It has support for introspection, reflection and metaprogramming, as well as support for interpreter-based^[19] threads. Ruby features dynamic typing, and supports parametric polymorphism.

According to the Ruby FAQ,^[20] "If you like Perl, you will like Ruby and be right at home with its syntax. If you like Smalltalk, you will like Ruby and be right at home with its semantics. If you like Python, you may or may not be put off by the huge difference in design philosophy between Python and Ruby/Perl."

Syntax

The syntax of Ruby is broadly similar to Perl and Python. Class and method definitions are signaled by keywords. In contrast to Perl, variables are not obligatorily prefixed with a sigil. When used, the sigil changes the semantics of scope of the variable. The most striking difference from C and Perl is that keywords are typically used to define logical code blocks, without braces (i.e., pair of { and }). For practical purposes there is no distinction between expressions and statements^[21]. Line breaks are significant and taken as the end of a statement; a semicolon may be equivalently used. Unlike Python, indentation is not significant.

One of the differences of Ruby compared to Python and Perl is that Ruby keeps all of its instance variables completely private to the class and only exposes them through accessor methods (attr_writer, attr_reader, etc.). Unlike the "getter" and "setter" methods of other languages like C++ or Java, accessor methods in Ruby are created with a single line of code via metaprogramming. As invocation of these methods does not require the use of parentheses, it is trivial to change an instance variable into a full function, without modifying a single line of code or having to do any refactoring achieving similar functionality to C# and VB.NET property members. Python's property descriptors are similar, but come with a tradeoff in the development process. If one begins in Python by using a publicly exposed instance variable and later changes the implementation to use a private instance variable exposed through a property descriptor, code internal to the class may need to be adjusted to use the private variable rather than the public property. Ruby removes this design decision by forcing all instance variables to be private, but also provides a simple way to declare set and get methods. This is in keeping with the idea that in Ruby, one never directly accesses the internal members of a class from outside of it. Rather one passes a message to the class and receives a response.

See the examples section for samples of code demonstrating Ruby syntax.

Deviations from behavior elsewhere

Some features which differ notably from languages such as C or Perl:

• The language syntax is sensitive to the capitalization of identifiers, in most cases treating capitalized variables as constants.
• The sigils $ and @ do not indicate variable data type as in Perl, but rather function as scope resolution operators.
• To denote a floating point without a decimal component, one must follow with a zero digit (99.0) or an explicit conversion (99.to_f). It is insufficient to append a dot (99.) since numbers are susceptible to method syntax.
• Boolean evaluation of non-boolean data is strict: 0, "" and [] are all evaluated to true. In C, the expression 0 ? 1 : 0 evaluates to 0 (i.e. false). In Ruby, however, it yields 1, as all numbers evaluate to true; only nil and false evaluate to false. A corollary to this rule is that Ruby methods by convention — for example, regular-expression searches — return numbers, strings, lists, or other non-false values on success, but nil on failure. This convention is also used in Smalltalk, where only the special objects true and false can be used in a boolean expression.
• Versions prior to 1.9 use plain integers to represent single characters, much like C. This may cause surprises when slicing strings: "abc"[0] yields 97 (the ASCII code of the first character in the string); to obtain "a" use "abc"[0,1] (a substring of length 1) or "abc"[0].chr.
• The notation statement until expression, unlike other languages' equivalent statements (e.g. do { statement } while (!(expression)); in C/C++/...), actually never runs the statement if the expression is already true. This is because statement until expression is actually syntactic sugar over until expression; statement; end, the equivalent of which in C/C++ is while (!(expression)) { statement; }, just like statement if expression is an equivalent to if (expression) { statement; }. However, the notation begin statement end until expression in Ruby will in fact run the statement once even if the expression is already true, acting similar to the "do-while" of other languages.
• Because constants are references to objects, changing what a constant refers to generates a warning, but modifying the object itself does not. For example, Greeting << " world!" if Greeting == "Hello" does not generate an error or warning. This is similar to final variables in Java or a const pointer to a non-const object in C++, but Ruby provides the functionality to "freeze" an object, unlike Java.

Some features which differ notably from other languages:

• The usual operators for conditional expressions, and and or, do not follow the normal rules of precedence: and does not bind tighter than or. Ruby also has expression operators || and && which work as expected.

A list of so-called gotchas may be found in Hal Fulton's book The Ruby Way, 2nd ed (ISBN 0-672-32884-4), Section 1.5. A similar list in the 1st edition pertained to an older version of Ruby (version 1.6), some problems of which have been fixed in the meantime. retry, for example, now works with while, until, and for, as well as iterators.

Interaction

The Ruby official distribution also includes "irb", an interactive command-line interpreter which can be used to test code quickly. The following code fragment represents a sample session using irb:

$ irb
irb(main):001:0> puts "Hello, World"
Hello, World
=> nil
irb(main):002:0> 1+2
=> 3

Examples

The following examples can be run in a Ruby shell such as Interactive Ruby Shell or saved in a file and run from the command line by typing ruby <filename>.

Classic Hello world example:

puts "Hello World!"

Some basic Ruby code:

# Everything, including a literal, is an object, so this works:
-199.abs                                                # 199
"ruby is cool".length                                   # 12
"Your mother is nice.".index("u")                       # 2
"Nice Day Isn't It?".downcase.split("").uniq.sort.join  # " '?acdeinsty"

Conversions:

puts "What's your favorite number?"
number = gets.chomp
output_number = number.to_i + 1
puts output_number.to_s + ' is a bigger and better favorite number.'

Strings

There are a variety of methods for defining strings in Ruby.

The following assignments are equivalent and support String interpolation:

a = "\nThis is a double-quoted string\n"
a = %Q{\nThis is a double-quoted string\n}
a = %{\nThis is a double-quoted string\n}
a = %/\nThis is a double-quoted string\n/
a = <<-BLOCK
 
This is a double-quoted string
BLOCK

The following assignments are equivalent and produce raw strings:

a = 'This is a single-quoted string'
a = %q{This is a single-quoted string}

Collections

Constructing and using an array:

a = [1, 'hi', 3.14, 1, 2, [4, 5]]
 
puts a[2]           # 3.14
puts a.[](2)        # 3.14
puts a.reverse      # [[4, 5], 2, 1, 3.14, 'hi', 1]
puts a.flatten.uniq # [1, 'hi', 3.14, 2, 4, 5]

Constructing and using an associative array (called hashes in Ruby):

hash = { :water => 'wet', :fire => 'hot' }
puts hash[:fire] # Prints:  hot
 
hash.each_pair do |key, value| # Or:  hash.each do |key, value|
  puts "#{key} is #{value}"
end
 
# Prints:  water is wet
#          fire is hot
 
hash.delete :water # Deletes :water => 'wet'
hash.delete_if {|key,value| value=='hot'} # Deletes :fire => 'hot'

Blocks and iterators

The two syntaxes for creating a code block:

{ puts "Hello, World!" } # Note the { braces }
#or
do puts "Hello, World!" end

When a code block is created it is always attached to a method as an optional block argument.

Parameter-passing a block to be a closure:

# In an object instance variable (denoted with '@'), remember a block.
def remember(&a_block)
  @block = a_block
end
 
# Invoke the above method, giving it a block which takes a name.
remember {|name| puts "Hello, #{name}!"}
 
# When the time is right (for the object) -- call the closure!
@block.call("Jon")
# => "Hello, Jon!"

Creating an anonymous function:

proc {|arg| print arg}
Proc.new {|arg| print arg}
lambda {|arg| print arg}

Returning closures from a method:

def create_set_and_get(initial_value=0) # Note the default value of 0
  closure_value = initial_value
  return Proc.new {|x| closure_value = x}, Proc.new { closure_value }
end
 
setter, getter = create_set_and_get  # ie. returns two values
setter.call(21)
getter.call # => 21
 
#You can also use a parameter variable as a binding for the closure. 
#So the above can be rewritten as...
 
def create_set_and_get(closure_value=0)
  proc {|x| closure_value = x } , proc { closure_value }
end

Yielding the flow of program control to a block which was provided at calling time:

def use_hello
  yield "hello"
end
 
# Invoke the above method, passing it a block.
use_hello {|string| puts string} # => 'hello'

Iterating over enumerations and arrays using blocks:

array = [1, 'hi', 3.14]
array.each {|item| puts item }
# => 1
# => 'hi'
# => 3.14
 
array.each_index {|index| puts "#{index}: #{array[index]}" }
# => 0: 1
# => 1: 'hi'
# => 2: 3.14
 
(3..6).each {|num| puts num }
# => 3
# => 4
# => 5
# => 6

A method such as inject() can accept both a parameter and a block. Inject iterates over each member of a list, performing some function on it while retaining an aggregate. This is analogous to the foldl function in functional programming languages. For example:

[1,3,5].inject(10) {|sum, element| sum + element} # => 19

On the first pass, the block receives 10 (the argument to inject) as sum, and 1 (the first element of the array) as element; this returns 11. 11 then becomes sum on the next pass, which is added to 3 to get 14. 14 is then added to 5, to finally return 19.

Blocks work with many built-in methods:

File.open('file.txt', 'w') do |file| # 'w' denotes "write mode".
  file.puts 'Wrote some text.'
end                                  # File is automatically closed here
 
File.readlines('file.txt').each do |line|
  puts line
end
# => Wrote some text.

Using an enumeration and a block to square the numbers 1 to 10:

(1..10).collect {|x| x*x} # => [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Classes

The following code defines a class named Person. In addition to 'initialize', the usual constructor to create new objects, it has two methods: one to override the <=> comparison operator (so Array#sort can sort by age) and the other to override the to_s method (so Kernel#puts can format its output). Here, "attr_reader" is an example of metaprogramming in Ruby: "attr_accessor" defines getter and setter methods of instance variables, "attr_reader" only getter methods. Also, the last evaluated statement in a method is its return value, allowing the omission of an explicit 'return'.

class Person
  attr_reader :name, :age
  def initialize(name, age)
    @name, @age = name, age
  end
  def <=>(person) # Comparison operator for sorting
    @age <=> person.age
  end
  def to_s
    "#@name (#@age)"
  end
end
 
group = [
  Person.new("Bob", 33), 
  Person.new("Chris", 16), 
  Person.new("Ash", 23) 
]
 
puts group.sort.reverse

The above prints three names in reverse age order:

Bob (33)
Ash (23)
Chris (16)

Open classes

In Ruby, classes are never closed: you can always add methods to an existing class. This applies to the classes you write as well as the standard, built-in classes. All you have to do is open up a class definition for an existing class, and the new contents you specify will be added to whatever's there. A simple example of adding a new method to the standard library's Time class:

# re-open Ruby's Time class
class Time
  def yesterday
    self - 86400
  end
end
 
today = Time.now # => Thu Aug 14 16:51:50 +1200 2008
yesterday = today.yesterday # => Wed Aug 13 16:51:50 +1200 2008

Adding methods to previously defined classes is often called monkey-patching. This practice, however, can lead to possible collisions of behavior and subsequent unexpected results, and is a concern for code scalability if performed recklessly.

Exceptions

An exception is raised with a raise call:

raise

An optional message can be added to the exception:

raise "This is a message"

You can also specify which type of exception you want to raise:

raise ArgumentError, "Illegal arguments!"

Alternatively, you can pass an exception instance to the raise method:

raise ArgumentError.new("Illegal arguments!")

This last construct is useful when you need to raise a custom exception class featuring a constructor which takes more than one argument:

class ParseError < Exception
  def initialize input, line, pos
    super "Could not parse '#{input}' at line #{line}, position #{pos}"
  end
end
 
raise ParseError.new("Foo", 3, 9)

Exceptions are handled by the rescue clause. Such a clause can catch exceptions which inherit from StandardError. Also supported for use with exceptions are else and ensure

begin
# Do something
rescue
# Handle exception
else
# Do this if no exception was raised
ensure
# Do this whether or not an exception was raised
end

It is a common mistake to attempt to catch all exceptions with a simple rescue clause. To catch all exceptions one must write:

begin
# Do something
rescue Exception # don't write just rescue -- that only catches StandardError, a subclass of Exception
# Handle exception
end

Or catch particular exceptions:

begin
# ...
rescue RuntimeError 
# handling
end

It is also possible to specify that the exception object be made available to the handler clause:

begin
# ...
rescue RuntimeError => e
# handling, possibly involving e, such as "print e.to_s"
end

Alternatively, the most recent exception is stored in the magic global $!.

You can also catch several exceptions:

begin
# ...
rescue RuntimeError, Timeout::Error => e
# handling, possibly involving e
end

Metaprogramming

Ruby code can programmatically modify, at runtime, aspects of its own structure that would be fixed in more rigid languages, such as class and method definitions. This sort of metaprogramming can be used to write more concise code and effectively extend the language.

For example, the following Ruby code generates new methods for the built-in String class, based on a list of colors. The methods wrap the contents of the string with an HTML tag styled with the respective color.

COLORS = { :black   => "000",
           :red     => "f00",
           :green   => "0f0",
           :yellow  => "ff0",
           :blue    => "00f",
           :magenta => "f0f",
           :cyan    => "0ff",
           :white   => "fff" }
 
class String
  COLORS.each do |color,code|
    define_method "in_#{color}" do
      "<span style=\"color: ##{code}\">#{self}</span>"
    end
  end
end

The generated methods could then be used like so:

"Hello, World!".in_blue
 
   => "<span style=\"color: #00f\">Hello, World!</span>"

To implement the equivalent in many other languages, the programmer would have to write each method (in_black, in_red, in_green, etc.) by hand.

Some other possible uses for Ruby metaprogramming include:

• intercepting and modifying method calls
• implementing new inheritance models
• dynamically generating classes from parameters
• automatic object serialization
• interactive help and debugging

More examples

More sample Ruby code is available as algorithms in the following articles:

• Exponentiating by squaring
• Trabb Pardo-Knuth algorithm

Implementations

The newest version of Ruby, the recently released version 1.9, has a single working implementation written in C that utilizes a Ruby-specific virtual machine.

Ruby version 1.8 has two main implementations: The official Ruby interpreter often referred to as the Matz's Ruby Interpreter or MRI, which is the most widely used, and JRuby, a Java-based implementation that runs on the Java Virtual Machine.

There are other less-known or upcoming implementations such as Cardinal (an implementation for the Parrot virtual machine), IronRuby (alpha version available since July 24, 2008),^[22] MacRuby (0.6 experimental), MagLev, Rubinius, Ruby.NET, XRuby and HotRuby (runs Ruby source code on a web browser and Flash).

The maturity of Ruby implementations tends to be measured by their ability to run the Ruby on Rails (Rails) framework, because it is a complex framework to implement, and it uses many Ruby-specific features. The point when a particular implementation achieves this goal is called The Rails singularity. As of May 2010, only the reference implementation (MRI) and JRuby are able to run Rails unmodified in a production environment. Rubinius recently released 1.0 and can run Rails,^[23] but may not be being used for production sites yet.^[24] IronRuby^[25][26] is starting to be able to run Rails test cases, but is still far from being production-ready.

Ruby is available on many operating systems such as Linux, Mac OS X, Microsoft Windows, Windows CE and most flavors of Unix.

Ruby 1.9 has recently been ported onto Symbian OS 9.x.^[27]

Repositories and libraries

The Ruby Application Archive (RAA), as well as RubyForge, serve as repositories for a wide range of Ruby applications and libraries, containing more than seven thousand items. Although the number of applications available does not match the volume of material available in the Perl or Python community, there are a wide range of tools and utilities which serve to foster further development in the language.

RubyGems has become the standard package manager for Ruby libraries. It is very similar in purpose to Perl's CPAN, although its usage is more like apt-get.

Recently, many new and existing libraries have found a home on GitHub, which is focused on Git and used to have native support for RubyGems packaging.^[28]

See also

• Duck typing
• Comparison of programming languages
• Ruby MRI, the reference C implementation
• JRuby
• MacRuby
• IronRuby
• Rubinius
• XRuby
• Ruby on Rails
• Why's (poignant) Guide to Ruby - an online ruby textbook in graphic novel format
• Ruby wikibook

References

Bibliography

External links

• Ruby language home page
• Ruby documentation site
• The Great Ruby Shootout (December 2008): Ruby implementations comparison.
• Collingbourne, Huw (June 17 2006), The Little Book Of Ruby, free PDF eBook 1.1MB, pp. 87
• Collingbourne, Huw (April 18 2009), The Book Of Ruby, free PDF eBook 2.9MB, pp. 425
• Ruby.on-page.net — simple Ruby manual with many samples
• Ruby at the Open Directory Project
• Ruby User Guide By Matz, the creator of Ruby. Translated into English.
• Ruby From Other Languages
• Wiki: Ruby language and implementation specification
• Writing C Extensions to Ruby (MRI 1.8)
• RubyFlow: Community Filtered Ruby News
• The Ruby Reflector Automated Ruby News