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Ruby
Paradigm	multi-paradigm (functional, imperative, object-oriented (class-based))
Appeared in	1995
Designed by	Yukihiro Matsumoto
Developer	Yukihiro Matsumoto (among others)
Latest release	1.9.0/ December 25, 2007
Typing discipline	dynamic ("duck")
Major implementations	Ruby, JRuby, Rubinius, IronRuby
Influenced by	Smalltalk, Perl, Lisp, Scheme, Python, CLU, Eiffel, Ada, Dylan
Influenced	Groovy
OS	Cross-platform
License	Ruby License and GPL
Website	www.ruby-lang.org

Ruby is a reflective, dynamic, object-oriented programming language. It combines syntax inspired by Perl with Smalltalk-like object-oriented features, and also shares some features with Python, Lisp, Dylan, and CLU. Ruby is a single-pass interpreted language. Its official implementation is free software written in C.

Yukihiro Matsumoto, the creator of Ruby.

The language was created by Yukihiro "Matz" Matsumoto, who started working on Ruby on February 24, 1993, and released it to the public in 1995. "Ruby" was named as a gemstone because of a joke within Matsumoto's circle of friends alluding to Perl's name ^[1].

As of December 2007, the latest stable version is 1.8.6. Ruby 1.9.0 was released in December, but it is considered a development release. Poor performance of the Ruby implementation prior to 1.9 in comparison to other more entrenched programming languages led to the development of several virtual machines for Ruby. These include JRuby, a port of Ruby to the Java platform, IronRuby, an implementation for the .NET Framework produced by Microsoft, and Rubinius, an interpreter modeled after self-hosting Smalltalk virtual machines. The main developers have thrown their weight behind the virtual machine provided by the YARV project, which was merged into the Ruby source tree on 31 December 2006, and released as part of Ruby 1.9.

The language's creator, Yukihiro "Matz" Matsumoto, has said that Ruby is designed for programmer productivity and fun, following the principles of good user interface design.^[2] He stresses that systems design needs to emphasize human, rather than computer, needs ^[3]:

Ruby is said to follow the principle of least surprise (POLS), meaning that the language should behave in such a way as to minimize confusion for experienced users. Matz has said his primary design goal was to make a language that he himself enjoyed using, by minimizing programmer work and possible confusion. He has said he hadn't applied the principle of least surprise to the design of Ruby,^[4] 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, Matz attempted to distance Ruby from POLS, explaining that since 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. [1]

Matz defined it this way in an interview [2]:

Ruby is object-oriented: every data type is an object, including even 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 functionally (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^[5] threads. Ruby features dynamic typing, and supports parametric polymorphism.

According to the Ruby FAQ ^[6], "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." ^[7]

• object-oriented
• four levels of variable scope: global, class, instance, and local
• exception handling
• iterators and closures (based on passing blocks of code)
• native, Perl-like regular expressions at the language level
• operator overloading
• automatic garbage collecting
• highly portable
• cooperative multi-threading on all platforms using green threads
• DLL/shared library dynamic loading on most platforms
• introspection, reflection and metaprogramming
• large standard library
• supports dependency injection
• supports object runtime alteration^[8]
• continuations and generators (examples in RubyGarden: continuations and generators)

Ruby currently lacks full support for Unicode, though it has partial support for UTF-8.

See also: Interactive Ruby Shell

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

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 }). 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 can be written with a single line of code. 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.

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

• Names that begin with a capital letter are treated as constants, so local variables should begin with a lowercase letter.
• The symbols $ @ are not sigils as in Perl, but rather function as scope resolution operators.
• To denote floating point numbers, one must follow with a zero digit (99.0) or an explicit conversion (99.to_f). It is insufficient to append a dot (99.) because 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 (e.g., mismatch). 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 lack a character data type (compare to C, which provides type char for characters). This may cause surprises when slicing strings: "abc"[0] yields 97 (an integer, representing 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", despite the English-language implication that the statement would be executed at least once, which follows the precedent used in other languages' equivalent statements (e.g. "do { statement } while (not(expression));" in C/C++/...), actually never runs the statement if the expression is already true.
• Because constants are references to objects, changing what a constant refers to generates a warning, but modifying the object itself does not. For example, if Greeting = "Hello" then Greeting << " world!" does not generate an error or warning. This is similar to final variables in Java, but Ruby does also have the functionality to "freeze" an object, unlike Java.

• In terms of speed, Ruby's performance is inferior to that of many compiled languages (as is any interpreted language) and other major scripting languages such as Python and Perl^[9]. However, in future releases (current revision: 1.9), Ruby will be bytecode compiled to be executed on YARV (Yet Another Ruby VM). Currently, Ruby's memory footprint for the same operations is higher than Perl's and Python's.^[9]
• Omission of parentheses around method arguments may lead to unexpected results if the methods take multiple parameters. Note that the Ruby developers have stated that omission of parentheses on multi-parameter methods may be disallowed in future Ruby versions, the Ruby interpreter currently (Nov 2007) throws a warning which encourages the writer to not omit (), to avoid ambiguous meaning of code. Not using () is however still common practise, and can be especially nice to use Ruby as a human readable domain-specific language itself, along with the method called method_missing().

A list of "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.

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 .

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
"Rick".index("c")                                       # 2
"Nice Day Isn't It?".downcase.split(//).sort.uniq.join  # " '?acdeinsty"

Conversions:

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

Constructing and using an array:

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

Constructing and using a hash:

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_if {|key, value| key == :water} # Deletes :water => 'wet'

The two syntaxes for creating a code block:

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

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 that takes a name.
remember {|name| puts "Hello, #{name}!"}
 
# When the time is right (for the object) -- call the closure!
@block.call("Jon")
# => "Hello, Jon!"

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

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.to_s + ": " + array[index] }
# => 0: one
# => 1: two
# => 2: three
 
(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 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]

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
        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
        attr_reader :name, :age
end
 
group = [
        Person.new("Jon", 20), 
        Person.new("Marcus", 63), 
        Person.new("Ash", 16) 
]
 
puts group.sort.reverse

The above prints three names in reverse age order:

Marcus (63)
Jon (20)
Ash (16)

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 constuct 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 that inherit from StandardError:

begin
# Do something
rescue
# Handle exception
end

Note that 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 -- this 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

Or catch an array of exceptions:

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

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

• Exponentiating by squaring
• Ruby associative arrays
• Trabb Pardo-Knuth algorithm
• Miller-Rabin primality test

Ruby has two main implementations: The official Ruby interpreter (often referred to as the MRI or Matz's Ruby Interpreter), which is the most widely used, and JRuby, a Java-based implementation.

There are other less known implementations such as IronRuby (pre-alpha sources available on August 31st, 2007^[10]), Rubinius, Ruby.NET, XRuby and YARV. YARV is Ruby 1.9's official new virtual machine and is no longer a separate project.

Ruby is available for the following operating systems:

Other ports may also exist.

The Ruby interpreter and libraries are distributed disjointedly (dual licensed) under the free and open source licenses GPL and Ruby License ^[11].

Version 1.8, the current stable version of the interpreter, has some limitations, which include:

• Performance -- the Ruby interpreter's performance trails that of comparable languages such as Perl, PHP, and Python [3] [4], mainly due to the design of the interpreter: To execute Ruby code, the interpreter builds a syntax tree from the source code and then evaluates the syntax tree directly, instead of first compiling it into more efficiently executable form.
• Threading -- the Ruby threading model uses green threads [5], and its model has some inherent limitations that render it difficult to use or unsafe in some scenarios.[6].
• Unicode -- Ruby does not yet have native support for Unicode or multibyte strings [7].

Ruby 2.0 aims to address all of the aforementioned problems:

• A new, faster interpreter, YARV, a virtual machine that executes bytecode instructions, which are in turn compiled into native processor instructions using a JIT compiler.
• Native threads will be used instead of green threads.[8]
• Full support for Unicode strings.

Version 1.9, the codebase that is considered the development version of 2.0, was released [9] on 26 December 2007.

Some problems that may not be solved in version 2.0 include:

• Ruby still lacks a specification, the current reference specification being the de facto C implementation. [10] [11] .

The Ruby Application Archive (RAA), as well as RubyForge, serve as repositories for a wide range of Ruby applications and libraries, containing more than two 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.

Free software Portal

• Duck typing
• Comparison of programming languages
• Watir
• JRuby
• IronRuby
• Rubinius
• XRuby
• Ruby On Rails

Wikibooks has a book on the topic of

Ruby Programming

At Wikiversity, you can learn about:

Topic:Ruby

• Ruby language home page
• Ruby.on-page.net — the simplest Ruby manual with many samples
• A Ruby documentation community
• Ruby documentation site
• Ruby programming language at the Open Directory Project
• Ruby User Guide Mirror
• Ruby From Other Languages
• Wiki: Ruby language and implementation specification
• Writing C Extensions to Ruby (MRI 1.8)