Ruby (programming language)

Ruby

File:Ruby-(programming-language)-logo-2008.png
Paradigm	multi-paradigm
Designed by	Yukihiro Matsumoto
Developer	Yukihiro Matsumoto (among others)
First appeared	1995
Stable release	1.9.0 / December 26, 2007 (2007-12-26)
Typing discipline	dynamic ("duck")
OS	Cross-platform
License	Ruby License GNU General Public License
Website	www.ruby-lang.org
Major implementations
Ruby MRI, JRuby
Influenced by
Smalltalk, Perl, Lisp, Scheme, Python, CLU, Eiffel, Ada, Dylan
Influenced
Groovy, Nu

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 initially developed and designed by Yukihiro "Matz" Matsumoto. It is based on Perl, Smalltalk, Eiffel, Ada, and Lisp

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

The standard 1.8.6 (stable) 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 2008, there are a number of complete or upcoming alternative implementations of the Ruby language, including YARV, JRuby, Rubinius, IronRuby, and MacRuby, each of which takes a different approach, with JRuby and IronRuby providing just-in-time compilation functionality. The official 1.9 (development) branch uses YARV, and so will 2.0, 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. According to Matsumoto he "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".

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. Initially two names were proposed: "Coral" and "Ruby", with the latter the one being chosen as the name by Matsumoto in a later email to Ishitsuka. Matsumoto has later stated that a factor in choosing the name "Ruby" was because it was the birthstone of one of his colleagues. Later it was recognized that pearl is the birthstone for the month of June, while ruby is the birthstone for July, implying Ruby as the successor of Perl.

First publication

The first public release of Ruby 0.95 was announced on Japanese domestic newsgroups on December 21, 1995. Subsequently three more versions of Ruby were released in two days. 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.

Ruby 1.0

Ruby reached version 1.0 on December 25, 1996.

Following the release of Ruby 1.3 in 1999 the first English language mailing list ruby-talk began, which signaled 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 releasedto the public further widening the adoption of Ruby amongst native English speakers.

Ruby 1.8.7

As of June 2008, the latest stable version of the reference implementation is 1.8.7. It adds some of the improvements to the standard library found in the 1.9.0 developmental release to the previous stable version, 1.8.6. It does not include some of the syntax changes.

Ruby 1.9.1

The next version will be Ruby 1.9.1, and it is scheduled to be released on 25 January 2009.

Ruby 1.9.1 introduces a lot of changes, compared to version 1.8.6, such as:

• 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

Philosophy

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. He stresses that systems design needs to emphasize human, rather than computer, needs:

Often people, especially computer engineers, focus on the machines. They think, "By doing this, the machine will run faster. By doing this, the machine will run more effectively. By doing this, the machine will something something something." They are focusing on machines. But in fact we need to focus on humans, on how humans care about doing programming or operating the application of the machines. We are the masters. They are the slaves.

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. 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 he had not applied the principle of least surprise to the design of Ruby, 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 POLS, 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.

Matsumoto defined it this way in an interview:

Everyone has an individual background. Someone may come from Python, someone else may come from Perl, and they may be surprised by different aspects of the language. Then they come up to me and say, 'I was surprised by this feature of the language, so Ruby violates the principle of least surprise.' Wait. Wait. The principle of least surprise is not for you only. The principle of least surprise means principle of least my surprise. And it means the principle of least surprise after you learn Ruby very well. For example, I was a C++ programmer before I started designing Ruby. I programmed in C++ exclusively for two or three years. And after two years of C++ programming, it still surprises me.

Features

• Object-oriented, "everything is an object"
• Six levels of variable scope: global, class, class instance, instance, local, and block
• 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
• Continuations and generators

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

Semantics

Ruby is object-oriented: every data type is an object, including classes and types which 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 threads. Ruby features dynamic typing, and supports parametric polymorphism.

According to the Ruby FAQ, "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. 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.

"Gotchas"

Language comparison

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

• Names which begin with a capital letter are treated as constants, so local variables should begin with a lowercase letter.
• The sigils $ and @ do not indicate variable data type 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, unlike 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. This is because statement until expression is actually syntactic sugar over until expression; statement; end, the equivalent of which in C/C++ is while (not(expression)) statement; just like statement if expression is an equivalent to if expression; statement; end. However, the notation begin statement end until expression in Ruby will in fact run the statement once even 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, Greeting << " world!" if Greeting == "Hello" 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.

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.

Language features

• Omission of parentheses around method arguments may lead to unexpected results if the methods take multiple parameters. The Ruby developers have stated that omission of parentheses on multi-parameter methods may be disallowed in future Ruby versions; the current (November 2007) Ruby interpreter throws a warning which encourages the writer not to omit (), to avoid ambiguous meaning of code. Not using () is still common practice, and can be especially nice to use Ruby as a human readable domain-specific programming 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.

Interaction

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

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
"Rick Astley".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.'

Strings

There are a variety of methods of defining strings in Ruby

The below conventions are equivalent for double quoted strings:

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

This is a multi-line double quoted string
BLOCK
a = %/\nThis is a double quoted string\n/

The below conventions are equivalent for single quoted 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]]
 
p a[2]           # 3.14
p a.[](2)        # 3.14
p a.reverse      # [[4, 5], 2, 1, 3.14, 'hi', 1]
p a.flatten.uniq # [1, 'hi', 3.14, 2, 4, 5]

Constructing and using an associative array:

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

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!"

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 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)

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:

begin
# Do something
rescue
# Handle exception
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

Or catch an array of exceptions:

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

Metaprogramming

Ruby provides a programmer the ability to add to or modify methods in the language's Standard Library during runtime, as well for a Ruby program to modify itself during its operation without generating source code, techniques known as metaprogramming. 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

More examples

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

• Exponentiating by squaring
• Trabb Pardo-Knuth algorithm

Implementations

See also: Ruby MRI § Operating systems

Ruby 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 IronRuby (alpha version available since July 24, 2008), Rubinius, Ruby.NET, XRuby, YARV, Cardinal and MagLev. YARV is Ruby 1.9's official new virtual machine and is no longer a separate project.

The maturity of Ruby implementations tend to be measured by their ability to run Rails (because this is a complex framework to implement, and it uses a lot of Ruby-specific features). The point when a particular implementation achieves this goal is called The Rails singularity. As of May 2008, only the reference implementation (MRI) and JRuby are able to run Rails unmodified in a production environment. IronRuby and Rubinius start to be able to run Rails test cases, but they still are far from production ready for this task.

As of Ruby MRI, Ruby is available on a lot of 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.

Criticism

• As variables are declared simply by assigning a value to them, typing errors can introduce new variables and cause unexpected behavior.
• Ruby's ability for metaprogramming allows a programmer to modify methods in the language's Standard Library during runtime, a practice known as monkey patching. This can lead to possible collisions of behavior and subsequent unexpected results, and is a concern for code scalability if done recklessly.
• The Ruby threading model uses green threads , and its model has some inherent limitations which render it difficult to use or unsafe in some scenarios.
• Ruby 1.8 does not yet have native support for Unicode or multibyte strings, although 1.9 now supports unicode and other encodings, as well as adding API for encoding agnostic string manipulation.
• Ruby suffers from backward compatibility problems.
• Ruby code runs slower than many compiled languages (as is typical for interpreted languages) and other major scripting languages such as Python and Perl. However, in future releases (current revision: 1.9), Ruby will be bytecode compiled to be executed on YARV (Yet Another Ruby VM). The performance of YARV is currently very similar to those of other scripting languages such as Python.

Ruby 2.0 aims to address some of the aforementioned problems:

• Native threads will be used instead of green threads.

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

• Ruby still lacks a specification, the current C implementation being the de facto reference specification. However the work for a specification is officially ongoing with a project named RubySpec based on the work started by the Rubinius project. The creator of Ruby has expressed his plans to make Ruby an ISO standard, which he deems will take some time to be completed.

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.

See also

• Duck typing
• Comparison of programming languages
• Watir
• Ruby MRI, the reference C implementation
• JRuby
• MacRuby
• IronRuby
• Rubinius
• XRuby
• Ramaze
• Ruby on Rails

External links

• Ruby language home page
• Ruby documentation site
• The Great Ruby Shootout (December 2008): Ruby implementations comparison.
• Ruby.on-page.net — the simplest Ruby manual with many samples
• Ruby programming language 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