Ruby (programming language)

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Ruby
Paradigm: multi-paradigm
Appeared in: 1995
Designed by: Yukihiro Matsumoto
Developer: Yukihiro Matsumoto (among others)
Latest release: 1.8.5 / August 25, 2006
Typing discipline: dynamic ("duck")
Major implementations: Ruby, JRuby
Influenced by: Smalltalk, Perl, Lisp, Python, CLU, Dylan
Influenced: Groovy
OS: Cross-platform
License: Ruby License and GPL
Website: www.ruby-lang.org

Ruby is a reflective, 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 main implementation is free software distributed under an open-source license.

Contents

[edit] History

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 after a colleague's birthstone. As of October 2006, the latest stable version is 1.8.5. Ruby 1.9 (with some major changes) is also in development.

[edit] Philosophy

Matz's primary design consideration is to make programmers happy by reducing the menial work they must do, following the principles of good user interface design.[1] He stresses that systems design needs to emphasize human, rather than computer, needs [2]:

   
“
 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 typically behaves intuitively or as the programmer assumes it should. The phrase did not originate with Matz and, generally speaking, Ruby may more closely follow a paradigm best termed as "Matz's Least Surprise", though many programmers have found it to be close to their own mental model as well.

Matz 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 surprised me.
   
”

[edit] Semantics

Ruby is object-oriented: every bit of data is an object, 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 everything done in Ruby procedurally (that is, outside of the scope of a particular object) is actually done to an Object instance named 'main'. 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 you to program procedurally (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 meta-programming, as well as support for 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." [3]

[edit] Features

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

[edit] Interaction

The Ruby official distribution also includes "irb", an interactive command-line interpreter which can be used to test code quickly. A session with this interactive program might be:

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

There also exist readline bindings (module Readline), easily allowing the user for custom shells with history support.

 Readline.readline(, true) # param true means ~  "enable history"

[edit] 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 brackets. Line breaks are significant and taken as the end of a statement; a semicolon may be equivalently used. Indentation is not significant (unlike Python).

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

[edit] Gotchas and possible surprises

Although Ruby's design is guided by the principle of least surprise, naturally, some features differ 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.
  • 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 however 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.

In addition, some issues with the language itself are commonly raised:

  • 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[4]. However, in future releases (current revision: 1.9), Ruby will be bytecode compiled to be executed on the YARV (Yet Another Ruby VM). Currently, Ruby's memory footprint for the same operations is better than Perl's and Python's.[4]
  • 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. Much existing literature, however, encourages parenthesis omission for single-argument methods.

Ruby differs from Python in how it treats named arguments in function invocation: in Ruby, C, and many other languages, calling a function by some_function(x=4, 5) binds x to 4, and passes 4 and 5 to some_function. In Python, however, this would behave differently; the argument named x in the function definition for some_function would have 4 passed to it, and 5 would be passed into the remaining argument, this all being regardless of the order of arguments.

A good list of "gotchas" may be found in Hal Fulton's book The Ruby Way, pages 48-64 (ISBN 0-672-32884-4). However, since the list in the book pertains to an older version of Ruby (version 1.6), some items have been fixed since the book's publication. For example, retry now works with while, until and for, as well as iterators.

[edit] Examples

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?".split(//).uniq.sort.join  # " '?DINaceinsty"

[edit] Collections

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'

[edit] Blocks and iterators

The two syntaxes for creating a code block:

{ puts "Hello, World!" }

do puts "Hello, World!" end

Parameter-passing a block to be a closure:

# In an object instance variable, remember a block.
def remember(&b)
   @block = b
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("John")
# Prints "Hello, John!"

Returning closures from a method:

def foo(initial_value=0)
  var = initial_value
  return Proc.new {|x| var = x}, Proc.new { var }
end

setter, getter = foo
setter.call(21)
getter.call           # => 21

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

def a( &block )
   yield block( "hello")
end

# Invoke the above method, passing it a block.
a {|s| puts s}        # Prints: "hello"

# Perhaps the following needs cleaning up.

# Breadth-first search
def bfs(e)                      # 'e' should be a block.
   q = []                       # Make an array.
   e.mark                       # 'mark' is a user-defined method. (??)
   yield e                      # Yield to the block.
   q.push e                     # Add the block to the array.
   while not q.empty?           # This could be made much more Ruby-like.
      u = q.shift
      u.edge_iterator do |v|
         if not v.marked?       # 'marked?' is a user-defined method.
            v.mark
            yield v
            q.push v
         end
      end  
   end
end
bfs(e) {|v| puts v}

Iterating over enumerations and arrays using blocks:

a = [1, 'hi', 3.14]
a.each {|item| puts item}                         # Prints each element
(3..6).each {|num| puts num}                      # Prints the numbers 3 through 6
[1,3,5].inject(0) {|sum, element| sum + element}  # Prints 9 (you can pass both a parameter and a block)

Blocks work with many built-in methods:

File.open('file.txt', 'w+b') do |file|
   file.puts 'Wrote some text.'
end                             # File is automatically closed here

Or:

File.readlines('file.txt').each do |line|
   # Process each line, here.
end

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]

[edit] 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 meta-programming in Ruby: "attr" 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)
    @age <=> person.age
  end

  def to_s
    "#{@name} (#{@age})"
  end

  attr_reader :name, :age
end

group = [ Person.new("John", 20), 
          Person.new("Markus", 63), 
          Person.new("Ash", 16) 
        ]

puts group.sort.reverse

The above prints three names in reverse age order:

Markus (63)
John (20)
Ash (16)

[edit] 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!"

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
   # Handle exception
 end

Or particular exceptions:

begin
  # ...
rescue RuntimeError 
  # handling
end

Finally, it is possible to specify that the exception object be made available to the handler clause:

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

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

[edit] More examples

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

[edit] Implementations

Ruby has two main implementations: the official Ruby interpreter, which is the most widely used, and JRuby, a Java-based implementation.

[edit] Operating systems

Ruby is available for the following operating systems:

Other ports may also exist.

[edit] Licensing terms

The Ruby interpreter and libraries are distributed disjointedly (dual licensed) under the free and open source licenses GPL and Ruby License [5].

[edit] 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 two thousand items. Although the number of applications available does not match the volume of material available in the Perl or Python community, there is 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.

[edit] Sample scripts

For sample ruby scripts, use the Google Code Search:

Start here: Google Code Search lang:ruby and if you need so, add additional parameters to the address. For example to find scripts which use Win32API use: Google Code Search lang:ruby sketchup Win32API

The Google Code Search will let you locate and preview ruby code which is stored on the Internet, including code stored in ZIP files.

[edit] See also

Portal:Free software
 Free software Portal

[edit] Notes

  1. ^ The Ruby Programming Language by Yukihiro Matsumoto on 2000-06-12 (informit.com)
  2. ^ The Philosophy of Ruby, A Conversation with Yukihiro Matsumoto, Part I by Bill Venners on 2003-09-29 (Artima Developer)
  3. ^ How Does Ruby Compare With Python (rubygarden.org)
  4. ^ a b The Computer Language Shootout Benchmarks
  5. ^ Ruby License (ruby-lang.org)

[edit] External links

Wikibooks
Wikibooks has more on the topic of

Ruby homepage

Ruby libraries

Ruby news

Ruby community

Ruby learning

Ruby references

Ruby applications

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