RSS (most commonly expanded as Really Simple Syndication) is a family of web feed formats used to publish frequently updated works—such as blog entries, news headlines, audio, and video—in a standardized format.[2] An RSS document (which is called a "feed", "web feed",[3] or "channel") includes full or summarized text, plus metadata such as publishing dates and authorship. Web feeds benefit publishers by letting them syndicate content automatically. They benefit readers who want to subscribe to timely updates from favored websites or to aggregate feeds from many sites into one place. RSS feeds can be read using software called an "RSS reader", "feed reader", or "aggregator", which can be web-based, desktop-based, or mobile-device-based. A standardized XML file format allows the information to be published once and viewed by many different programs. The user subscribes to a feed by entering into the reader the feed's URI or by clicking an RSS icon in a web browser that initiates the subscription process. The RSS reader checks the user's subscribed feeds regularly for new work, downloads any updates that it finds, and provides a user interface to monitor and read the feeds.
RSS formats are specified using XML, a generic specification for the creation of data formats. Although RSS formats have evolved from as early as March 1999,[4] it was between 2005 and 2006 when RSS gained widespread use, and the ("Feed-icon.svg") icon was decided upon by several major Web browsers.[5]
History
Main article: History of web syndication technology
The RSS formats were preceded by several attempts at web syndication that did not achieve widespread popularity. The basic idea of restructuring information about websites goes back to as early as 1995, when Ramanathan V. Guha and others in Apple Computer's Advanced Technology Group developed the Meta Content Framework.[6] For a more detailed discussion of these early developments, see the history of web syndication technology.
RDF Site Summary, the first version of RSS, was created by Guha at Netscape in March 1999 for use on the My.Netscape.Com portal. This version became known as RSS 0.9.[4] In July 1999, Dan Libby of Netscape produced a new version, RSS 0.91,[2] which simplified the format by removing RDF elements and incorporating elements from Dave Winer's scriptingNews syndication format.[7] Libby also renamed RSS "Rich Site Summary" and outlined further development of the format in a "futures document".[8]
This would be Netscape's last participation in RSS development for eight years. As RSS was being embraced by web publishers who wanted their feeds to be used on My.Netscape.Com and other early RSS portals, Netscape dropped RSS support from My.Netscape.Com in April 2001 during new owner AOL's restructuring of the company, also removing documentation and tools that supported the format.[9]
Two entities emerged to fill the void, with neither Netscape's help nor approval: The RSS-DEV Working Group and Winer, whose UserLand Software had published some of the first publishing tools outside of Netscape that could read and write RSS.
Winer published a modified version of the RSS 0.91 specification on the UserLand website, covering how it was being used in his company's products, and claimed copyright to the document.[10] A few months later, UserLand filed a U.S. trademark registration for RSS, but failed to respond to a USPTO trademark examiner's request and the request was rejected in December 2001.[11]
The RSS-DEV Working Group, a project whose members included Guha and representatives of O'Reilly Media and Moreover, produced RSS 1.0 in December 2000.[12] This new version, which reclaimed the name RDF Site Summary from RSS 0.9, reintroduced support for RDF and added XML namespaces support, adopting elements from standard metadata vocabularies such as Dublin Core.
In December 2000, Winer released RSS 0.92[13] a minor set of changes aside from the introduction of the enclosure element, which permitted audio files to be carried in RSS feeds and helped spark podcasting. He also released drafts of RSS 0.93 and RSS 0.94 that were subsequently withdrawn.[14]
In September 2002, Winer released a major new version of the format, RSS 2.0, that redubbed its initials Really Simple Syndication. RSS 2.0 removed the type attribute added in the RSS 0.94 draft and added support for namespaces. To preserve backward compatibility with RSS 0.92, namespace support applies only to other content included within an RSS 2.0 feed, not the RSS 2.0 elements themselves.[15] (Although other standards such as Atom attempt to correct this limitation, RSS feeds are not aggregated with other content often enough to shift the popularity from RSS to other formats having full namespace support.)
Because neither Winer nor the RSS-DEV Working Group had Netscape's involvement, they could not make an official claim on the RSS name or format. This has fueled ongoing controversy in the syndication development community as to which entity was the proper publisher of RSS.
One product of that contentious debate was the creation of an alternative syndication format, Atom, that began in June 2003.[16] The Atom syndication format, whose creation was in part motivated by a desire to get a clean start free of the issues surrounding RSS, has been adopted as IETF Proposed Standard RFC 4287.
In July 2003, Winer and UserLand Software assigned the copyright of the RSS 2.0 specification to Harvard's Berkman Center for Internet & Society, where he had just begun a term as a visiting fellow.[17] At the same time, Winer launched the RSS Advisory Board with Brent Simmons and Jon Udell, a group whose purpose was to maintain and publish the specification and answer questions about the format.[18]
In December 2005, the Microsoft Internet Explorer team[19] and Outlook team[20] announced on their blogs that they were adopting the feed icon first used in the Mozilla Firefox browser (Feed-icon.svg). In February 2006, Opera Software followed suit.[21] This effectively made the orange square with white radio waves the industry standard for RSS and Atom feeds, replacing the large variety of icons and text that had been used previously to identify syndication data.
In January 2006, Rogers Cadenhead relaunched the RSS Advisory Board without Dave Winer's participation, with a stated desire to continue the development of the RSS format and resolve ambiguities. In June 2007, the board revised their version of the specification to confirm that namespaces may extend core elements with namespace attributes, as Microsoft has done in Internet Explorer 7. According to their view, a difference of interpretation left publishers unsure of whether this was permitted or forbidden.
Variants
There are several different versions of RSS, falling into two major branches (RDF and 2.*).
The RDF (or RSS 1.*) branch includes the following versions:
* RSS 0.90 was the original Netscape RSS version. This RSS was called RDF Site Summary, but was based on an early working draft of the RDF standard, and was not compatible with the final RDF Recommendation.
* RSS 1.0 is an open format by the RSS-DEV Working Group, again standing for RDF Site Summary. RSS 1.0 is an RDF format like RSS 0.90, but not fully compatible with it, since 1.0 is based on the final RDF 1.0 Recommendation.
* RSS 1.1 is also an open format and is intended to update and replace RSS 1.0. The specification is an independent draft not supported or endorsed in any way by the RSS-Dev Working Group or any other organization.
The RSS 2.* branch (initially UserLand, now Harvard) includes the following versions:
* RSS 0.91 is the simplified RSS version released by Netscape, and also the version number of the simplified version originally championed by Dave Winer from Userland Software. The Netscape version was now called Rich Site Summary; this was no longer an RDF format, but was relatively easy to use.
* RSS 0.92 through 0.94 are expansions of the RSS 0.91 format, which are mostly compatible with each other and with Winer's version of RSS 0.91, but are not compatible with RSS 0.90.
* RSS 2.0.1 has the internal version number 2.0. RSS 2.0.1 was proclaimed to be "frozen", but still updated shortly after release without changing the version number. RSS now stood for Really Simple Syndication. The major change in this version is an explicit extension mechanism using XML namespaces.[22]
For the most part, later versions in each branch are backward-compatible with earlier versions (aside from non-conformant RDF syntax in 0.90), and both versions include properly documented extension mechanisms using XML Namespaces, either directly (in the 2.* branch) or through RDF (in the 1.* branch). Most syndication software supports both branches. "The Myth of RSS Compatibility", an article written in 2004 by RSS critic and Atom advocate Mark Pilgrim, discusses RSS version compatibility issues in more detail.
The extension mechanisms make it possible for each branch to track innovations in the other. For example, the RSS 2.* branch was the first to support enclosures, making it the current leading choice for podcasting, and as of 2005[update] is the format supported for that use by iTunes and other podcasting software; however, an enclosure extension is now available for the RSS 1.* branch, mod_enclosure. Likewise, the RSS 2.* core specification does not support providing full-text in addition to a synopsis, but the RSS 1.* markup can be (and often is) used as an extension. There are also several common outside extension packages available, including a new proposal from Microsoft for use in Internet Explorer 7.
The most serious compatibility problem is with HTML markup. Userland's RSS reader—generally considered as the reference implementation—did not originally filter out HTML markup from feeds. As a result, publishers began placing HTML markup into the titles and descriptions of items in their RSS feeds. This behavior has become expected of readers, to the point of becoming a de facto standard, though there is still some inconsistency in how software handles this markup, particularly in titles. The RSS 2.0 specification was later updated to include examples of entity-encoded HTML; however, all prior plain text usages remain valid.
As of January 2007[update], tracking data from www.syndic8.com indicates that the three main versions of RSS in current use are 0.91, 1.0, and 2.0. Of these, RSS 0.91 accounts for 13 percent of worldwide RSS usage and RSS 2.0 for 67 percent, while RSS 1.0 has a 17 percent share.[23] These figures, however, do not include usage of the rival web feed format Atom. As of August 2008[update], the syndic8.com website is indexing 546,069 total feeds, of which 86,496 were some dialect of Atom and 438,102 were some dialect of RSS.[24]
Modules
The primary objective of all RSS modules is to extend the basic XML schema established for more robust syndication of content. This inherently allows for more diverse, yet standardized, transactions without modifying the core RSS specification.
To accomplish this extension, a tightly controlled vocabulary (in the RSS world, "module"; in the XML world, "schema") is declared through an XML namespace to give names to concepts and relationships between those concepts.
Some RSS 2.0 modules with established namespaces are:
* Ecommerce RSS 2.0 Module
* Media RSS 2.0 Module
* OpenSearch RSS 2.0 Module
Tuesday, August 17, 2010
Ruby (programming language)
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]
[edit] 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]
[edit] 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]
[edit] 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.
[edit] 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]
[edit] Ruby 1.9.1
The latest stable version of the reference implementation is 1.9.1.
Ruby 1.9.1 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
[edit] 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]
“ 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 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]
“ 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 [sic]. 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. ”
[edit] 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
[edit] 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."
[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 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.
[edit] Deviations from behaviour 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.
[edit] 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
[edit] 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.
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.'
[edit] 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}
[edit] 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'
[edit] 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]
[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 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)
[edit] 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.
[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!"
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
[edit] 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
"#{self}"
end
end
end
The generated methods could then be used like so:
"Hello, World!".in_blue
=> "Hello, World!"
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
[edit] More examples
More sample Ruby code is available as algorithms in the following articles:
* Exponentiating by squaring
* Trabb Pardo-Knuth algorithm
[edit] Implementations
See also: Ruby MRI#Operating systems
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]
[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 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]
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]
[edit] 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]
[edit] 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]
[edit] 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.
[edit] 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]
[edit] Ruby 1.9.1
The latest stable version of the reference implementation is 1.9.1.
Ruby 1.9.1 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
[edit] 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]
“ 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 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]
“ 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 [sic]. 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. ”
[edit] 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
[edit] 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."
[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 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.
[edit] Deviations from behaviour 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.
[edit] 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
[edit] 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
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.'
[edit] 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}
[edit] 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'
[edit] 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]
[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 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)
[edit] 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.
[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!"
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
[edit] 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
"#{self}"
end
end
end
The generated methods could then be used like so:
"Hello, World!".in_blue
=> "Hello, World!"
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
[edit] More examples
More sample Ruby code is available as algorithms in the following articles:
* Exponentiating by squaring
* Trabb Pardo-Knuth algorithm
[edit] Implementations
See also: Ruby MRI#Operating systems
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]
[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 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]
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