Ruby (programming language)

Ruby

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, Rubinius
Influenced by
Smalltalk, Perl, Lisp, Scheme, Python, CLU, Eiffel, Ada, Dylan
Influenced
Groovy

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.

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

In its current, official implementation, written in C, Ruby is a single-pass interpreted language. As of 2008, there are a number of alternative implementations of the Ruby language, including Rubinius, JRuby, YARV, and IronRuby, each of which takes a different approach, with JRuby providing just-in-time compilation functionality.

History

Yukihiro Matsumoto, the creator of Ruby.

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

As of December 2007, the latest stable version of the reference implementation is 1.8.6. Appart from the reference, several other virtual machines are being developed for Ruby. These include JRuby, a port of Ruby to the Java platform, IronRuby, an implementation for the .NET Framework produced by Microsoft, and Rubinius, an interpreter modeled after self-hosting Smalltalk virtual machines.

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 hadn't 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.

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."

Type system

Ruby checks the type of each expression and sub-expression dynamically, as they are encountered during execution, and will raise a runtime error whenever a type error is found.

Is Ruby type-safe?

There is no universally agreed upon definition of type safe (see TypeSafe at C2.com Wiki), and in the case of Ruby, it depends on which definition one refers to.

One definition of type-safe language requires that "no operation will be applied to a variable of a wrong type." In this respect, Ruby is probably type safe (given Ruby's semantics, it would be very difficult to prove theoretically, but might be assumed as long as no contradictory code example is found).

Another definition of type-safe program requires that "the program will not have type errors when it runs". In this respect, Ruby is obviously not type safe, since it may by design raise type errors during execution.

Is Ruby strongly typed?

Again, there is no universally agreed upon definition of Strongly Typed. C2.com Wiki lists at least 8 different definitions from different sources.

According to some of these definitions, Ruby is strongly typed, while according to others it is weakly typed:

1. Def: A language is strongly typed if type annotations are associated with variable names, rather than with values. If types are attached to values, it is weakly typed. => Ruby is weakly typed.
2. Def: A language is strongly typed if it contains compile-time checks for type constraint violations. If checking is deferred to run time, it is weakly typed. => Ruby is weakly typed.
3. Def: A language is strongly typed if there are compile-time or run-time checks for type constraint violations. If no checking is done, it is weakly typed. => Ruby is strongly typed.
4. Def: A language is strongly typed if conversions between different types are forbidden. If such conversions are allowed, it is weakly typed. => Ruby is weakly typed.
5. Def: A language is strongly typed if conversions between different types must be indicated explicitly. If implicit conversions are performed, it is weakly typed. => Ruby is weakly typed. (Examples: (2+3.5) (implicit conversion Fixnum to Float) and (if 42 then "a" else "b" end) (implicit conversion from Fixnum to TrueClass))
6. Def: A language is strongly typed if there is no language-level way to disable or evade the type system. If there are casts or other type-evasive mechanisms, it is weakly typed. => Ruby is assumed to be strongly typed, but very difficult to prove mathematically.
7. Def: A language is strongly typed if it has a complex, fine-grained type system with compound types. If it has only a few types, or only scalar types, it is weakly typed. => Ruby is strongly typed.
8. Def: A language is strongly typed if the type of its variables is fixed and does not vary over the lifetime of the variable. If the type of the datum stored in a variable can change, the language is weakly typed. => Ruby is weakly typed. (Example: x = 1; x = "s". Type of x changes from Fixnum to String.)

Features

• object-oriented
• five levels of variable scope: global, class, 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 (examples in RubyGarden: continuations and generators)

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

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

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 }). 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.
• 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

• 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). Ruby's current memory footprint for the same operations is higher than Perl's and Python's.
• 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 (Nov 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.

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".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 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 :water # Deletes :water => 'wet'
hash.delete_if {|k,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

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

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

Iterating over enumerations and arrays using blocks:

array = [1, 'hi', 3.14]
array.each { |item| puts item }
# => 1
# => 'hi'
# => 3.14

array.each_index { |index| puts index.to_s + ": " + array[index] }
# => 0: 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
	def initialize(name, age)
		@name, @age = name, age
	end
	def <=>(person) # Comparison operator for sorting
		@age <=> person.age
	end
	def to_s
		"#@name (#@age)"
	end
	attr_reader :name, :age
end
 
group = [
        Person.new("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 constuct is useful when you need to raise a custom exception class featuring a constructor which takes more than one argument:

class ParseError < Exception
        def initialize input, line, pos
                super "Could not parse '#{input}' at line #{line}, position #{pos}"
        end
end

raise ParseError.new( "Foo", 3, 9 )

Exceptions are handled by the rescue clause. Such a clause can catch exceptions 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 -- this only catches StandardError, a subclass of Exception
# Handle exception
end

Or catch particular exceptions:

begin
# ...
rescue RuntimeError 
# handling
end

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

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

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

You can also catch several exceptions:

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

Or catch an array of exceptions:

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

More 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.

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

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.

Criticism

A number of the design choices made for Ruby have well-known disadvantages:

• As in BASIC, because variables are not required to be declared before use, typing errors can introduce new variables and cause unexpected behavior.
• Dynamic typing can cause type errors to be found later in the development process, making them more expensive to fix, and add runtime overhead compared to static typing.
• The runtime extensible environment enabled by metaprogramming can make programs more difficult to reason about statically and inhibit some types of optimizations.
• 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 does not yet have native support for Unicode or multibyte strings.
• Ruby suffers from backward compatibility problems.

Ruby 2.0 aims to address all of the aforementioned problems:

• Native threads will be used instead of green threads.
• Full support for Unicode strings.

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.

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 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
• JRuby
• IronRuby
• Rubinius
• XRuby
• Ramaze
• Ruby on Rails

External links

• Ruby language home page
• Ruby documentation site
• Ruby programming language at the Open Directory Project
• Ruby User Guide Mirror
• Ruby From Other Languages
• Wiki: Ruby language and implementation specification
• Writing C Extensions to Ruby (MRI 1.8)
• RubyConf 2007 Conference Videos