Common Ruby Idioms

Question

One thing I love about ruby is that mostly it is a very readable language (which is great for self-documenting code)

However, inspired by this question: http://stackoverflow.com/questions/609612/ruby-code-explained and the description of how ||= works in ruby, I was thinking about the ruby idioms I don't use, as frankly, I don't fully grok them.

So my question is, similar to the example from the referenced question, what common, but not obvious, ruby idioms do I need to be aware of to be a truly proficient ruby programmer?

By the way, from the referenced question

a ||= b

is equivalent to

if a == nil || a == false
  a = b
end

(Thanks to Ian Terrell for the correction)

Edit: It turns out this point is not totally uncontroversial. The correct expansion is in fact

(a || (a = (b)))

See these links for why:

• http://DABlog.RubyPAL.Com/2008/3/25/a-short-circuit-edge-case/
• http://DABlog.RubyPAL.Com/2008/3/26/short-circuit-post-correction/
• http://ProcNew.Com/ruby-short-circuit-edge-case-response.html

Thanks to Jörg W Mittag for pointing this out.

Answer 1

I like this:

str = "Something evil this way comes!"
regexp = /(\w[aeiou])/

str[regexp, 1] # <- This

Which is (roughly) equivalent to:

str_match = str.match(regexp)
str_match[1] unless str_match.nil?

Or at least that's what I've used to replace such blocks.

Answer 2

This slideshow is quite complete on the main Ruby idioms, as in:

• Swap two values:

  x, y = y, x

• Parameters that, if not specified, take on some default value

  def somemethod(x, y=nil)

• Batches up extraneous parameters into an array

  def substitute(re, str, *rest)

And so on...

Answer 3

The magic if clause that lets the same file serve as a library or a script:

if __FILE__ == $0
  # this library may be run as a standalone script
end

Packing and unpacking arrays:

# put the first two words in a and b and the rest in arr
a,b,*arr = *%w{a dog was following me, but then he decided to chase bob}
# this holds for method definitions to
def catall(first, *rest)
  rest.map { |word| first + rest }
end
catall( 'franken', 'stein', 'berry', 'sense' ) #=> [ 'frankenstein', 'frankenberry', 'frankensense' ]

The syntatical sugar for hashes as method arguments

this(:is => :the, :same => :as)
this({:is => :the, :same => :as})

Hash initializers:

# this
animals = Hash.new { [] }
animals[:dogs] << :Scooby
animals[:dogs] << :Scrappy
animals[:dogs] << :DynoMutt
animals[:squirrels] << :Rocket
animals[:squirrels] << :Secret
animals #=> {}
# is not the same as this
animals = Hash.new { |_animals, type| _animals[type] = [] }
animals[:dogs] << :Scooby
animals[:dogs] << :Scrappy
animals[:dogs] << :DynoMutt
animals[:squirrels] << :Rocket
animals[:squirrels] << :Secret
animals #=> {:squirrels=>[:Rocket, :Secret], :dogs=>[:Scooby, :Scrappy, :DynoMutt]}

metaclass syntax

x = Array.new
y = Array.new
class << x
  # this acts like a class definition, but only applies to x
  def custom_method
     :pow
  end
end
x.custom_method #=> :pow
y.custom_method # raises NoMethodError

class instance variables

class Ticket
  @remaining = 3
  def self.new
    if @remaining > 0
      @remaining -= 1
      super
    else
      "IOU"
    end
  end
end
Ticket.new #=> Ticket
Ticket.new #=> Ticket
Ticket.new #=> Ticket
Ticket.new #=> "IOU"

Blocks, procs, and lambdas. Live and breathe them.

 # know how to pack them into an object
 block = lambda { |e| puts e }
 # unpack them for a method
 %w{ and then what? }.each(&block)
 # create them as needed
 %{ I saw a ghost! }.each { |w| puts w.upcase }
 # and from the method side, how to call them
 def ok
   yield :ok
 end
 # or pack them into a block to give to someone else
 def ok_dokey_ok(&block)
    ok(&block)
    block[:dokey] # same as block.call(:dokey)
    ok(&block)
 end
 # know where the parentheses go when a method takes arguments and a block.
 %w{ a bunch of words }.inject(0) { |size,w| size + 1 } #=> 4
 pusher = lambda { |array, word| array.unshift(word) }
 %w{ eat more fish }.inject([], &pusher) #=> ['fish', 'more', 'eat' ]

Answer 4

I would suggest reading through the code of popular and well designed plugins or gems from people you admire and respect.

Some examples I've run into:

if params[:controller] == 'discussions' or params[:controller] == 'account'
  # do something here
end

corresponding to

if ['account', 'discussions'].include? params[:controller]
  # do something here
end

which later would be refactored to

if ALLOWED_CONTROLLERS.include? params[:controller]
  # do something here
end

Answer 5

Here's a few, culled from various sources:

use "unless" and "until" instead of "if not" and "while not". Try not to use "unless" when an "else" condition exists, though.

Remember you can assign multiple variables at once:

a,b,c = 1,2,3

and even swap variable without a temp:

a,b = b,a

Use trailing conditionals where appropriate, e.g.

do_something_interesting unless want_to_be_bored?

Be aware of a commonly-used but not instantly obvious (to me at least) way of defining class methods:

class Animal
  class<<self
    def class_method
      puts "call me using Animal.class_method"
    end
  end
end

Some references:

• http://cbcg.net/talks/rubyidioms/index.html
• http://www.therailsway.com/2006/12/8/idiomatic-ruby
• http://www.therailsway.com/2007/1/21/more-idiomatic-ruby
• http://www.caliban.org/ruby/rubyguide.shtml

Answer 6

By the way, from the referenced question

a ||= b

is equivalent to

if a == nil   
  a = b 
end

That's subtly incorrect, and is a source of bugs in newcomers' Ruby applications.

Since both (and only) nil and false evaluate to a boolean false, a ||= b is actually (almost*) equivalent to:

if a == nil || a == false
  a = b
end

Or, to rewrite it with another Ruby idiom:

a = b unless a

(*Since every statement has a value, these are not technically equivalent to a ||= b. But if you're not relying on the value of the statement, you won't see a difference.)

Answer 7

Array.pack and String.unpack for working with binary files:

# extracts four binary sint32s to four Integers in an Array
data.unpack("iiii") 

Answer 8

You can deepcopy with Marshaling object easily. - taken from The Ruby Programming Language

def deepcopy(o)
  Marshal.load(Marshal.dump(o))
end

Note that files and I/O streams, as well as Method and Binding objects, are too dynamic to be marshaled; there would be no reliable way to restore their state.

Answer 9

Some more idioms:

Use of the %w, %r and %( delimiters

%w{ An array of strings %}
%r{ ^http:// }
%{ I don't care if the string has 'single' or "double" strings }

Type comparison in case statements

def something(x)
  case x
    when Array
      # Do something with array
    when String
      # Do something with string
    else
      # You should really teach your objects how to 'quack', don't you?
  end
end

... and overall abuse of the === method in case statements

case x
  when 'something concrete' then ...
  when SomeClass then ...
  when /matches this/ then ...
  when (10...20) then ...
  when some_condition >= some_value then ...
  else ...
end

Something that should look natural to Rubyists, but maybe not so to people coming from other languages: the use of each in favor of for .. in

some_iterable_object.each{|item| ... }

In Ruby 1.9+, Rails, or by patching the Symbol#to_proc method, this is becoming an increasingly popular idiom:

strings.map(&:upcase)

Conditional method/constant definition

SOME_CONSTANT = "value" unless defined?(SOME_CONSTANT)

Query methods and destructive (bang) methods

def is_awesome?
  # Return some state of the object, usually a boolean
end

def make_awesome!
  # Modify the state of the object
end

Implicit splat parameters

[[1, 2], [3, 4], [5, 6]].each{ |first, second| puts "(#{first}, #{second})" }