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Eloquent Ruby Note

By Jiazhen Xie on Mar 23, 2014
Eloquent Ruby Note


  • Use Hash if would like to search

  • Cancel the new line

    yet_another = %Q{another multi-line string with 
    no newline}
  • kind_of?, which will return true if the object is an instance of DocumentIdentifier or a subclass of DocumentIdentifier

  • include module, inject a super class to the object

  • When you tack a block onto the end of a method call, Ruby will package up the block as sort of a secret argument and (behind the scenes) passes this secret argument to the method. Inside the method you can detect whether your caller has actually passed in a block with the block_given? method and fire off the block (if there is one) with yield

    def each_word
      words.each { |word| yield( word ) }
  • Enumerable also contributes the each_cons method to your class. The each_cons method takes an integer and a block, and will repeatedly call the block, each time passing in an array of consecutive elements from the collection.

  • Benefits for block: execute around

    def with_logging(description)
        @logger.debug( "Starting #{description}" )
        @logger.debug( "Completed #{description}" )
        @logger.error( "#{description} failed!!")
  • Code blocks drag along the scope in which they were created wherever they go. In the last example, this means that @doc object is automatically visible inside the code block?no need to pass it down as an argument.

  • Proc vs. Lambda

    1. Proc.new object is very forgiving of the number of arguments passed to its call method.
    2. a Proc.new block executes an explicit return, Ruby will try to return not just from the block but from the method that created the block. While a return from a lambda wrapped block will simply return from the block and no further
  • inherited, will get called just after each subclass is define

    def self.included( host_class )
      host_class.extend( ClassMethods )
  • at_exit is last in / first out

  • $! is a global variable that Ruby sets to the last exception raised

  • If you have many different ways that you might create an object, a set of wellnamed class methods is generally clearer than making the user supply all sorts of clever arguments to the new method

  • Including modules into the singleton class is a common enough task that Ruby has a special shortcut for it in the form of extend

    class Document
      extend Finders
      # Most of the class omitted...
  • Execute around with a block. All of the variables that are visible just before the opening do or { are still visible inside the code block.

  • A good rule of thumb is that the only arguments you should pass from the application into an execute around method are those that the execute around method itself, not the block.

  • Compose methods

    class TextCompressor
      attr_reader :unique, :index
      def initialize( text )
        @unique = []
        @index = []
        add_text( text )
      def add_text( text )
        words = text.split
        words.each { |word| add_word( word ) }
      def add_word( word )
        i = unique_index_of( word ) || add_unique_word( word )
        @index << i
      def unique_index_of( word )
      def add_unique_word( word )
        @unique << word
        unique.size - 1
    • First, each method should do a single thing.
    • Each method needs to operate at a single conceptual level: Simply put, don’t mix high-level logic with the nitty-gritty details.
    • Finally, each method needs to have a name that reflects its purpose.
  • alias_method actually copies a method implementation, giving it a new name along the way.

    class String
      alias_method :old_addition, :+
      def +( other )
        if other.kind_of? Document
          new_content = self + other.content
          return Document.new(other.title, other.author, new_content)
  • __FILE__ is supplied via the magic of Ruby and is always set to the path of the source file of the current class

  • the class_eval method takes a string and evaluates it as if it were code that appeared in the class body

  • Pass instance_eval a block and, just like call, it will execute the block. The difference is that instance_eval changes the value of self as it executes the block. self is the receiver of the block

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