# パーフェクトRuby

# setter/getter
    class Ruler
      # setter
      def length=(val)
        @length = val
      end

      # getter
      def length
        @length
      end end

    ruler = Ruler.new
    ruler.length = 30
    ruler.length # 30

**equivalent:**

    class Ruler
      attr_accessor :length
    end
# self
    class Ruler
      attr_accessor :length

      def display_length
        # equivalent to `self.length`
        puts length
      end
    end

**Can't omit** `**self**`  **on assigning value**


    class Ruler
      attr_accessor :length

      def set_default_value
        # interpreted as assigning value to local variable if `self` ommited
        self.length = 30
      end
    end
# Proc / lambda
## Proc
    # called on `Proc#call`
    greeter = Proc.new { |name| puts "Hello, #{name}!" }
    greeter.call "Proc" #=> "Hello, Proc!"
## lambda
    # proc method
    by_proc    = proc { |name| puts "Hello, #{name}!" }
    # lambda method
    by_lambda  = lambda { |name| puts "Hello, #{name}!" }
    # syntax sugar of lambda
    by_literal = ->(name) { |name| puts "Hello, #{name}!" }
# === / case,when

`===` **method changes behavior depends on the object**

| method object | behavior                                                             |
| ------------- | -------------------------------------------------------------------- |
| Range         | returns true if the given argument is in range                       |
| Regexp        | returns true if the given argument(String) is matched                |
| Proc          | invokes the block with the given argument as the proc’s parameter    |
| Module, Class | returns true if the given argument is self or its subclass’ instance |

`when` ‘s argument is compared using `===`  under the hood


    stone = "ruby"

    detected =
      case stone
      when /ruby/
        "7月"
      when /peridot|sardonyx/
        "8月"
      else
        "unknown"
      end

    puts detected # "7月"
# When to use `()`  with methods

One criteria is use `()`  when you want to use returned value, and not use for just a statement


    member = find_member_by(name)
    set_current member


# Block
## yield

use `yield`  to invoke parameter’s block

    def block_sample
      puts "stand up"
      yield
      puts "sit down"
    end

    block_sample do
      puts "walk"
    end

parameter of `yield`  gets to be block’s parameter

    def with_current_time
      yield Time.now
    end

    # `now` is `Time.now`
    with_current_time do |now|
      puts now.year
    end
## &block

use `&block`  to pass block as a parameter
`block`  is Proc object


    def block_sample(&block)
      puts "stand up"
      block.call if block
      puts "sit down"
    end

    block_sample do
      puts "walk"
    end
# when to use `do … end`  and `{…}`

One criteria is:

- Us `do … end` for multi-line block
- Use `{…}`  for single-line block

And the other is:

- Use `do … end` for expression to perform the procedure
- Use `{…}`  for for expression to return value
# Class method
## class << self
    class Ruler
      attr_accessor :length

      # returns two Ruler objects array
      def self.pair
        [new, new]
      end
    end

    # equivalent
    class Ruler
      attr_accessor :length

      class << self
        def pair
          [new, new]
        end

        def trio
          [new, new, new]
        end
      end
    end
# Enumerable module
    class FriendList
      include Enumerable

      def initialize(*friends)
        @friends = friends
      end

      # returns undefined method `each` error unless defined
      def each
        for v in @friends
          yield v
        end
      end
    end

    friend_list = FriendList.new("Alice", "Bob", "Charlie")
    upper_cased_friends = friend_list.map { |f| f.upcase }
    puts upper_cased_friends
# extend

`extend`  gives behavors to the specific object, like class method


    module Greetable
      def greet_to(name)
        puts "Hello, #{name}"
      end
    end

    class Alice
      extend Greetable
    end

    Alice.greet_to "Bob"