[Ruby] Block


Method call with block

There are three main uses.

  1. Loop abstraction
  2. Addition of functions to blocks. Typically resource management.
  3. Callback function / event handler

Block as a closure

The block is a closure. That is, ** free variables appearing in the code inside the block follow the block's external environment **. Free variables in a block are resolved in the context outside the block, which is the external environment.

In the example below, the local variable count is shared outside and inside the block.

str = 'Hello, World'
count = 0
str.each_line do |line|
  print line
  count += 1
print count # => 1

Local variables, self, instance variables associated with self, method calls, etc. can all be used in the block in the same way as outside.

def some_method
  3.times { p self } #self is not 3 but some_method self

Preservation of environment

** The external environment referenced by the block is preserved as long as the block exists **. Even if the method execution is completed, the internal code block uses the local variable at the time of method execution.

In the example below, create_counter converts the code block inside the method into a Proc object and returns it to the caller. Calling the Proc # call method executes a block of code.

The local variable count in the execution context of the create_counter method can only be referenced by the Proc returned by the method. That is, the internal state can be concealed.

def create_counter
  count = 1
  return Proc.new do
    count += 1
    p count
counter = create_counter
p counter.class # => Proc
counter.call # => 2
counter.call # => 3

counter2 = create_counter
counter2.call # => 2
counter.call # => 4
counter2.call # => 3

Block parameters

The block passed to the method can receive arguments when it is called back from the method. Blocks can also have their own local variable = block parameter.

a = "str"
[1, 2, 3].each { |a| p a } #This a is different from the above a
p a # => "str"

There are two points to be aware of from what we have learned so far.

Definition of method with block

yield expression

def foo_bar_baz
  yield "foo"
  yield "bar"
  yield "baz"


def foo_bar_baz
  #Enum when not given a block_Generate and return Enumerator with for
  return enum_for(:foo_bar_baz) unless block_given?
  %w[ foo bar baz ].each do |item|
    yield item

foo_bar_baz do |item|
  puts item
# => foo
#    bar
#    baz


def my_map
  [yield(1), yield(2), yield(3)]
p my_map { |i| i + 1 }
# => [2, 3, 4]


If you want to get the calling block as an object, use a Proc object.

The & handler in the example below is called the block argument.

class SleepyPerson
  def register_handler(&handler)
    @event_handler = handler
  def wake_up!
    @event_handler.call Time.now, "woke up"
john = SleepyPerson.new
john.register_handler { |time, message| p [time, message] }

Block from Proc

proc = Proc.new { puts "Proc was called" }
# => Proc was Called
#    Proc was Called
#    Proc was Called


For the time being, this seems to be important


-First Ruby

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