Ruby Tutorial: Blocks

Code Blocks

Ref. WGR Section 6.3, "Iterators and code blocks"

do...end

@@@ruby
3.times do 
  puts "Hip! Hip! Hooray!"
end
  • Anywhere you see "do" in Ruby, it's the start of a block
  • Also known as a closure or a proc or a lambda
  • Blocks

{...}

@@@ruby
3.times { puts "Hip! Hip! Hooray!" }
  • Blocks can also be wrapped in curly braces
  • By convention, braces are for a single line, do...end for multiple lines

What are blocks like?

  • anonymous functions
  • function pointers
  • callbacks
  • runnable objects

What are blocks for?

A block is a piece of code that is declared but not run in the place it's written. The idea is to leave is up to the receiver of the block to decide when to call it.

-- Wolfram Arnold

The Default Block

  • Every method, no matter what its parameter list, might get an optional magic block parameter

  • This is called "the default block" and the method can call it using yield

      @@@ ruby
  def foo
    yield
  end

  foo do
    puts "hi"
  end

yield calls the default block

@@@ ruby
def twice
   yield
   yield
end

twice do
  puts "hi"
end

"twice do" kind of almost resembles English a little, right?

Block parameters

@@@ ruby
def foo
  yield "alice"
  yield "bob"
end

foo do |name|
  puts "hi, #{name}"
end
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