Ruby Blocks and yield 2020

A Ruby Code block (aka closure) s a way of grouping statements so that we can associate with method invocations as if they were parameters.

The Ruby standard is to use braces for single-line blocks and do-end for multi-line blocks.

Once we have created a block, we can associate it with a call to a method. Usually the code blocks passed into methods are anonymous objects, created on the spot. For example, in the following code, the block containing puts "Hello" is associated with the call to a method hello.

hello {puts 'Hello'}  

If the method has parameters, they appear before the block:

hello("RubyOnRails") {puts 'Hello'}  

A method can then invoke an associated block using the Ruby yield statement. So, any method that wants to take a block as a parameter can use the yield keyword to execute the block at any time.

If we provide a code block when we call a method, then inside the method, we can yield control to that code block - suspend execution of the method; execute the code in the block; and return control to the method body, right after the call to yield:

#!/usr/bin/env ruby

# block1.rb

def rb_block
  puts 'Start'
  # you can call the block using the yield keyword
  yield
  yield
  puts 'End'
end
# Code blocks may appear only in the source adjacent to a method call
rb_block {puts "We're in the block"}

Output:

Start
We're in the block
We're in the block
End

We can provide parameters to the call to yield: these will be passed to the block. Within the block, we list the names of the arguments to receive the parameters between vertical bars (|).

#!/usr/bin/env ruby

# block2.rb

def rb_block
    yield('hello', 2014)
end
# Code blocks may appear only in the source adjacent to a method call
rb_block { |s, num| puts s + ' ' + num.to_s}

Output:

hello 2014

As we can see, the code in the block is not executed at the time it is encountered by the Ruby interpreter. Instead, Ruby remembers the context in which the block appears and then enters the method.

Ruby on Rails

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