Code Blocks

Ruby code blocks are from the closure family

Ruby blocks are found throughout ruby. They are a powerful feature that allow you to pass snippets of code to enumerable methods (e.g. each, select, detect) as well as custom methods useing the yield keyword.

Blocks are nothing new, they use the computer science concept called closures. This concept was invented by Peter J. Landin in 1964. Closures were adopted by a version of a Lisp called Scheme in 1975.

What is a block

Blocks are a concept that allows programmers to extend their applications to have a domain specific language (DSL). A DSL is a form of metaprogramming that makes it easier to program and configure complex systems.

If you have ever used a method on an Array to loop through elements in that Array you have used a block.

["alex", "gina", "pam"].each do |inserted_name|
  puts "Hello, #{inserted_name}"
end

A block starts with do and terminates with end. A block can have a parameter defined between the pipes |x| at the begin of a block definition. When defining a block on a single line use braces arr.each { |i| }.

This single line line block is often called an inline block.

["alex", "gina", "pam"].each { |inserted_name| puts "Hello, #{inserted_name}" }

When writing a block that will need to be more than one line it do / end should be used. These are called multi-line blocks.

Methods

All ruby methods can be associated with a block. A block allows programmers to group statements together with a method call.

def something
  puts "do something"
end

If a the method does not call yield the block will not be called. The method will ignore the block being passed.

something do
  puts "do something else"
end

Here the method something is being passed a block. However as seen below shows the output of the puts statement in the method is called and the statement in the block is not called.

do something

To execute the block passed to method it will need to be yielded from within the method.

Yield

An associated block can be invoked within the method using the keyword yield. This allows the programmer to control the flow of the block’s execution. Without calling yield the block is ignored, as seen above.

def something_else
  puts "Begin"
  yield
  puts "End"
end

The contents of the block will be executed when the yield statement is called. Once the block is yielded the method is returned control of execution.

something_else do
  puts "yielded code"
end

The method executing the block can run any code before or after the block is invoked.

Begin
yielded code
End

When something_else is called with a block the method prints out the string Begin. The block is then invoked, printing out yielded code. The method is then returned control printing out the final string End.

Passing Arguments

Yield blocks can be passed arguments within a method. These arguments can then be used within the blocks as a parameter.

def run_three
  [1,2,3].each do |i|
    yield(i)
  end
end

The parameter will be defined between the pipes |x|. The arity of the parameters should match the number of arguments being passed into the yield statement.

run_three do |i|
  puts "and a #{i}"
end

The output:

and a 1
and a 2
and a 3

Above the we can see that the method run_three is being envoked with a block that accepts an argument. Within the run_three method we have an array of 3 numbers [1,2,3]. The method iterates through the array and calls yield with each value in the array.

Blocks as Arguments

A block can be passed around as an argument to a method. This can be done by adding an argument to the method with an ampersand &block.

def pass_method(&block)
  block.call
end

Invoking the method call on the block will execute the block. When the passed block is stored as a variable it is changed to a Proc.

pass_method { puts "yielded code" }

The block can still have params passed to it, like a block that is yielded.

def my_iterator(things, &block)
  things.each do |thing|
    block.call(thing)
  end
end

van = []

my_iterator ['John', 'Paul', 'George', 'Ringo'] do |beatle|
  van << beatle
end

puts "#{van.join(', ')} are in the van"

A block is not execute at the time it is encountered. The current context is stored with the block for later use before entering the method. This mean local variables called in the block will have the same value a when the block was passed to the method call.

Conclusion

A block can be used to create some commonly used code that allows you to inject custom code where it is used. This comes in handy when you need to dry up your code but find that the code differs in a few key places.

Brian Mehrman

Ruby Code Blocks