Blocks and yields in Ruby

Question

I am trying to understand blocks and yield and how they work in Ruby.

How is yield used? Many of the Rails applications I've looked at use yield in a weird way.

Can someone explain to me or show me where to go to understand them?

Answer 1

Yes, it is a bit puzzling at first.

In Ruby, methods can receive a code block in order to perform arbitrary segments of code.

When a method expects a block, you can invoke it by calling the yield function.

Example:

Take Person, a class with a name attribute and a do_with_name method. When the method is invoked it will pass the name attribute to the block.

class Person 
    def initialize( name ) 
         @name = name
    end

    def do_with_name   # expects a block
        yield( @name ) # invoke the block and pass the `@name` attribute
    end
end

Now you can invoke this method and pass an arbitrary code block.

person = Person.new("Oscar")

# Invoking the method passing a block to print the value
person.do_with_name do |value|
    puts "Got: #{value}"
end

Would print:

Got: Oscar

Notice the block receives as a parameter a variable called value. When the code invokes yield it passes as argument the value of @name.

yield( @name )

The same method can be invoked with a different block.

For instance to reverse the name:

reversed_name = ""

# Invoke the method passing a different block
person.do_with_name do |value| 
    reversed_name = value.reverse
end

puts reversed_name

=> "racsO"

Other more interesting real life examples:

Filter elements in an array:

 days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]  

 # Select those which start with 'T' 
 days.select do | item |
     item.match /^T/
 end

=> ["Tuesday", "Thursday"]

Or sort by name length:

 days.sort do |x,y|
    x.size <=> y.size
 end

=> ["Monday", "Friday", "Tuesday", "Thursday", "Wednesday"]

If the block is optional you can use:

yield(value) if block_given?

If is not optional, just invoke it.

You can try these examples on your computer with irb (Interactive Ruby Shell)

Here are all the examples in a copy/paste ready form:

class Person 
    def initialize( name ) 
         @name = name
    end

    def do_with_name   # expects a block
        yield( @name ) # invoke the block and pass the `@name` attribute
    end
end


person = Person.new("Oscar")

# Invoking the method passing a block to print the value
person.do_with_name do |value|
    puts "Got: #{value}"
end


reversed_name = ""

# Invoke the method passing a different block
person.do_with_name do |value| 
    reversed_name = value.reverse
end

puts reversed_name



# Filter elements in an array:    
days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]  

# Select those which start with 'T' 
days.select do | item |
    item.match /^T/
end



# Sort by name length:     
days.sort do |x,y|
   x.size <=> y.size
end