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 

Answer 2

In Ruby, methods can check to see if they were called in such a way that a block was provided in addition to the normal arguments. Typically this is done using the block_given? method but you can also refer to the block as an explicit Proc by prefixing an ampersand (&) before the final argument name.

If a method is invoked with a block then the method can yield control to the block (call the block) with some arguments, if needed. Consider this example method that demonstrates:

def foo(x)   puts "OK: called as foo(#{x.inspect})"   yield("A gift from foo!") if block_given? end  foo(10) # OK: called as foo(10) foo(123) {|y| puts "BLOCK: #{y} How nice =)"} # OK: called as foo(123) # BLOCK: A gift from foo! How nice =) 

Or, using the special block argument syntax:

def bar(x, &block)   puts "OK: called as bar(#{x.inspect})"   block.call("A gift from bar!") if block end  bar(10) # OK: called as bar(10) bar(123) {|y| puts "BLOCK: #{y} How nice =)"} # OK: called as bar(123) # BLOCK: A gift from bar! How nice =)