Mastering Ruby Closures

Executing Blocks with the yield Keyword

When you see yield anywhere in a Ruby block, you should think “execute the block.” Try out the following in irb:

>> def do_it
>>   yield
>> end
=> :do_it

This is a pretty plain-looking piece of code that just executes any block you give it:

>> do_it { puts "I'm doing it" }
I'm doing it
=> nil

Outputting a string doesn’t return any value. In other words, this block is executed merely for its side effects. Now, let’s make a block that returns a value and pass it into the do_it method:

>> do_it { [1,2,3] << 4 }
=> [1, 2, 3, 4]

What happens when we don’t pass in a block to do_it?

>> do_it
LocalJumpError: no block given (yield)
  from (irb):28:in `do_it'

irb helpfully ...