yield is a special thing in Ruby.
Using yield, a method can
ask a block to be executed.
Here, the method first prints Good morning.
Then, the method yields the control to the block.
Once the block is processed, then the method gets
the control back
and
then the method prints Good evening.
If the method does not get a block to execute, then
the method will raise an error.
Let's try it out and this time no block will be
passed to the method.
You should get an error in the case given below:
We want to yield to a block
only if a block is passed.
There is no point in yielding
to a block if there is no block.
We can solve this by using the block_given? method.
block_given? would return
true if a block is passed.
If a block is passed, then that block
will be evaluated.