Ruby’s Symbol

To iterate over a collection doing only one action, we can explicitly write the action:

[1, 2, 3, 4].map { |num| num.to_s }

Or we can pass &:to_s as an argument to the method instead of a block, which is fundamentally the same thing:

[1, 2, 3, 4].map(&:to_s)

We can see & in front of an object. This tells Ruby to try to convert said object into a block. However it’s expecting a Proc object therefore if it’s not a Proc object, Ruby will call #to_proc on the object.

What is happening here is:

  • Ruby convert the object after & to a Proc with #to_proc if it is not one already. (An error is thrown if the object cannot be converted)
  • If successful, & turns the Proc into a block.

The code bellow illustrates illustrates what is happening step by step:

def my_method

my_proc = :to_s.to_proc # Call `to_proc` on symbol `to_s`
my_method(&my_proc)     # Convert Proc into a block then pass to method
# => "2"