to_proc.rb

["hello", "world", "hahahahaha"].map(&:length)
#=> [5, 5, 10]

# So, what's happening is, :length gets #to_proc called on it, which results in map doing the following:

["hello", "world", "hahahahaha"].map do |str|
  :length.to_proc.call(str)
end

# Now, :length.to_proc.call(str) does the following:

str = "hello"
str.send(:length)
#=> 5

# try it yourself!

:+.to_proc.call(2, 3) == 2.send(:+, 3)
#=> true

# str.send(:length) is (basically) the same as

str.length
#=> 5