Ruby && is control flow

ruby_&&_is_control_flow.rb

# Note that we can use parentheses to group expressions together,
# we'll use this to make a "side-effect" of incrementing a number,
# and then return a boolean value (true / false)
i = 0
(i += 1; true)  # => true
(i += 1; false) # => false
i               # => 2



# Lets look at the short-circuiting of &&

# When the left-hand side is true, it evaluates the right hand side
i = 0                             # => 0
(i += 1; true) && (i += 1; true)  # => true
i                                 # => 2
(i += 1; true) && (i += 1; false) # => false
i                                 # => 4

# When the left-hand side is false, it doesn't need to check the right-hand side
i = 0                              # => 0
(i += 1; false) && (i += 1; true)  # => false
i                                  # => 1
(i += 1; false) && (i += 1; false) # => false
i                                  # => 2

# Note that this is very different from how things like methods work, if it was
# a method, it would evaluate its arguments before being called, so it would
# always have the side-effect of incrementing our number!
def my_and(left, right)
  left && right
end
i = 0                                    # => 0
my_and((i += 1; false), (i += 1; true))  # => false
i                                        # => 2
my_and((i += 1; false), (i += 1; false)) # => false
i                                        # => 4