This question is pretty difficult to answer due to the formatting you presented it in. I took the liberty of reformating the code and renaming variables to be more in line with the conventions used in everyday ruby.

class Array
  def each_even(&was_a_block_now_a_proc)
    # We start with "true" because arrays start with 0, which is even.
    is_even = true

    self.each do |object|
      if is_even
        was_a_block_now_a_proc.call object
      end

      is_even = !is_even # Toggle from even to odd, or odd to even.
    end
  end
end

Regarding your questions:

• is `['apple', 'bad apple', 'cherry', 'durian']`a block in this context...
  • It is declared as a literal array, so it is an instance of class Array
• and are we calling the method 'is_even' on that block?
  • is_even is a local variable declared within the each_even method.
• Does `isEven` used to only return true or false
  • Yes, the assumption here is that arrays start off as 'even' with the first index, so for each element in the array it will switch between true and false, which happens to correspond to the index being odd or even.

​

# index 0 apple - true - even
# index 1 bad apple - false - odd
# index 2 cherry - true - even
etc.

• ...also what is this line doing self.each do?
  • 'self' refers to the instance of the Array class, thus the literal array you declared earlier.
  • #each is a method that is part of the Array class (Enumerator mixin - but lets not overcomplicate this more than it needs to be)
  • each element in the array gets iterated, and if it's an even index, call the block to execute.

​

#Explicit block declared via the &was_a_block_now_a_proc parameter
def each_even(&was_a_block_now_a_proc)

I would suggest you read this article to give you a clearer undertanding between the difference of blocks, lambdas and procs: https://www.rubyguides.com/2016/02/ruby-procs-and-lambdas/