Passing procs and methods

Question

I came across this code:

squareIt = Proc.new do |x|
  x * x
end

doubleIt = Proc.new do |x|
  x + x
end

def compose proc1, proc2
  Proc.new do |x|
    proc2.call(proc1.call(x))
  end
end

doubleThenSquare = compose(doubleIt, squareIt)
squareThenDouble = compose(squareIt, doubleIt)

doubleThenSquare.call(5)
squareThenDouble.call(5)

doubleThenSquare is called with 5. doubleThenSquare is equal to the return value of compose, which has its two parameters doubleIt and squareIt passed.

I don't see how 5 is passed all its way into the different procs Proc.new do |x|. How does it know what x is in each case?

Answer 1

Let's step through it.

doubleIt = Proc.new do |x|
  x + x
end
  #=> #<Proc:0x00000002326e08@(irb):1429>

squareIt = Proc.new do |x|
  x * x
end
  #=> #<Proc:0x00000002928bf8@(irb):1433>

proc1 = doubleIt
proc2 = squareIt

compose returns the proc proc3.

proc3 = Proc.new do |x|
    proc2.call(proc1.call(x))
end
  #=> #<Proc:0x000000028e7608@(irb):1445>

proc3.call(x) is executed in the same way as

proc3_method(x)

where

def proc3_method(x)
  y = proc1.call(x)
  proc2.call(y)
end

When x = 5,

y = proc1.call(5)
  #=> 10
proc2.call(10)
  #=> 100

proc3_method(5) therefore returns 100, as does proc3.call(5).