weirich.rb

# https://www.youtube.com/watch?v=FITJMJjASUs

puts ->() {

  pred = ->(n) { n - 1 }     # let's pretend no operators

  fact_improver = ->(partial) {
    ->(n) { n.zero? ? 1 : n * partial.(pred.(n)) }
  }

  fib_improver = ->(partial) {
    ->(n) { n.zero? ? 0 : ((pred.(n)).zero? ?
                           1 : partial.(pred.(n)) + partial.(pred.(pred.(n)))) }
  }

  z = ->(improver) {
    ->(generator) { generator.(generator) }.(
      ->(generator) { improver.(->(v) { generator.(generator).(v) }) } ) }
      # no v wrapper needed in normal-order lang

  # more canonical applicative form
  # some no-op immediately-invoked functional wrappers for symmetry
  z_combinator = ->(f) {
    ->(x) { f.(->(v) { x.(x).(v) }) }.(
    ->(x) { f.(->(v) { x.(x).(v) }) }) }

  fact = z_combinator.(fact_improver)
  fact = fact_improver.(fact)          # what it even means to be a fixpoint

  fib = z.(fib_improver)
  fib = fib_improver.(fib)             # what it even means to be a fixpoint

  [fact.(5), fib.(7)]                  # expect => 120, 13

}.()