Mutual Recursion

Tail-call optimization for Ruby

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README.md

Tail call optimization for mutually (indirectly) and directly recursive functions in Ruby.

The current design uses a trampoline. However, it is implemented in a way that still allows a tail recursive function to easily return a Proc as its terminal value.

examples

require 'mutual_recursion'
include MutualRecursion
def mutual_one(x, y = 0)
  return terminal_value(y) if x.negative?
  tail_call { mutual_two(x, y + 1) }
end
def mutual_two(x, y)
  tail_call { mutual_one(x - 1, y) }
end
mutual_one(50_000).invoke
# => 50001

require 'mutual_recursion'
include MutualRecursion
def direct(x, y = 0)
  return terminal_value(y) if x.negative?
  tail_call { direct(x - 1, y + 1) }
end
direct(50_000).invoke
# => 50001

require 'mutual_recursion'
include MutualRecursion
def proc_returning(x, y = 0)
  return terminal_value(proc { "|#{y}|" }) if x.negative?
  tail_call { proc_returning(x - 1, y + 1) }
end
generated_proc = proc_returning(20).invoke
generated_proc.call
# => "|21|"

require 'mutual_recursion'
def without_include(x, y = 0)
  return MutualRecursion.terminal_value(y) if x.negative?
  MutualRecursion.tail_call { without_include(x - 1, y + 1) }
end
without_include(50_000).invoke
# => 50001