Recursion

Warning: This article is a work in progress! No refunds if it moves!

Recursion is a fantastic and often ignored feature of programming languages. Most introductions show an example you'd never use in practice, so this article is my attempt at showing some better ones.

Loops

Recursion can create loops!

Here's an infinite loop:

function infinite_loop()
  print("Hello there!")
  return infinite_loop()
end
infinite_loop()

Here's a counting loop:

function count_down(number)
  if number == 0 then return end
  print(number)
  return count_down(number - 1)
end
count_down(100)

Here's a loop that asks a user to pick a valid choice:

function get_choice(choices)
  local line = io.read()
  choice = choices[line]
  if choice then
    return choice
  else
    print("Invalid choice! Try again")
    return get_choice(choices)
  end
end
print("Select a letter to get a number: A, B, C")
choice = get_choice({A=1, B=2, C=3})
print("You picked number " .. choice)

State machines

Mutual recursion can be used to make state machines!

Here's a tiny adventure game with the player choosing state transitions:

function dark_room()
  print("You are in a dark room.")
  print("Pick a door: fuzzy or metal")
  choice = get_choice({fuzzy=1,metal=2})
  if choice == 1 then return fuzzy_room()
  elseif choice == 2 then return metal_room()
  end
end
function fuzzy_room()
  print("This room feels pretty fuzzy...")
  print("Pick a door: dark, metal")
  choice = get_choice({dark=1,metal=2})
  if choice == 1 then return dark_room()
  elseif choice == 2 then return metal_room()
  end
end
function metal_room()
  print("This room feels really metallic.")
  print("Pick a door: dark, fuzzy or win")
  choice = get_choice({dark=1, fuzzy=2, win=3})
  if choice == 1 then return dark_room()
  elseif choice == 2 then return fuzzy_room()
  elseif choice == 3 then return metal_room()
  end
end
function win_room()
  print("You found the treasure!")
  return
end
dark_room()

Mainstream support

- functional programming languages

- lua

- clang mustcall

- webassembly