Recursion

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()

This is a bit longer than a non-recursive example.

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)

A non-recursive version of this would likely use some kind of for or while loop.

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)

Without recursion this code would look a lot more confusing, at least to me.

State machines

Not all recursion has to be direct. Indirect recursion lets you represent state machines easily.

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()

Without recursion you'd likely need to put everything in a single function with a loop and state variable.

Mainstream support

- functional programming languages

- lua

- clang mustcall

- webassembly