Editing 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