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 using Lua.

Loops
Recursion can create loops without language constructs.

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.

Some things just make more sense when implemented recursively, to me at least.

Tail call optimization
There is a caveat with recursive programs: Each function call takes up stack space. The deeper you recurse, the more likely you are to run out of stack space and crash your program. This makes recursion useless in most programming languages.

However there is a compromise: If a return in a function is just a call to another function then that return call is a 'tail call'. Languages that implement tail call optimization will re-use the current function call's stack for the function you're calling, solving the issue of stack space.

All the examples on this page use tail calls and run in Lua which implements tail call optimization. This means every program on this page is immune to stack overflows.

The 1977 AI Lab Memo 443 talks more broadly about how tail calls are like goto statements that you can pass arguments to. Huge shoutout to the folks at Wikisource that transcribed this to an accessible text form.

The significant downside of tail call optimization is that it can make debugging more difficult as you lack a proper stack trace.

Mainstream support
Despite mainstream languages slowly adding features from functional languages developed 40 years ago, tail call optimization is still unpopular. I'm guessing that the reason is because not many people see the use of recursion.

Here's an incomplete list of languages that support it automatically:

Here's an incomplete list of languages that require explicit support:
 * Haskell
 * Erlang (and Elixir)
 * Any Scheme implementation (Chez Scheme, Chibi Scheme, Chicken Scheme, TinyScheme)
 * Lua (see Programming in Lua 6.3 - Proper Tail Calls)
 * Steel Bank Common Lisp (See SBCL Debug Tail Recursion)
 * Squirrel (See Squirrel's Threads page)
 * Racket (See The Racket Guide 2.3.3 - Tail Recursion)
 * Clang C and C++ (see the Clang musttail attribute)
 * Tcl (see Tcl's tailcall manual page)
 * OCaml (See OCaml's tailcall attribute)
 * Perl (See Perl's goto function, specifically the goto &NAME variant)
 * Unix (See exec and Tail-call Optimization)
 * Assembly (Instead of returning set up registers and jump)
 * Ruby (See Ruby's tailcall_optimization compile option)
 * Zig (See Zig's always_tail call option)

Here's an incomplete list of popular languages that don't support it:

Things look decent for desktops, but not so much for phones or web browsers.
 * C and C++
 * Go
 * Rust
 * Swift
 * PHP
 * Python
 * Raku
 * Anything running on the JVM (Java, Clojure, Scala, Kotlin)
 * Anything running on .NET (C#, F#)
 * Anything running on WebAssembly
 * Anything JavaScript or transpiling to JavaScript (TypeScript, CoffeeScript)