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 shout-out to the folks at Wikisource that transcribed this to an accessible text form.

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

From an implementer perspective the problem is that stack cleanup is tricky: A function that tail calls another cannot clean up any temporary variables it passes along. The solution to this is to make sure function stack use is identical and have the caller clean up, or to implement garbage collection.

Language support

Despite 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:

• 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)

Here's an incomplete list of languages that require explicit support:

• 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:

• 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)

Things look decent for desktops, but not so much for phones or web browsers.

Personally I lean towards the idea of languages adding new keywords or explicit support for this, such as a 'goto' or 'jump' keyword. It helps in a debugger when you have stack frames by default, and it helps make it clear that it's important that this tail call is optimized.