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| [[Category:Research]]
| | 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 using Lua.
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| | - introduction/overview |
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| | - 'magic of recursion'? |
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| | ==A bad example== |
| | - fibonacci example in lua |
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| | - intertwines recursive algorithm and recursion |
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| | - mention stack overflow |
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| | - imagine an infinite stack |
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| ==Loops== | | ==Loops== |
| Recursion can create loops without language constructs.
| | - recursion can implement loops! |
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| | - implementing a for loop |
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| Here's an infinite loop:
| | - implementing a while loop |
| function infinite_loop()
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| print("Hello there!")
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| return infinite_loop()
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| end
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| infinite_loop()
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| This is a bit longer than a non-recursive example.
| | - implementing a do while loop |
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| Here's a counting loop:
| | - each loop iteration only shares global and function args |
| function count_down(number)
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| if number == 0 then return end
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| print(number)
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| return count_down(number - 1)
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| end
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| count_down(100)
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| A non-recursive version of this would likely use some kind of for or while loop.
| | ==State machines== |
| | - implementing a stateful algorithm |
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| Here's a loop that asks a user to pick a valid choice:
| | - some kind of menu system |
| function get_choice(choices)
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| local line = io.read()
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| choice = choices[line]
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| if choice then
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| return choice
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| else
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| print("Invalid choice! Try again")
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| return get_choice(choices)
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| end
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| end
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| print("Select a letter to get a number: A, B, C")
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| choice = get_choice({A=1, B=2, C=3})
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| print("You picked number " .. choice)
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| Without recursion this code would look a lot more confusing, at least to me.
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| ==State machines==
| | - the code makes sense to read |
| Not all recursion has to be direct. Indirect recursion lets you represent state machines easily.
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| Here's a tiny adventure game with the player choosing state transitions:
| | - this is mutual recursion |
| function dark_room()
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| print("You are in a dark room.")
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| print("Pick a door: fuzzy or metal")
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| choice = get_choice({fuzzy=1, metal=2})
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| if choice == 1 then return fuzzy_room()
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| elseif choice == 2 then return metal_room()
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| end
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| end
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| function fuzzy_room()
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| print("This room feels pretty fuzzy...")
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| print("Pick a door: dark, metal")
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| choice = get_choice({dark=1, metal=2})
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| if choice == 1 then return dark_room()
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| elseif choice == 2 then return metal_room()
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| end
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| end
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| function metal_room()
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| print("This room feels really metallic.")
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| print("Pick a door: dark, fuzzy or win")
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| choice = get_choice({dark=1, fuzzy=2, win=3})
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| if choice == 1 then return dark_room()
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| elseif choice == 2 then return fuzzy_room()
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| elseif choice == 3 then return metal_room()
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| end
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| end
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| function win_room()
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| print("You found the treasure!")
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| return
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| end
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| dark_room()
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| Without recursion you'd likely need to put everything in a single function with a loop and state variable.
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| Some things just make more sense when implemented recursively, to me at least.
| | - very hard to do in a traditional structured language |
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| == Tail call optimization == | | == Lambdas == |
| 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.
| | - lambdas to actually replace looping constructs/switch statements |
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| 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.
| | - most mainstream languages support lambas |
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| 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.
| | - recursion-based control flow |
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| The 1977 [https://en.wikisource.org/wiki/Index:AIM-443.djvu 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.
| | ==Tail call elimination== |
| | - floating back down to reality |
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| 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.
| | - we've been writing code as there's no stack |
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| 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.
| | - tail call elimination |
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| ==Language support==
| | - hints deeper at function calls vs jumps |
| 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.
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| Here's an incomplete list of languages that support it automatically:
| | - structured programming, goto wars |
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| * Haskell
| | == Mainstream support == |
| * Erlang (and Elixir)
| | - functional programming languages |
| *Any Scheme implementation (Chez Scheme, Chibi Scheme, Chicken Scheme, TinyScheme)
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| *Lua (see [https://www.lua.org/pil/6.3.html Programming in Lua 6.3 - Proper Tail Calls])
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| *Steel Bank Common Lisp (See [http://www.sbcl.org/manual/#Debug-Tail-Recursion SBCL Debug Tail Recursion])
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| *Squirrel (See [http://squirrel-lang.org/squirreldoc/reference/language/threads.html Squirrel's Threads page])
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| *Racket (See [https://docs.racket-lang.org/guide/Lists__Iteration__and_Recursion.html#%28part._tail-recursion%29 The Racket Guide 2.3.3 - Tail Recursion])
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| Here's an incomplete list of languages that require explicit support:
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| * Clang C and C++ (see the [https://clang.llvm.org/docs/AttributeReference.html#musttail Clang musttail attribute])
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| * Tcl (see [https://www.tcl.tk/man/tcl/TclCmd/tailcall.html Tcl's tailcall manual page])
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| *OCaml (See [https://ocaml.org/manual/attributes.html OCaml's tailcall attribute])
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| * Perl (See [https://perldoc.perl.org/functions/goto Perl's goto function], specifically the goto &NAME variant)
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| *Unix (See [https://jeapostrophe.github.io/2012-05-28-exec-vs--post.html exec and Tail-call Optimization])
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| *Assembly (Instead of returning set up registers and jump)
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| *Ruby (See [https://nithinbekal.com/posts/ruby-tco/ Ruby's tailcall_optimization compile option])
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| *Zig (See [https://ziglang.org/documentation/master/#call Zig's always_tail call option])
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| Here's an incomplete list of popular languages that don't support it:
| | - lua |
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| * C and C++
| | - clang mustcall |
| * Go
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| * Rust
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| * Swift
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| * PHP
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| *Python
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| *Raku
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| * Anything running on the JVM (Java, Clojure, Scala, Kotlin)
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| * Anything running on .NET (C#, F#)
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| * Anything running on WebAssembly
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| * Anything JavaScript or transpiling to JavaScript (TypeScript, CoffeeScript)
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| Things look decent for desktops, but not so much for phones or web browsers.
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| 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.
| | - webassembly |