Editing Recursion
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print("You are in a dark room.") | print("You are in a dark room.") | ||
print("Pick a door: fuzzy or metal") | print("Pick a door: fuzzy or metal") | ||
choice = get_choice({fuzzy=1, metal=2}) | choice = get_choice({fuzzy=1,metal=2}) | ||
if choice == 1 then return fuzzy_room() | if choice == 1 then return fuzzy_room() | ||
elseif choice == 2 then return metal_room() | elseif choice == 2 then return metal_room() | ||
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print("This room feels pretty fuzzy...") | print("This room feels pretty fuzzy...") | ||
print("Pick a door: dark, metal") | print("Pick a door: dark, metal") | ||
choice = get_choice({dark=1, metal=2}) | choice = get_choice({dark=1,metal=2}) | ||
if choice == 1 then return dark_room() | if choice == 1 then return dark_room() | ||
elseif choice == 2 then return metal_room() | elseif choice == 2 then return metal_room() | ||
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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. | 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 | However there is a compromise: If a return in a function is just a call to another function then return is a 'tail call'. Languages that implement tail call optimization will re-use the current function call's stack for the tail call, 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. | 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 [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 | 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 shoutout 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. | 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. | ||
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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. | 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. | ||
== | ==Mainstream 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. | 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 | Here's an incomplete list of languages that support it: | ||
* Lua (see [https://www.lua.org/pil/6.3.html Programming in Lua 6.3 - Proper Tail Calls]) | |||
* Clang C and C++ (see the [https://clang.llvm.org/docs/AttributeReference.html#musttail musttail attribute]) | |||
* Tcl (see Tcl's [https://www.tcl.tk/man/tcl/TclCmd/tailcall.html tailcall manual page]) | |||
* Haskell | * Haskell | ||
* Erlang (and Elixir) | * Erlang (and Elixir) | ||
*Any Scheme implementation (Chez Scheme, Chibi Scheme, Chicken Scheme, TinyScheme) | *Any Scheme implementation (Chez Scheme, Chibi Scheme, Chicken Scheme, TinyScheme) | ||
Here's an incomplete list of popular languages that don't support it: | Here's an incomplete list of popular languages that don't support it: | ||
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* Rust | * Rust | ||
* Swift | * Swift | ||
* Perl | |||
* PHP | * PHP | ||
* Anything running on the JVM (Java, Clojure, Scala, Kotlin) | * Anything running on the JVM (Java, Clojure, Scala, Kotlin) | ||
* Anything running on .NET (C#, F#) | * Anything running on .NET (C#, F#) | ||
* Anything running on WebAssembly | * Anything running on WebAssembly | ||
* Anything JavaScript or transpiling to JavaScript (TypeScript, CoffeeScript) | * Anything JavaScript or transpiling to JavaScript (TypeScript, CoffeeScript) | ||
The situation looks a little bleak at this point. Maybe the future will be better? | |||