Specific to the world of programming, what does "Turtles all the way down" mean?

Question

I hear this phrase often and do not fully understand it's meaning. What does it mean? And if possible, is there an example?

thank you!

Well, it's really more of [a phrase from popular culture](http://en.wikipedia.org/wiki/Turtles_all_the_way_down) that can be applied to many areas in CS. — Michael Petrotta, Aug 18 '12 at 20:56
The channels documentation defines it here: https://channels.readthedocs.io/en/latest/introduction.html#turtles-all-the-way-down — MattCochrane, Oct 25 '20 at 05:58

Mark Byers · Accepted Answer · 2012-08-18T21:04:05.013

One use of this expression refers to a style of programming where there is a very deep call stack. You might see a method called Grobble and wonder what it does, so you open up the definition and see this:

class FooHandler
{
    void Grobble(Foo foo)
    {
        foo.Grobble();
    }
}

So then you look at Foo.Grobble:

class Foo
{
    FooImpl _fooImpl;

    void Grobble()
    {
        _fooImpl.Grobble();
    }
}

That takes you too FooImpl which looks like this:

class FooImpl
{
    void Grobble()
    {
        this.Grobble(false);
    }

    // etc...
}

After going deeper and deeper into the code, still unable to see the end, you are allowed to frustratedly exclaim "It's turtles all the way down!"

The reference is to the metaphor of the Earth being on the back of a turtle. What is the turtle standing on? Another turtle... etc.

Perhaps literally if you manage to pass a continuation to itself. — Antimony, Aug 18 '12 at 21:02
Well shit, knowing this, and knowing the legacy code i've spent 2 years working on... I have missed at least 100 opportunities to exclaim the phrase!!! — Mario Zigliotto, Aug 19 '12 at 19:17

score 5 · Answer 2 · answered Aug 18 '12 at 20:58

5

This usually refers to self-hosting programming languages, where the interpreter or compiler is written in the same language that is being interpreted/compiled. It can also refer to the libraries for the language being written in the language itself.

Smalltalk and Lisp are well known for this kind of thing.

answered Aug 18 '12 at 20:58

Adam B

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In the context of Smalltalk, it could also be a reference to the LOGO programming language from Seymour Papert where LOGO's Turtle can be instructed to move and produce graphics. Seeing all kinds of graphical objects in Smalltalk, one would exclaim: "There are no pixels - it's turtles all the way down!" – kiplantt Oct 29 '18 at 10:13

score 1 · Answer 3 · answered Aug 18 '12 at 20:56

Turtles all the way down is a phrase sometimes used to refer to infinite recursion. For example, what is the least integer? There isn't one. -2 is less than -1, -3 is less than -2 and so on. There is no bottom. The original source of the quote was an answer to the question of "If the world is on the back of a turtle, what does the turtle stand on?". Anyway, it doesn't have any programming specific meaning that I know of.

score 1 · Answer 4 · answered Aug 21 '12 at 21:47

1

It is sometimes used to refer to 'pure' object oriented (or functional) languages. In a.o. java, c#, c++, Objective c and Delphi, there are native types (int) that don't behave like objects. The illusion is maintained much better in Smalltalk.

answered Aug 21 '12 at 21:47

Stephan Eggermont

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And in JavaScript / node.js – CommaToast Jan 12 '13 at 07:27

Specific to the world of programming, what does "Turtles all the way down" mean?

4 Answers4