Ask HN: How do you explain closures? | Hacker News Reader

Ask HN: How do you explain closures?

69 pointsdstorrs64 comments

In the past year or two, I've had to explain closures--what they are and why they are interesting--several times. Usually, at the end of my explanation, the person doesn't get it, and they go back to doing whatever they are familiar with.

So, I ask HN to put yourselves in this scenario and tell me what your answer would be:

Joe is a talented but inexperienced developer. He's got a year or two of experience, maybe as much as 5, but he programs for a job, not for fun. Also, for whatever reason he's never encountered closures--most likely he's only worked in languages that don't really support them.

You want to explain to him (a) what they are and (b) why he would want to use them. You have about 10 seconds to get his interest before he starts thinking about his WoW raid tonight, and about 5 minutes for the whole explanation.

What do you say?

View on HN

You have about 10 seconds to get his interest before he starts thinking about his WoW raid tonight, and about 5 minutes for the whole explanation.

Not going to happen. First, you need a person that wants to better themselves as a programmer, and understanding closures is just a part of that. Discounting the time it takes to put someone in that state of mind, closures can be explained in under a minute (edit: OK, maybe two minutes ;)).

A closure is just a (function, bindings) pair. If someone has a Java/C++ background, it might help to think of a closure as an object which has some code with references to variables defined outside of that code. Those variables are dependencies without which you can't run the code.

JavaScript example:

    function foo() {
        var x = 1;
        function bar() {
            return x + 1;
        }
        return bar;
    }

What does foo() return? It returns bar. But it can't just return bar, since bar has a dependency on x, so it returns bar and everything bar references: the closure (bar, { x: 1 }). In practice, you call a closure as you would call a function, but underneath the abstraction, you need to realize what's returned is both bar and the value of x.

So when does "x" get evaluated? What if x was a global variable instead? (Honest questions here, not trying to sidetrack the discussion. No, I don't "get" closures.)

Every time you call bar, x gets evaluated again. If x was a global variable, the same thing would happen.

Closures don't change the scope of any existing thing. What they do is force the program to keep scopes around because something still refers to them. Those scopes become a place to stick private data. If you create multiple functions in that scope, they can communicate through that private data. Here is a simple example in JavaScript.

  function closure_demo (x) {
      return {
           set: function (s) {x = s}
         , get: function () {return x}
      };
  }

  var foo = closure_demo("hello");
  var bar = closure_demo("world");

  alert("Bar is " + bar.get());
  alert("Foo is " + foo.get());

  foo.set("goodbye");
  bar.set("earthlings");

  alert("Bar is now " + bar.get());
  alert("Foo is now " + foo.get());

What is happening here? Each function call has a scope. We return an object with 2 functions that can access this scope. Each time you call the function you get a separate scope, so if you call it twice you get different closures. The scope must exist as long as those functions exist.

This is a good example of the problem with saying "closure" rather than "lexical scope". What most people mean by "closure" is a single anonymous function with state attached. What you've done here is returned an anonymous object containing two named functions.

So, it's obviously a closure because x is being "closed over". But if we're being pedantic it's actually not a closure. It's a lexically scoped anonymous object. And a closure is just an anonymous function, no more no less. A function in a language with lexical scope.

Sorry, but your attempted pedantry betrays some fundamental misunderstandings.

You claim that a closure is just an anonymous function, no more no less. But as http://en.wikipedia.org/wiki/Closure_%28computer_science%29 points out, this is a common misunderstanding that arises because people are introduced to the concepts at the same time. But the concepts of closures and anonymous functions really are distinct.

A closure is a function that closes over some lexical environment. Whether the function is named or not is irrelevant. In JavaScript if I type

  foo = some_function_that_returns_a_closure();

then foo winds up being a named function, but it is still a closure. Most languages require more gyrations to wind up with a named function. But dynamic languages that allow metaprogramming at run-time usually have some way to make a closure into a named function. For example:

  ; Scheme
  (define foo some_closure)
  
  # Perl
  *foo = $some_closure;

  ; Common Lisp
  (setf (symbol-function ’foo) #’some-closure)

Going the other way, anonymous functions are not always closures. What if you create an anonymous function outside of any lexical scope, it isn't a closure. What if you create an anonymous function in a language without the idea of lexical scope? Such languages exist. You can play with function pointers in C, anonymous functions in elisp (the Lisp built into emacs), and anonymous functions in vimsh (the scripting language built into vim) all you want but you won't get closures in those languages.

Now back to my example. I returned a data structure that had two closures that closed over the exact same lexical scope. The functions foo.set, foo.get, bar.set and bar.get are all closures. Since pairs of them share environments they can communicate through those environments. But the fact that they appear in a more complex data structure doesn't change the fact that they are closures. If you prefer you can pull them out into variables and manipulate them that way:

  var foo_set = foo.set;
  var foo_get = foo.get;

  // time passes
  foo_set("hello");
  alert(foo_get());

See? Closures!

Don't be confused by the fact that JavaScript built an object system on top of data structures with closures. That detail is a red herring. Anyways you've got data structures with cooperating closures, anyone can build object systems. It is easier than you might think, as pg demonstrated nicely in his book On Lisp. (In section 25.2 he implements a reasonable object system in just 2 pages of code.) And the people at Netscape who invented JavaScript had been around Lisp enough that doing so was a natural step for them.

The moral is that what matters are the mechanic Any function that has variables bound to a lexical scope is a closure. No matter what context it appears in. Whether it is named, part of an object, some other data structure, etc, it is still a closure.

If x was a global variable then you don't have a closure, just an anonymous function.

Closures require closed over variables.

>Closures require closed over variables.

Is "over variables" a term here? Is that the same as "upvalues"?

Yup, the "upvalues" of a closure are the dependencies of the function that the closure "closes over".

So in OO terms, the reason "bar closes over x" is because bar's execution has a dependency on the value of x. Since bar needs x to execute, we "close bar over x (and whatever other dependencies bar has)" to create the "closure".

To demonstrate some other uses, when we take "the closure [of|over] bar", we "close over bar", meaning "find all of bar's dependencies, and bundle them". The resulting closure is "closed over x", as well. I guess in this context "over" means "including". So you've got a bunch of references and a function laying around. You take a "closure" like you would a tarp or something, and put it "over" all that stuff. Now you've got the function and the references "closed over" by the "closure".

Personally, I think this terminology is a little weird and immediately intuitive to someone without a math background [1]. On the other hand, getting good at comprehending less-than-intuitive definitions seems to be a big part of CS and Math, so it's important to get used to it :)

1. There are "closed" things are all over mathematics, meaning vaguely similar things: http://en.wikipedia.org/wiki/Closed

Your explanation here is very, very helpful. I just copied and pasted it into a file for later reference. Thank you for taking the time to explain this!

Cool! Glad to be helpful :) I feel like I'm rephrasing the same thing a lot, but in hindsight, closures took me quite a bit of repetition/rephrasing to wrap my mind around. To fully grok them, you really need to have a clear understanding of how scope and variable references work.

Great question! Every time you call foo, a new scope containing x is created. That scope "travels" with bar, and is evaluated (dereferenced might be a better word) just as any other variable inside bar would be - during the execution of bar. A global variable is just a variable in the topmost ("global") scope.

But my example doesn't make the distinction to clarify that at all. Here's a better one:

    function colors() {
        var cs = ['red', 'green', 'blue'],
            i = 0;

        function nextcolor() {
            i = (i+1) % cs.length;
            return cs[i];
        }

        return nextcolor;
    }

Every time you call colors, a new local scope is created. Easiest way to think of scopes is as maps from strings to values. In this case, a new scope { cs: ['red', 'green', 'blue'], i: 0 } is created every time you calls colors. If the language didn't support closures, that scope would be destroyed when colors finishes execution, but in JS, that scope is bound to nextcolor and returned along with nextcolor in a closure. Now, every time you call a particular nextcolor returned by a call to colors, the cs and i variables will reference those same values. Example using firebug:

    >>> A = colors()
    >>> B = colors()
    >>> A() => "green"
    >>> A() => "blue"
    >>> B() => "green"
    >>> B() => "blue"
    >>> A() => "red"
    >>> A() => "green"
    ...

Keep asking if anything is unclear. I was wrong about this taking 1 minute, I suppose... :)

Your two javascript examples are fantastic. Thank you for explaining this in a very clear way.

For the record: on second reading, I should have said "that scope is bundled with nextcolor and returned as a closure", not "bound to nextcolor", since the word "bound" is usually used to talk about variable names ("identifiers") and their values.

Similarly, in the next sentence, I should have pointed out that when you call the returned value, you're not just calling nextcolor but the closure over nextcolor, which includes the bindings for cs and i.

They are functions you can make up on the spot. That's usually the extent of my explanation.

Surprisingly, most of the people I talk to assume that the "fancy" parts of closures (such as variable value capture and upvalues) are the way such a construct should work and require no further explanation to use them. People who are surprised by capture (and assume, for instance, that the variable will take on the most recent value assigned to it by outside code instead of "remembering the value when the closure was made") get a more lengthy computer science-esque explanation. But I generally find that explanation to be unhelpful if a person can't see why closures are useful; "An anonymous function with some teeth" is often how people I talk to come to closures first, so it's how I let them think about it until the model is no longer helpful.

Right. That's exactly how it was with me when I first encountered closures. Explaining something that would be guessed anyway just makes it seem more complicated than it is.

I implemented anonymous methods in Delphi, and I faced a similar kind of problem trying to sell them, as it were, to the Delphi community.

What they are: functions with state.

Why they're useful: callbacks where a whole separate method or class would be overkill.

Motivating examples:

* Delegating work to a background thread. Captured variables are an easy way to pass along necessary state.

* A generic benchmarking routine. Benchmarking requires taking a measurement before the code, running the code to be measured possibly multiple times, and taking another measurement after the code. If you can turn the code to be measured into a callback, then the benchmarking code can be written once and reused elsewhere. But having to put all your benchmarks into their own methods, much less worry about state and possibly creating classes and initializing data in constructors, is pretty tedious. Closures avoid this tediousness.

(Yes, I'm conflating several concepts here, anonymous functions and lambda closures, for colloquial and didactic reasons. Nitpicking doesn't buy you much in terms of practicality when learning here.)

Are they similar in any way to Python generators? They are functions with a state, at least.

Multiple closures can share the same state ("environment"). Consider:

  (let ((i 0))
    (list (lambda () (incf i))
          (lambda () (decf i))
          (lambda () i)))

If you evaluate the first function this form returns ("increment") and then the third form ("get"), the result is 1. If you evaluate the whole form again, you now have another "instance", with its own separate shared i.

In Python, you would probably just make this a class. (But sometimes it's nice to have something equivalent to a class that's automatically built for you, and closures are that something.)

I usually go with a simple counter-generator as a first example:

  function counterMaker(next) {
    var value = next;
    return function() { value++; return value }
  }

  var myCounter = counterMaker(42);
  alert(myCounter());
  alert(myCounter());
  alert(myCounter());

I'm not sure it is necessary to explain what they are/how they work right at the start. An example like this is pretty clear that something "weird" is going on and if "Joe" is interested at all, questions should probably come up which can then be answered with more details. I think the "what to use them for" stuff is going to depend quite heavily on the environment, language, libraries, and product you're working on.

I usually find it's better to introduce a language idea to someone and let them try to find out where they might put it on their own - they may surprise you. If you're doing code reviews, you could use that as an opportunity to point out how something like a closure (or any other construct) might have improved a situation, or, at least, how it may have changed it.

I've found that the concept of anonymous functions is the biggest hurdle to get over -- closures seem to follow as a natural extension of that.

I explained this to someone recently in the context of event handlers in ActionScript/JavaScript, starting with the idea that when you do:

  something.onclick = function(){...};

you're passing a function around just like it was any other value.

Then I moved to an artificial example of a 'magic function-making machine':

  function giveMeAnAdder(baseNumber) {
    return function (i) {
      return baseNumber + i;
    };
  }

with a couple of examples of storing and using the functions that giveMeAnAdder returns.

Then I pointed out that, normally, local variables in a function disappear when the function returns, so how does my adder 'remember' what baseNumber is? And the answer is that the anonymous function is actually a bundle of a function body and the local context, and we call this a closure.

Thanks, this looks like a good way to present it. Next time I'm in this position, I'll use this as my jumping off point for the discussion.

This is my approach as well. JavaScript provides a very natural syntax for it, and the use case of "make me an event handler with some stuff baked into it" is extremely obvious and prevalent.

Can people please stop using the "make adder" examples? Can't you come up with something original and fresh? The triviality of what is acomplished in such case actually obscures the usefulness and power of the concept. As a result, the novice will interpret the end product as some useless, magical mambo jumbo that doesn't give him much. So who cares if I can get a function that adds a number to something. I have a "+" for that. Give me a use case that I can admire...

Why do you want me to appreciate a ferrari if all you do with it is go pick up some groceries...

That's why I use a counter as a simple demo - you see state changing with each call and it's done without using a global or passing in some kind of context object. It's not a lot more powerful than the adder, but in my experience it more often generates a "what the hell..." kind of reaction to the uninitiated.

I agree that for a novice a trivial example doesn't inspire them to learn more. But an experienced programmer who just wasn't familiar with this concept might appreciate more a stripped-down example like this that demonstrates the essentials of the idea. And in general, different people have different learning styles.

"Original and fresh" is generally not conducive to understanding. People are more likely to understand a new concept when that new concept is applied to concepts they already understand. It lets them filter out everything but that concept.

If they don't already understand object-oriented programming, you make sure they understand that first.

If they understand objects, you explain that closures are the same as objects. They are bundles of state that carry around functions that operate on that state. The only real difference is that a single closure only contains a single function (method) that operates on its data, where a single object can have several methods.

An interesting demonstration of why closures are useful is to start from a hypothetical language that supports closures but not objects and then implement a (simple!) object/class system. For me, it was extremely clarifying to have something in my mind click and go "Oh, OOP doesn't need to be a language feature ... there's this much more powerful concept available as a building block."

The venerable master Qc Na was walking with his student, Anton. Hoping to prompt the master into a discussion, Anton said "Master, I have heard that objects are a very good thing - is this true?" Qc Na looked pityingly at his student and replied, "Foolish pupil - objects are merely a poor man's closures."

Chastised, Anton took his leave from his master and returned to his cell, intent on studying closures. He carefully read the entire "Lambda: The Ultimate..." series of papers and its cousins, and implemented a small Scheme interpreter with a closure-based object system. He learned much, and looked forward to informing his master of his progress.

On his next walk with Qc Na, Anton attempted to impress his master by saying "Master, I have diligently studied the matter, and now understand that objects are truly a poor man's closures." Qc Na responded by hitting Anton with his stick, saying "When will you learn? Closures are a poor man's object." At that moment, Anton became enlightened.

You have much to teach us, great one.

No link to the source?

http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/m...

The idea is really nice, but I'm not sure it would work on the OP's Joe, who probably wouldn't even care about the hypothetical language and would regard the whole concept hypothetical, only for those who have way too much free time. (With WoW on the table, you don't have that luxury.)

I would take the route of first order functions -> anonymous functions -> closures, but I think it is really a lost cause for the typical 9-5 programmers.

I would take the route of first order functions -> anonymous functions -> closures

But those are three completely different concepts. A closure need not be anonymous or even first-order.

Yes. This approach works. I used it when I actually did work in a proprietary language which had no usable objects, but did have half-decent lambdas. Most people did not understand or use them at all, but one new guy with a Java background asked me how to cope. I explained to him how to use closures to implement a simple bank account class, and he got it immediately.

I don't think this guy played WoW, though. :)

You still want applicable objects, though, and closures don't quite cut it. (I say this as a schemer.)

See http://www.r6rs.org/formal-comments/comment-6.txt, for example.

Closures are essentially a way to create private variables.

For example, in Javascript:

    var fn = null;
    
    (function(){
        
        var temporary = 789;
        alert(temporary);
        
        var secret_private = 5;
        fn = function(){
            alert(secret_private);
        };
    
    })();
    
    fn();

In javascript,

    (function(){ ... })();

defines and executes kamikaze function. It's a function which dies as soon as its job is completed.

We can use the kamikaze to hide secret_private from all but the functions that we want to have access to it.

Since fn is defined in the kamikaze function, fn can access secret_private.

Since fn is assigned to a global variable, it can be accessed from outside the kamikaze.

When the kamikaze dies, it takes all of its variables with it, except for the ones that are referred to outside of the kamikaze.

temporary disappears because it is referred to only inside the kamikaze.

But, since secret_private is referred to in fn, and fn is a global, secret_private does not disappear, but now only fn can refer to it.

So sceret_private is essentially a private variable of fn.

Kamikaze functions? Sounds dangerous...

Dangerous, maybe, but high affect.

I'm referring to all my anonymous functions from now on as kamikaze functions. Brilliant.

You could try an example showing what a closure does 'under the hood' by showing an equivalent non-closure example, like:

With Closure:

  foreach (var obj in objects) {
    AddButton("Delete " + obj.ToString(), () => obj.Delete());
  }

Without closure:

  class DeleteHandler {
    public DeleteHandler(TYPE obj) {
      _obj = obj;
    }
    private TYPE obj;
    public void DoDelete() {
      obj.delete();
    }
  }
  ...
  foreach(var obj in objects) {
    var deleteHandler = new DeleteHandler(obj);
    AddButton("Delete " + obj.ToString(), deleteHandler.DoDelete);
  }

I'm not convinced by the utility of your example. What's wrong with:

    foreach (var obj in objects) {
      AddButton("Delete " + obj.ToString(), obj.Delete);
    }

This is even simpler and doesn't use closures at all.

Doh!

It would have made more sense if I had some other piece of information to inject in, like if you had a database reference that needed to be passed in to the Delete() method.

Then:

  AddButton("Delete " + obj.ToString(), () => obj.Delete(db));

Vs.

  var d = new DeleteHandler(obj, db);
  AddButton("Delete " + obj.ToString(), d.DoDelete);

EDIT: fixed

Yup, makes more sense now! This is a good example, I think, to give to people who are used to objects being used for everything. Nice, clean, functional syntax.

(And of course you mean "d.DoDelete")

A closure enables you to keep local state in a functional language. Think of it as an object (a really abused word, I know, but it conveys the idea).

So what are the mechanics? Follow me (I'll use Arc):

Lesson 1) A function can return a function:

  arc> (def adder-factory () [+ 1 _])
  #<procedure: adder-factory>
  arc> (= add-1 (adder-factory))
  #<procedure>
  arc> (add-1 10)
  11

Lesson 2) A created function can hang on to information which you pass in:

  arc> (def adder-factory (c) [+ c _])
  #<procedure: adder-factory>
  arc> (= add-17 (adder-factory 17))
  #<procedure>
  arc> (add-17 13)
  30

Lesson 3) The information a function hangs on to can be mutable local state.

  arc> (def make-point ((o x 0) (o y 0))
            "Constructor for creating point objects"
            (let methods (obj
                    get-x (fn () x)
                    get-y (fn () y)
                    set-x [= x _ ]
                    set-y [= y _ ])
                (fn (method . args)
                    (apply (methods method) args))))
  #<procedure: make-point>
  arc> (= pt (make-point))
  #<procedure>
  arc> (= pt (make-point))
  #<procedure>
  arc> (pt 'get-x)
  0
  arc> (pt 'set-x 17)
  17
  arc> (pt 'get-x)
  17

And that is a "closure". The name comes from the fact that the scope of the created function "closes over" the local variable.

Helpful or noisy?

Closures are functions with state.

Your claim that Joe is a talented programmer isn't very persuasive. Edit: maybe I should say why. I have known many programmers of the 9-5 type who do a passable job, but are not interested in learning anything. It is as if they had a limited supply of energy for learning and used it up when they were getting started. This is a common type of working programmer and you made it sound like Joe might be one.

Closures are different from objects in only a couple of ways: (1) the creation protocol is extremely concise (lambda) (2) the access interface is the function call

Say that closures lower the syntactic overhead of accessing the same flexibility he normally turns to objects for.

So if he uses closures he will effectively use objects in more situations simply because it becomes easier and this will increase the flexibility of his code.

Closures are the context in which a function executes. The best way to demonstrate this is with a silly example that sticks in the mind:

  def learn_how_to_greet(formal=True):
    if formal:
      string = "Good day, Mr. {last_name}."
    else:
      string = "What's up, {first_name}?"
    def greet(**kwds):
      return string.format(**kwds)
    return greet

What happens if you do:

  def learn_how_to_greet(formal=True):
    if formal:
      string = "Good day, Mr. {last_name}."
    else:
      string = "What's up, {first_name}?"
    def greet(**kwds):
      return string.format(**kwds)
    ret = greet
    string = "WHOA! Look at me!"
    return ret

In other words, at what point do the variables used in the "greet" function get saved off? And what is saved, the variable name itself, or the data in it?

  >>> learn_how_to_greet()(first_name="Joe", last_name="Smith")
  'WHOA! Look at me!'

It should be saved as its value at the point of greet's definition, but Python appears to not have real closures, so the change in the value of string afterwards changes it in greet too.

Why do you say that the value should be saved at its definition? I realize Java does that (with anonymous classes & final), but allowing mutation of closured state lets you fairly easily write things - like an OO class system, if you want to.

What does "real" mean? When you use closures (or not-quite-closures, or however you want to call them) in python, they work just like "real" closures, except for the pathological case of defining functions in loops.

I think it happens when the closing scope stops being local (i.e. at the time the defined function gets returned).

A lexical closure is similar in many ways to the class of an object-oriented language. In fact, lexical closures can be used to create an object system.

A class in PHP, for example, can have any number of class variables and for this demonstration consider these variables to be "private" (not accessible outside of the class or its own instances). One can, however, write methods or functions in the class that can provide an interface to operate on or access these private class variables.

That is, essentially, what a lexical closure is but without the overhead of an object system (remember, an object system can be created using closures!). They are a remarkably useful pattern of encapsulating certain kinds of logic.

Continuations, are another fun topic related to lexical closures too...

I have been in this position, and what I've found is that formal definitions don't make sense. You have to encounter an example that makes you sit up and realize that something is going on that you don't know.

Two examples where I tried to induce that kind of brain lock in Perl programmers are http://www.perlmonks.org/index.pl?node_id=34786 and http://www.perlmonks.org/index.pl?node_id=50132.

I've got the following matlab code:

  eta1=0.7;
  omega=10;
  f = @(t) eta1*sin(omega*t) + (1-eta1)*sin(2*omega*t)

I can call

  f(2)

and return a scaler value. I could just make a function in a separate document, but then I would have to pass in my parameters eta1 and omega. By using a closure, I just have to set those parameters in my code beforehand. Also, if I want to change my forcing term, by adding new parameters, I don't have to change the function definition, I just add those parameters right in.

I wrote an article about the closure aspect of Ruby blocks that was intended for beginners:

http://darwinweb.net/articles/41-ruby_blocks_as_closures

It really focuses on just one aspect of closures that Ruby's syntax makes especially convenient, which is that they enable you to write generic functions that don't need to know a single detail about what kind of data they are being applied to.

http://www.iode.co.uk/blog/pdrummond/i_finally_understand_cl...

A method remembers its context? I am not sure how to not understand it. Understanding all the implications might be another problem.

I wrote this to answer that question: http://www.jerf.org/iri/post/2542

In particular, I focus on showing an example that is noticeably more complicated without closures, but also a real case that could conceivably come up.

Simple Clojure example:

  user=> (defn add-n [n] #(+ n %))
  #'user/add-n
  user=> (def add-2 (add-n 2))
  #'user/add-2
  user=> (add-2 5)
  7

Imagine you need a task to be done. Normally, you go and do it yourself. With a closure, you write the instructions and just hand it off to someone :)

I'm going to assume you mean closures as in Scheme, not closures in a pure functional language.

For that, what I would try is an operational approach:

"Joe, you know how local variables are allocated on a stack?" If he says "no" I'm not sure what the next move is but if he says yes, get him to briefly explain it back to you. E.g., get him to say, at least in vague terms, that on entry to some function the local variables get "pushed on the stack".

"Ok, good. So, what happens when the function returns?" Hopefully he can come up with "the variables are popped off the stack".

"Right. Now, how does the program find the variables on the stack?" Hopefully, Joe can come up with the idea of a stack pointer.

"Cool. [Draw a little picture] So, the stack pointer is kind of like a pointer to a structure. The variables are the fields of the structure. See?" Hopefully Joe says "yes".

"Here's an alternative: instead of using a stack, we could allocate that structure just like any other structure. Instead of popping things off the stack at the end, we could just free/delete/whatever the structure. Make sense?" Hopefully he's still with you.

"Only, we don't actually have to free/delete the structure right away. Suppose I write a function inside this other function, like this [write pseudo-code for an anonymous function with some free variables bound to locals. Have the containing function return this anonymous function.]"

"You might think that when we return a 'function pointer' that we're just returning a pointer to the code. But in this case, our 'function pointer' is really two pointers: a pointer to the code and another pointer to the structure containing the local variables it refers to. So now, we DON'T free/delete that structure of local variables when returning. It sticks around. It's used by this 'function pointer' where returning. When we call that function pointer, it gets passed a sekrit hidden magic parameter which is a pointer to that structure. So the function can still keep using those variables." Hopefully Joe is still with you but ask some questions to find out.

"It's a little bit like that structure of local variables is an object, but an object with only one 'method' and that one method is the nested function we wrote. Get it?"

I think that should get a typical Joe (at least one who has some clue about stacks and stack pointers) to a first-level, rough and ready, operational understanding of closures. If it does, you can mention that actual optimized implementations tend to be a lot hairier, that GC is important, that there are deep connections to lambda calculus, etc.

Again, if on the other hand Joe doesn't even have the slightest awareness of an operational model of how a stack-based language works - I'm not sure what you can do for him.

Closures are functions that exist inside a variable. Therefore an entire different level of scope, now exists inside that variable. The variable then becomes like an object in that respect.

Easy.

Reply on news.ycombinator.com