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README.markdown

Closures In Ruby

Paul Cantrell
http://innig.net
Email: username "cantrell", domain name "pobox.com"

I recommend executing this file, then reading it alongside its output.

Alteratively, you can give yourself a sort of Ruby test by deleting all the comments, then trying to guess the output of the code!

A closure is a block of code which meets three criteria:

  • It can be passed around as a value and
  • executed on demand by anyone who has that value, at which time
  • it can refer to variables from the context in which it was created (i.e. it is closed with respect to variable access, in the mathematical sense of the word "closed").

(The word "closure" actually has an imprecise meaning, and some people don't think that criterion 1 is part of the definition. I think it is.)

Closures are a mainstay of functional languages, but are present in many other languages as well (e.g. Java's anonymous inner classes). You can do cool stuff with them: they allow deferred execution, and some elegant tricks of style.

Ruby is based on the "principle of least surprise," but I had a really nasty surprise in my learning process. When I understood what methods like "each" were doing, I thought, "Aha! Ruby has closures!" But then I found out that a function can't accept multiple blocks -- violating the principle that closures can be passed around freely as values.

This document details what I learned in my quest to figure out what the deal is.

def example(num)
  puts
  puts "------ Example {num} ------"
end

Section 1: Blocks

Blocks are like closures, because they can refer to variables from their defining context:

Example 1

def thrice
  yield
  yield
  yield
end

x = 5
puts "value of x before: {x}"
thrice { x += 1 }
puts "value of x after: {x}"

A block refers to variables in the context it was defined, not the context in which it is called:

Example 2

def thrice_with_local_x
  x = 100
  yield
  yield
  yield
  puts "value of x at end of thrice_with_local_x: {x}"
end

x = 5
thrice_with_local_x { x += 1 }
puts "value of outer x after: {x}"

A block only refers to existing variables in the outer context; if they don't exist in the outer, a block won't create them there:

Example 3

thrice do  # note that {...} and do...end are completely equivalent
    y = 10
    puts "Is y defined inside the block where it is first set?"
    puts "Yes." if defined? y
end
puts "Is y defined in the outer context after being set in the block?"
puts "No!" unless defined? y

OK, so blocks seem to be like closures: they are closed with respect to variables defined in the context where they were created, regardless of the context in which they're called.

But they're not quite closures as we've been using them, because we have no way to pass them around: "yield" can only refer to the block passed to the method it's in.

We can pass a block on down the chain, however, using &:

Example 4

def six_times(&block)
  thrice(&block)
  thrice(&block)
end

x = 4
six_times { x += 10 }
puts "value of x after: {x}"

So do we have closures? Not quite! We can't hold on to a &block and call it later at an arbitrary time; it doesn't work. This, for example, will not compile:

def save_block_for_later(&block)
  saved = █
end

But we can pass it around if we use drop the &, and use block.call(...) instead of yield:

Example 5

def save_for_later(&b)
  @saved = b   # Note: no ampersand! This turns a block into a closure of sorts.
end

save_for_later { puts "Hello!" }
puts "Deferred execution of a block:"
@saved.call
@saved.call

But wait! We can't pass multiple blocks to a function! As it turns out, there can be only zero or one &block_params to a function, and the &param must be the last in the list.

None of these will compile:

def f(&block1, &block2) ...
def f(&block1, arg_after_block) ...
f { puts "block1" } { puts "block2" }

What the heck?

I claim this single-block limitation violates the "principle of least surprise." The reasons for it have to do with ease of C implementation, not semantics.

So: are we screwed for ever doing anything robust and interesting with closures?

Section 2: Closure-Like Ruby Constructs

Actually, no. When we pass a block &param, then refer to that param without the ampersand, that is secretly a synonym for Proc.new(&param):

Example 6

def save_for_later(&b)
  @saved = Proc.new(&b)  # same as: @saved = b
end

save_for_later { puts "Hello again!" }
puts "Deferred execution of a Proc works just the same with Proc.new:"
@saved.call

We can define a Proc on the spot, no need for the &param:

Example 7

@saved_proc_new = Proc.new { puts "I'm declared on the spot with Proc.new." }
puts "Deferred execution of a Proc works just the same with ad-hoc Proc.new:"
@saved_proc_new.call

Behold! A true closure!

But wait, there's more.... Ruby has a whole bunch of things that seem to behave like closures, and can be called with .call:

Example 8

@saved_proc_new = Proc.new { puts "I'm declared with Proc.new." }
@saved_proc = proc { puts "I'm declared with proc." }
@saved_lambda = lambda { puts "I'm declared with lambda." }

def some_method 
  puts "I'm declared as a method."
end

@method_as_closure = method(:some_method)

puts "Here are four superficially identical forms of deferred execution:"

@saved_proc_new.call
@saved_proc.call
@saved_lambda.call
@method_as_closure.call

So in fact, there are no less than seven -- count 'em, SEVEN -- different closure-like constructs in Ruby:

  1. block (implicitly passed, called with yield)
  2. block (&b => f(&b) => yield)
  3. block (&b => b.call)
  4. Proc.new
  5. proc
  6. lambda
  7. method

Though they all look different, some of these are secretly identical, as we'll see shortly.

We already know that (1) and (2) are not really closures -- and they are, in fact, exactly the same thing. Numbers 3-7 all seem to be identical. Are they just different syntaxes for identical semantics?

Section 3: Closures and Control Flow

No, they aren't! One of the distinguishing features has to do with what "return" does.

Consider first this example of several different closure-like things without a return statement. They all behave identically:

Example 9

def f(closure)
  puts
  puts "About to call closure"
  result = closure.call
  puts "Closure returned: {result}"
  "Value from f"
end

puts "f returned: " + f(Proc.new { "Value from Proc.new" })
puts "f returned: " + f(proc { "Value from proc" })
puts "f returned: " + f(lambda { "Value from lambda" })
def another_method
  "Value from method"
end

puts "f returned: " + f(method(:another_method))

But put in a "return," and all hell breaks loose!

Example 10

begin f(Proc.new { return "Value from Proc.new" }) rescue Exception => e puts "Failed with {e.class}: {e}" end

The call fails because that "return" needs to be inside a function, and a Proc isn't really quite a full-fledged function:

example 11

def g result = f(Proc.new { return "Value from Proc.new" }) puts "f returned: " + result never executed "Value from g" never executed end

puts "g returned: {g}"

Note that the return inside the "Proc.new" didn't just return from the Proc -- it returned all the way out of g, bypassing not only the rest of g but the rest of f as well! It worked almost like an exception.

This means that it's not possible to call a Proc containing a "return" when the creating context no longer exists:

example 12

def make_proc_new begin Proc.new { return "Value from Proc.new" } this "return" will return from make_proc_new ensure puts "make_proc_new exited" end end

begin puts make_proc_new.call rescue Exception => e puts "Failed with {e.class}: {e}" end

(Note that this makes it unsafe to pass Procs across threads.)

A Proc.new, then, is not quite truly closed: it depends in the creating context still existing, because the "return" is tied to that context.

Not so for lambda:

example 13

def g result = f(lambda { return "Value from lambda" }) puts "f returned: " + result "Value from g" end

puts "g returned: {g}"

And yes, you can call a lambda even when the creating context is gone:

example 14

def make_lambda begin lambda { return "Value from lambda" } ensure puts "make_lambda exited" end end

puts make_lambda.call

Inside a lambda, a return statement only returns from the lambda, and flow continues normally. So a lambda is like a function unto itself, whereas a Proc remains dependent on the control flow of its caller.

A lambda, therefore, is Ruby's true closure.

As it turns out, "proc" is a synonym for either "Proc.new" or "lambda." Anybody want to guess which one? (Hint: "Proc" in lowercase is "proc.")

example 15

def g result = f(proc { return "Value from proc" }) puts "f returned: " + result "Value from g" end

puts "g returned: {g}"

Hah. Fooled you.

The answer: Ruby changed its mind. If you're using Ruby 1.8, it's a synonym for "lambda." That's surprising (and also ridiculous); somebody figured this out, so in 1.9, it's a synonym for Proc.new. Go figure.

I'll spare you the rest of the experiments, and give you the behavior of all 7 cases:

"return" returns from caller: 1. block (called with yield) 2. block (&b => f(&b) => yield)
3. block (&b => b.call)
4. Proc.new 5. proc in 1.9

"return" only returns from closure: 5. proc in 1.8 6. lambda
7. method

---------------------------- Section 4: Closures and Arity ----------------------------

The other major distinguishing of different kinds of Ruby closures is how they handle mismatched arity -- in other words, the wrong number of arguments.

In addition to "call," every closure has an "arity" method which returns the number of expected arguments:

example 16

puts "One-arg lambda:" puts (lambda {|x|}.arity) puts "Three-arg lambda:" puts (lambda {|x,y,z|}.arity)

...well, sort of:

puts "No-args lambda: " puts (lambda {}.arity) This behavior is also subject to change in 1.9. puts "Varargs lambda: " puts (lambda {|*args|}.arity)

Watch what happens when we call these with the wrong number of arguments:

example 17

def call_with_too_many_args(closure) begin puts "closure arity: {closure.arity}" closure.call(1,2,3,4,5,6) puts "Too many args worked" rescue Exception => e puts "Too many args threw exception {e.class}: {e}" end end

def two_arg_method(x,y) end

puts; puts "Proc.new:"; call_with_too_many_args(Proc.new {|x,y|}) puts; puts "proc:" ; call_with_too_many_args(proc {|x,y|}) puts; puts "lambda:" ; call_with_too_many_args(lambda {|x,y|}) puts; puts "Method:" ; call_with_too_many_args(method(:two_arg_method))

def call_with_too_few_args(closure) begin puts "closure arity: {closure.arity}" closure.call() puts "Too few args worked" rescue Exception => e puts "Too few args threw exception {e.class}: {e}" end end

puts; puts "Proc.new:"; call_with_too_few_args(Proc.new {|x,y|}) puts; puts "proc:" ; call_with_too_few_args(proc {|x,y|}) puts; puts "lambda:" ; call_with_too_few_args(lambda {|x,y|}) puts; puts "Method:" ; call_with_too_few_args(method(:two_arg_method))

Yet oddly, the behavior for one-argument closures is different....

example 18

def one_arg_method(x) end

puts; puts "Proc.new:"; call_with_too_many_args(Proc.new {|x|}) puts; puts "proc:" ; call_with_too_many_args(proc {|x|}) puts; puts "lambda:" ; call_with_too_many_args(lambda {|x|}) puts; puts "Method:" ; call_with_too_many_args(method(:one_arg_method)) puts; puts "Proc.new:"; call_with_too_few_args(Proc.new {|x|}) puts; puts "proc:" ; call_with_too_few_args(proc {|x|}) puts; puts "lambda:" ; call_with_too_few_args(lambda {|x|}) puts; puts "Method:" ; call_with_too_few_args(method(:one_arg_method))

Yet when there are no args...

example 19

def no_arg_method end

puts; puts "Proc.new:"; call_with_too_many_args(Proc.new {||}) puts; puts "proc:" ; call_with_too_many_args(proc {||}) puts; puts "lambda:" ; call_with_too_many_args(lambda {||}) puts; puts "Method:" ; call_with_too_many_args(method(:no_arg_method))

For no good reason that I can see, Proc.new, proc and lambda treat a single argument as a special case; only a method enforces arity in all cases. Principle of least surprise my ass.

---------------------------- Section 5: Rant ----------------------------

This is quite a dizzing array of syntactic options, with subtle semantics differences that are not at all obvious, and riddled with minor special cases. It's like a big bear trap from programmers who expect the language to just work.

Why are things this way? Because Ruby is:

(1) designed by implementation, and (2) defined by implementation.

The language grows because the Ruby team tacks on cool ideas, without maintaining a real spec apart from CRuby. A spec would make clear the logical structure of the language, and thus help highlight inconsistencies like the ones we've just seen. Instead, these inconsinstencies creep into the language, confuse the crap out of poor souls like me who are trying to learn it, and then get submitted as bug reports. Something as fundamental as the semantics of proc should not get so screwed up that they have to backtrack between releases, for heaven's sake! Yes, I know, language design is hard -- but something like this proc/lambda issue or the arity problem wasn't so hard to get right the first time. Yammer yammer.

---------------------------- Section 6: Summary ----------------------------

So, what's the final verdict on those 7 closure-like entities?

                                                 "return" returns from closure
                                True closure?    or declaring context...?         Arity check?
                                ---------------  -----------------------------    -------------------
  1. block (called with yield) N declaring no
  2. block (&b => f(&b) => yield) N declaring no
  3. block (&b => b.call) Y except return declaring warn on too few
  4. Proc.new Y except return declaring warn on too few
  5. proc <<< alias for lambda in 1.8, Proc.new in 1.9 >>>
  6. lambda Y closure yes, except arity 1
  7. method Y closure yes

    The things within each of these groups are all semantically identical -- that is, they're different syntaxes for the same thing:

    1. block (called with yield)
    2. block (&b  =>  f(&b)  =>  yield)  
    -------
    3. block (&b  =>  b.call)
    4. Proc.new  
    5. proc in 1.9
    -------
    5. proc in 1.8
    6. lambda    
    -------
    7. method (may be identical to lambda with changes to arity checking in 1.9)
    

    Or at least, this is how I think it is, based on experiment. There's no authoritative answer other than testing the CRuby implementation, because there's no real spec -- so there may be other differences I haven't discovered.

    The final verdict: Ruby has four types of closures and near-closures, expressible in seven syntactic variants. Not pretty. But you sure sure do cool stuff with them! That's up next....

    This concludes the "Ruby sucks" portion of our broadcast; from here on, it will be the "Ruby is awesome" portion.

    ---------------------------- Section 7: Doing Something Cool with Closures ----------------------------

    Let's make a data structure containing all of the Fibonacci numbers. Yes, I said all of them. How is this possible? We'll use closures to do lazy evaluation, so that the computer only calculates as much of the list as we ask for.

    To make this work, we're going to use Lisp-style lists: a list is a recursive data structure with two parts: "car," the next element of the list, and "cdr," the remainder of the list.

    For example, the list of the first three positive integers is [1,[2,[3]]]. Why? Because:

    [1,[2,[3]]] <--- car=1, cdr=[2,[3]] [2,[3]] <--- car=2, cdr=[3] [3] <--- car=3, cdr=nil

    Here's a class for traversing such lists:

example 20

class LispyEnumerable include Enumerable

def initialize(tree)
    @tree = tree
end

def each
    while @tree
        car,cdr = @tree
        yield car
        @tree = cdr
    end
end

end

list = [1,[2,[3]]] LispyEnumerable.new(list).each do |x| puts x end

So how to make an infinite list? Instead of making each node in the list a fully built data structure, we'll make it a closure -- and then we won't call that closure until we actually need the value. This applies recursively: the top of the tree is a closure, and its cdr is a closure, and the cdr's cdr is a closure....

example 21

class LazyLispyEnumerable include Enumerable

def initialize(tree)
    @tree = tree
end

def each
    while @tree
        car,cdr = @tree.call  <--- @tree is a closure
        yield car
        @tree = cdr
    end
end

end

list = lambda{[1, lambda {[2, lambda {[3]}]}]} same as above, except we wrap each level in a lambda LazyLispyEnumerable.new(list).each do |x| puts x end

example 22

Let's see when each of those blocks gets called: list = lambda do puts "first lambda called" [1, lambda do puts "second lambda called" [2, lambda do puts "third lambda called" [3] end] end] end

puts "List created; about to iterate:" LazyLispyEnumerable.new(list).each do |x| puts x end

Now, because the lambda defers evaluation, we can make an infinite list:

example 23

def fibo(a,b) lambda { [a, fibo(b,a+b)] } <---- this would go into infinite recursion if it weren't in a lambda end

LazyLispyEnumerable.new(fibo(1,1)).each do |x| puts x break if x > 100 we don't actually want to print all of the Fibonaccis! end

This kind of deferred execution is called "lazy evaluation" -- as opposed to the "eager evaluation" we're used to, where we evaluate an expression before passing its value on. (Most languages, including Ruby, use eager evaluation, but there are languages (like Haskell) which use lazy evaluation for everything, by default! Not always performant, but ever so very cool.)

This way of implementing lazy evaluation is terribly clunky! We had to write a separate LazyLispyEnumerable that knows we're passing it a special lazy data structure. How unsatisfying! Wouldn't it be nice of the lazy evaluation were invisible to callers of the lazy object?

As it turns out, we can do this. We'll define a class called "Lazy," which takes a block, turns it into a closure, and holds onto it without immediately calling it. The first time somebody calls a method, we evaluate the closure and then forward the method call on to the closure's result.

class Lazy def initialize(&generator) @generator = generator end

def method_missing(method, *args, &block) evaluate.send(method, *args, &block) end

def evaluate @value = @generator.call unless @value @value end end

def lazy(&b) Lazy.new &b end

This basically allows us to say:

lazy {value}

...and get an object that looks exactly like value -- except that value won't be created until the first method call that touches it. It creates a transparent lazy proxy object. Observe:

example 24

x = lazy do puts "<<< Evaluating lazy value >>>" "lazy value" end

puts "x has now been assigned" puts "About to call one of x's methods:" puts "x.size: {x.size}" <--- .size triggers lazy evaluation puts "x.swapcase: {x.swapcase}"

So now, if we define fibo using lazy instead of lambda, it should magically work with our original LispyEnumerable -- which has no idea it's dealing with a lazy value! Right?

example 25

def fibo(a,b) lazy { [a, fibo(b,a+b)] } end

LispyEnumerable.new(fibo(1,1)).each do |x| puts x end

Oops! That didn't work. What went wrong?

The failure started in this line of LispyEnumerable (though Ruby didn't report the error there):

  car,cdr = @tree

Let's zoom in on that result, and see what happened:

example 26

car,cdr = fibo(1,1) puts "car={car} cdr={cdr}"

Here's the problem. When we do this:

x,y = z

...Ruby calls z.respond_to?(to_a) to see if z is an array. If it is, it will do the multiple assignment; if not, it will just assign x=z and set y=nil.

We want our Lazy to forward the respond_to? call to our fibo list. But it doesn't forward it, because we used the method_missing to do the proxying -- and every object implements respond_to? by default, so the method isn't missing! The respond_to? doesn't get forwarded; instead, out Lazy says "No, I don't respond to to_a; thanks for asking." The immediate solution is to forward respond_to? manually:

class Lazy def initialize(&generator) @generator = generator end

def method_missing(method, *args, &block) evaluate.send(method, *args, &block) end

def respond_to?(method) evaluate.respond_to?(method) end

def evaluate @value = @generator.call unless @value @value end end

And now our original Lispy enum can work:

example 27

LispyEnumerable.new(fibo(1,1)).each do |x| puts x break if x > 200 end

Of course, this only fixes the problem for respond_to?, and we have the same problem for every other method of Object. There is a more robust solution -- frightening, but it works -- which is to undefine all the methods of the Lazy when it's created, so that everything gets forwarded.

And guess what? There's already a slick little gem that will do it:

 http://moonbase.rydia.net/software/lazy.rb/

Read the source. It's fascinating.

---------------------------- Section 8: Wrap-Up ----------------------------

So sure, this was all entertaining -- but is it good for anything?

Well, suppose you have an object which requires a network or database call to be created, or will use a lot of memory once it exists. And suppose that it may or may not be used, but you don't know at the time it's created whether it will be. Making it lazy will prevent it from consuming resources unless it needs to. Hibernate does this to prevent unnecessary DB queries, and it does it with more or less arbitrary Java objects (i.e. unlike ActiveRecord, it doesn't depend on a base class to do its lazy loading). Ruby can do the same thing, but with a lot less code!

That's just an example. Use your imagination.

If you're a functional langauge geek, and enjoyed seeing Ruby play with these ideas from Lisp and Haskell, you may enjoy this thread:

 http://redhanded.hobix.com/inspect/curryingWithArity.html

OK, I'll stop making your brain hurt now. Hope this has been a bit enlightening! The experience of working it out certainly was for me.

Paul

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