Closures Presentation

closures_presentation.rb

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

# 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.