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CLOSURES IN RUBY Paul Cantrell https://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 = &block
# 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 ¶m *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 ¶m, then refer to that param without the ampersand, that
# is secretly a synonym for Proc.new(¶m):
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 ¶m:
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 on 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 makes Paul crazy" 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_with_index do |x|
puts x
break if x > 100 # we don't actually want to print all of the Fibonaccis!
end
# Oops! If you're using Ruby 1.8, 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. (Ruby 1.9 helps make
# this easier by providing the BasicObject class, which defines only the most minimal set of methods.)
#
# 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
#
#
# P.S. I have a new album out:
#
# http://music.innig.net
#
# If you enjoyed the way I think about code, you might also enjoy the way I think about music.
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