[转]ruby中的闭包

原文：

• http://kenbeit.com/posts/87/ruby%E4%B8%AD%E7%9A%84%E9%97%AD%E5%8C%85

感谢作者的辛勤劳动。

# CLOSURES IN RUBY     Paul Cantrell    http://innig.net
# Email: username "cantrell", domain name "pobox.com"
#
# 翻译: kenshin54		http://kenbeit.com

# I recommend executing this file, then reading it alongside its output.
# 我强烈建议执行此脚本，然后根据它的输出来理解它。
#
# Alteratively, you can give yourself a sort of Ruby test by deleting all the comments,
# then trying to guess the output of the code!
# 或者，你可以给自己做一个ruby测试，删除所有的注释，然后猜测代码输出的结果

# A closure is a block of code which meets three criteria:
# 闭包是一个满足3个标准的代码块：
# 
#     * It can be passed around as a value and
#     * 它可以作为一个值被传入
# 
#     * executed on demand by anyone who has that value, at which time
#     * 在任何时候根据不同人的需要来执行它
# 
#     * 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").
#     * 它可以使用创建它的那个上下文中的变量（即它是对封闭变量的访问，数学意义上的词“封闭”）
#
# (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.)
# （词“闭包”实际上有一个不明确的意义，有一些人不认为标准#1是它定义的一部分，但我认为它是。）
# 
# 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.
# 闭包是一个函数式语言的主体，但是也存在与其他很多语言中（比如Java中的匿名内部类）。
# 你可以用它来做一些很酷的东西：他们可以延后执行，和一些优雅的技巧。
# 
# 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.
# Ruby基于“最小惊讶原则”，但是在我的学习过程中却是感到了惊讶。当我理解像“each”
# 这样的方法在做什么时，我想，“啊，Ruby有闭包！”。但是我发现一个函数无法接收
# 多个块——违反闭包可以被任意的传递值的原则。
# 
# This document details what I learned in my quest to figure out what the deal is
# 这个文档详述了在我弄清闭包的处理时我所学到的东西。

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

# ---------------------------- Section 1: Blocks ----------------------------
# -------------------------------- 章节一：块 -------------------------------

# Blocks are like closures, because they can refer to variables from their defining context:
# 块就像闭包，因为他们可以使用定义它们的那个上下文中的变量。

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:
# 一个块使用定义它的上下文中的变量，而不是被调用的上下文中的变量。

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:
# 一个块仅仅使用定义它的上下文中已经存在的变量，如果变量不存在外于部上下文，块不会去创建他们。

example 3

thrice do # note that {...} and do...end are completely equivalent 注意{...}和do...end是完全相等的
	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 &:
# 所以块看起来像闭包：他们封闭了定义它们的上下文中的变量，不管他们在哪里调用。
# 但是在我们使用它们时，它们不完全是闭包，因为我们没有办法传递他们。
# “yield”仅仅可以将块传给它所在的方法。

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:
# 所以我们是否有闭包？不完全！我们不能保存一个&block，然后稍后在任意时间来调用它。
# 但是如果我们丢掉&，我们就有办法传递它了，使用block,call(...)代替yield。

example 5

def save_for_later(&b)
	@saved = b  # Note: no ampersand! This turns a block into a closure of sorts. 注意：没有&符号！这个做法将一个块变成了一个闭包。
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?
# 等等！我们不能传递多个块给一个函数！一个函数只能接收0或1个&block_params参数，并且&block_params必须是最后一个参数。
# 我觉得这个单block违反了最小惊讶原则，因为这很容用C来实现，不是语义上的。


# ---------------------------- Section 2: Closure-Like Ruby Constructs ----------------------------
# --------------------------------- 章节二：闭包风格的Ruby构造器 ----------------------------------

# 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):
# 实际上，没有。当我们传递一个块&param，然后不带&来使用param，这是Proc.new(&param)的隐式同义词。

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:
# 我们可以随意定义一个Proc，不需要&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:
# 看！一个真的闭包！
# 等等，还有跟精彩的。。。Ruby还有一堆东西的行为看起来像闭包，并且能用.call来调用。

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 ----------------------------
# ---------------------------------- 章节三：闭包后控制流 --------------------------------------

# 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:
# 实际上Ruby有不少于7中不同的闭包风格的构造器
# ...
# 尽管他们看起来不一样，有些其实是完全相同的，我们后面马上会看到。
# 我们已经知道(1)和(2)不是真正的闭包，其实(1)和(2)是同样的东西。
# 3-7看上去是完全一样的，他们是不是语义相同的不用语法？

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!
# 但是加上一个“return”语句，一切都变的不可收拾。

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:
# 这样的调用会抛出异常因为“return”必须在一个函数内，而一个Proc不是一个真正的完全独立的函数。

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:
# 注意在“Proc.new”中的return语句不仅仅从Proc中返回，它将从g方法中返回，不仅绕过g方法中剩余的代码
# 而且f方法中也是一样的!它就像一个异常一样。
#
# 这意味着当创建Proc的上下文不存在时，无法调用一个包含“return”的Proc
#
# 本人的理解：
# 在Proc.new中的return语句，会尝试返回到创建它的上下文之后，继续执行，如果创建它的上下文已经“消失”或者返回到了全局上下文，就会出现LocalJumpError。
# 
# 在#example10中，在全局上下文中用Proc.new创建了一个proc，在f方法中使用call，然后proc会返回到创建它的上下文，即全局上下文，于是出了LocalJumpError。
# 在#example11中，在g方法的上下文中用Proc.new创建了一个proc，在f方法中使用call，然后proc会返回到创建它的上下文，即g方法之后，这种情况不会出异常。

example 12

def make_proc_new
	begin
		Proc.new { return "Value from Proc.new" } # this "return" will return from make_proc_new 这个“return”会返回到make_proc_new方法之外
	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.)
# （注意跨先辰的传递Proc会导致它不安全。）

# 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.
# Proc.new，不是一个真正完全的闭包。它依赖于创建它的上下文要一直存在，因为“return”和那个上下文联席在了一起。
#
# 本人的理解：
# 就和刚刚讲的一样，如果创建它的上下文已经“消失”，在这里make_proc_new方法把创建的proc返回了出去，所以make_proc_new的上下文就不存在了，
# 这时候再使用call，也会发生LocalJumpError。
#
# Not so for lambda:
# lambda就不是这个样子：

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:
# 你可以调用lambda，即使创建它的上下文已经不在。

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.
# 在lambda内部，return语句仅仅从lambda中返回，代码流程不会改变。
# 所以lambda就像一个方法一样，而Proc需要依赖于它调用者的控制流。(擦 什么烂翻译
#
# A lambda, therefore, is Ruby's true closure.
# 所以lambda才是ruby真正的闭包。
#
# 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.")
# 事实证明，"proc"是“Proc.new”或者“lambda”的同义词。
# 有人想猜一猜到底是哪个吗？（提示：“Proc”的小写是“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.
# 哈哈，你被耍了。
#
# 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.
# 答案是如果你使用ruby1.8，那么它是“lambda”的同义词。
# 这真得让人惊讶（而且荒谬），有些人指出了这一点，所以在ruby1.9，它是“Proc.new”的同义词了。

# I'll spare you the rest of the experiments, and give you the behavior of all 7 cases:
# 我就不进行剩下的实验了，但是会给出所有的7种示例。
#
#
# "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 ----------------------------
# --------------------------------- 章节四：闭包和元数 ----------------------------------
#
# 注：arity => 一个方法或者函数可以接受的参数个数

# The other major distinguishing of different kinds of Ruby closures is how they handle mismatched
# arity -- in other words, the wrong number of arguments.
# 另外一个主要辨别不同种类的ruby闭包是他们如何处理不匹配的元数，换句话说，就是不正确的参数个数。
#
# In addition to "call," every closure has an "arity" method which returns the number of expected
# arguments:
# 除了“call”以外，每个闭包还有一个“arity”方法可以返回期望的参数个数。

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. #这个行为在1.9中也会有变化。 1.8中是-1，1.9中是0
puts "Varargs lambda: "
puts (lambda {|*args|}.arity)

# Watch what happens when we call these with the wrong number of arguments:
# 看看当我们使用不真确的参数个数来调用他们时，会发生什么。

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....
# 奇怪的是，当闭包只有一个参数时，它们的行为又不一样了。。。

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...
# 当他们没有参数时。。。

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.
# 没办法解释你和我看到的结果，Proc.new，proc和lambda在面对一个参数时有特殊的处理；
# 只有method在任何情况下都强制执行了参数的个数。真是草尼马的最小惊讶原则。
#
# 注：#example17-19的结果在ruby1.8和1.9下，1.8下proc和lambda的执行结果完全一样，1.9下proc和Proc.new的结果完全相同。


# ---------------------------- Section 5: Rant ----------------------------
# ------------------------------ 章节五：咆哮 -----------------------------
#
# 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.
# 真是让人蛋疼的语法，这些语法上的微妙差异不是那么显而易见的，而且还有不少小的特殊情况。
#
#
# Why are things this way? Because Ruby is:
# 为什么会这样呢？因为ruby是：
#
#   (1) designed by implementation, and
#   (2) defined by implementation.
#   通过实现来设计和定义 （看不明白
#
# 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.
# （此处省略原作者抱怨的100字。。。）


# ---------------------------- Section 6: Summary ----------------------------
# ------------------------------- 章节六：总结 -------------------------------
#
# So, what's the final verdict on those 7 closure-like entities?          
# 所以，那7个闭包风格的实体最终结论如下：
#
#                                                     "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:
# 下面的分组中的每个在语义上都是相同的，也就是说，只是语法不同的同一个东西。
#   
#      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) 在1.9中参数检查和lambda一样
#
# 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.
# 至少，根据实验，我是这么认为的。除了测试CRuby实现外，也没有其他的官方答案。因为没有真实的规范--所以也许和我发现的有些不同。
#
# 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....
# 最终结论：Ruby有四个类型的闭包和近似闭包，使用7中语法变形来体现，不是很好，但是你能他们来做一些很cool的东西。
#
# 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 ----------------------------
# ----------------------------------- 章节七：用闭包来做些Cool的东西 ------------------------------------

# 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.
# 我们创建一个数据结构包含所有的菲波那契数。是的，我说“所有的”。
# 这怎么可能？我们将使用闭包来做到延迟求值，所以计算机仅仅会计算我们所要的。

# 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.
# 要让他工作，我们要使用Lisp风格的lists：一个包含2部分的可递归的数据结构：“car”，list中的下一个元素，“cdr”，list中剩余的元素。
#
# 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:
# 这有一个类来遍历这样的list

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....
# 所以如和去创建一个无限的list？我们通过创建一个闭包来代替原来在list中内建的基本数据结构。
# 除非我们真得须要数据，否则我们不会调用它。它会是一个递归。。。

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 和前面的一样，只是多包了一个lambda
LazyLispyEnumerable.new(list).each do |x|
	puts x
end

example 22

# Let's see when each of those blocks gets called:
# 让我们来看看这些block是什么时候被调用的
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:
# 现在，因为lamdba的延迟求值，我们可以创建无限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 这个会进入死循环如果它不在lambda内部
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! 我们实际上不会要打印所有的菲波那契数。
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.)
# 这就是延迟求值，和我们通常用得立即求值相反，我们在传送值之前就求值了表达式。
# 大部分语言，包括ruby，都是立即求值，但是有些其他语言，比如Haskell，他默认就是对任何都延迟求值。
# 这不是一直是高性能的，但是这非常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?
# 这种实现延迟求值的方式是非常笨拙的。我们不得不写一个单独的LazyLispyEnumerable，让他知道我们传了一个
# 特别的延迟数据结构给他。能不能让他更好的处理延迟求值，让他对于调用者隐藏延迟对象？
#
# 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.
# 事实证明，我们可以这么做。我们定一个叫“Lazy”的类，然后使用block，把它转成闭包，然后保存它。
# 第一次一些人调用了它，我们求取闭包的值，然后跳转到下一个方法调用。

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:
# 你可以这样来使用：
#
#   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:
# 。。。然后就能得到一个看起来像指定的value一样的对象—— 除了value只有在一次调用method的以后才会创建。
# 它创建了一个透明的延迟代理对象。

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 .size会触发延迟求值
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?
# 现在，我们使用lazy代替lambda来创建菲波那契函数，它应该可以神奇地和我们来原的LispyEnumerable工作。
# LispyEnumerable并不知道如何处理一个lazy value！是不是？

example 25

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

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

# 这个方法在ruby 1.8和1.9中不一样
# 1.8中会调用respond_to?(to_a)方法来找to_a，而respond_to又是Object的方法，每个对象都会有，所以不会经过method_missing
# 1.9中会直接调用method_missing(to_a)方法来找to_a
# 所以1.8只会打印出一个Lazy对象，而1.9中可以顺利执行

# Oops! That didn't work. What went wrong?
# 噢，它并没有如我们想的那样工作，哪里出错了？
#
# The failure started in this line of LispyEnumerable (though Ruby didn't report the error there):
# 错误发生在LispyEnumerable的这一行（尽管Ruby没有报错）
#
#      car,cdr = @tree
#
# Let's zoom in on that result, and see what happened:
# 让我们看看到底发生了什么！

example 26

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

# Here's the problem. When we do this:
# 问题就在这里，当我们这样赋值时：
#
#   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.
# Ruby会调用z.respond_to?(to_a)来看看z是否能变成一个数组。如果可以，它会进行多次赋值，
# 否则，它只会赋值 x=z 和 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:
# 我们想要的Lazy来转发respond_to?，让它调用到我们的菲波那契。但是它没有做到，因为我们用method_missing
# 来进行代理 -- 而每个对象默认都有respond_to?方法，所以无法触发到method_missing！respond_to?方法没有被转发，
# 所以Lazy说：“我没有to_a方法，谢谢你的调用。”最快捷的办法是手动转发respond_to?方法。

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:
# 现在我们原来的Lispy enum就能工作了。

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.
# 当然，这只是修改了respond_to?的问题，Object的其他方法也有同样的问题。这里有一个更健壮的办法，虽然有点可怕，
# 但是它能工作。那就是在Lazy对象创建时取消定义Lazy所有的方法，那样就都能被转发了。
#
# And guess what? There's already a slick little gem that will do it:
# 其实已经有一个gem做了这样的事：
#
#     http://moonbase.rydia.net/software/lazy.rb/
#
# Read the source. It's fascinating.
# 看看它的源码把，非常让人着迷。

# ---------------------------- Section 8: Wrap-Up ----------------------------
# ------------------------------- 章节八：总结 -------------------------------

# So sure, this was all entertaining -- but is it good for anything?
# 这就是所有了 -- 那么是不是它有利于任何事？
#
# 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!
# 假设你有一个对象须要一个网络或者数据库的调用才能被创建，或者会一旦创建会占用大量内存。你也不知道什么
# 时候会用到它。使用lazy将防止它消耗资源，除非它真得需要。Hibernate使用延迟加载来阻止不必要的数据库查询，
# 而Hibernate做到它需要或多或少的Java对象。（不像ActiveRecord，它依赖于一个base class来做到延迟加载）
# Ruby可以使用更少的代码做到相同的事情。
#
#
# That's just an example. Use your imagination.
# 这只是个示例，发挥你的想象力。
#
# If you're a functional langauge geek, and enjoyed seeing Ruby play with these ideas from Lisp and
# Haskell, you may enjoy this thread:
# 如果你是个函数式语言的极客，并且喜欢使用Ruby的这些来自于Lisp和Haskell的特性，你也许会想加入
#
#     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.
# 好了，我不再伤害你的大脑了。希望这能够给你一点启发！理解这些的经验对我非常有用。
#
# Paul