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Ruby's closures #ruby
# 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}" # 5
thrice { x += 1 }
puts "value of x after: #{x}" # 8
# 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}" # 8
# 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
@monsterooo
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翻译的很好

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