Scheme中的参数传递 [英] Parameter Passing in Scheme
问题描述
谁能帮我理解Scheme中的各种参数传递模式?我知道 Scheme 实现了按值传递的参数.但是其他模式呢?
Can anyone help me out understanding the various parameter passing modes in Scheme? I know Scheme implements parameter passing by value. But how about other modes?
Scheme 中的参数传递有什么好的文档吗?
Is there any good documentation for parameter passing in Scheme?
推荐答案
Scheme 只有 call-by-value 函数调用.还有其他替代方案可以在该语言中实现，但如果您是初学者，那么此时最好不要尝试它们.如果您正在寻找一种通过引用"传递值的方法——那么一个可以实现的选项是使用宏，但您真的不应该去那里.相反，像 PLT Scheme 这样的一些 Scheme 实现提供了一个框值":这是一种容器，使用方式如下:
Scheme has only call-by-value function calls. There are other alternatives that can be implemented within the language, but if you're a beginner then it's best to not even try them at this point. If you're looking for a way to pass values "by reference" -- then one option that can sort of make it is to use macros, but you really shouldn't go there. Instead, some Scheme implementations like PLT Scheme provide a "box value": this is a kind of a container that is used like this:
- 您使用
(box <something>) 创建一个包含 - 您使用
(unbox <some-box>) 获取存储在框中的值 - 您可以使用
(set-box! <some-box> <new-value>) 更改存储在框中的值
- You create a box holding
<something>with(box <something>) - You get the value that is stored in a box with
(unbox <some-box>) - You change the value that is stored in a box with
(set-box! <some-box> <new-value>)
鉴于这两个，你可以按值"使用这样的框对象，但它们的内容实际上是一个引用.这与 C 非常相似，其中所有值(实际上是大多数)都是按值传递的，但其中一些值可以是可以改变的指针.顺便说一句，最好避免这些:在 Scheme 中，函数式编程是更常见的选择，因此最好从它开始.
Given these two, you can use such box objects "by value", but their contents is actually a reference. This is very much like C, where all values (most, actually) are passed by-value, yet some of these values can be pointers that you can mutate. BTW, it's best to avoid even these: in Scheme, functional programming is the more common choice and it is therefore better to start with that.
(一旦您更熟练地使用 Scheme，并且如果您使用具有足够抽象的 Scheme，那么您也可以学习如何模仿许多替代方案.)
(Once you are more fluent with Scheme, and if you're using a Scheme with sufficient abstractions, then you can learn how to mimic lots of alternatives too.)
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