请注意，这个“意义”的概念是发生在观察者头脑中的事情。因此，同样的“参考”对不同的人可能意味着不同的事情。例如，我们在维基百科上有一个爱丁堡消歧页面。

在编程上下文中出现的一个相关问题可能是多态性。

也许我们应该为特殊情况下的多态(或者甚至是强制转换)取一个名字，其中不同的多态情况在语义上是等价的(而不是完全相似)。例如，数字1——可以用整数类型、复杂类型或任何其他类型表示——可以用多态方式处理)。

引用透明函数是只依赖于其输入的函数。

引用透明函数的作用类似于数学函数;给定相同的输入，它总是会产生相同的输出。它意味着传入的状态没有被修改，并且函数本身没有状态。

引用透明性可以简单地表述为:

一个表达式在任何上下文中总是求相同的结果[1]， 一个函数，如果给定相同的参数两次，必须产生相同的结果两次。

例如，编程语言Haskell是一种纯函数式语言;这意味着它是引用透明的。

编程中的引用透明性指的是，当一个函数提供了一个输入时，它总是会为给定的输入返回相同的值。以下面的示例函数为例。

Int plusFive(int x){

return x+5

}

对于给定的输入整数x，该函数总是返回相同的值。上述函数的输出可以被其返回值所替代，并且代码的操作应该相同。例如，如果x=10，那么代码可以写成:

Int输出= + 5 (10)

OR

Int输出= 15

The term "referential transparency" comes from analytical philosophy, the branch of philosophy that analyzes natural language constructs, statements and arguments based on the methods of logic and mathematics. In other words, it is the closest subject outside computer science to what we call programming language semantics. The philosopher Willard Quine was responsible for initiating the concept of referential transparency, but it was also implicit in the approaches of Bertrand Russell and Alfred Whitehead.

就其核心而言，“参考透明度”是一个非常简单明了的概念。“指涉物”一词在分析哲学中用来谈论一个表达所指代的事物。它与我们在编程语言语义中所说的“意义”或“外延”大致相同。以Andrew Birkett的博客文章为例，“苏格兰的首都”指的是爱丁堡市。这是“referent”的一个简单例子。

一个句子中的上下文是“引用透明的”，如果用另一个引用同一实体的术语替换该上下文中的一个术语不会改变其含义。例如

苏格兰议会在苏格兰首都开会。

意思和

苏格兰议会在爱丁堡开会。

因此，“苏格兰议会在……开会”是一个指涉透明的上下文。我们可以把“苏格兰的首府”换成“爱丁堡”而不改变它的意思。换句话说，上下文只关心术语所指的内容，而不关心其他内容。也就是说，上下文是“引用透明的”。

另一方面，在句子中，

自1999年以来，爱丁堡一直是苏格兰的首府。

我们不能做这样的替换。如果我们这样做，我们会得到“Edinburgh has been Edinburgh since 1999”，这是一个疯狂的说法，并且不能传达与原句子相同的意思。所以，“Edinburgh has been…”“自1999年以来”是指不透明的(指透明的反义词)。显然，它关心的东西比这个词所指的东西更重要。是什么?

像“苏格兰的首都”这样的词被称为“限定名词”，在很长一段时间里，它们并没有让逻辑学家和哲学家感到头痛。Russell和Quine把它们整理出来，说它们实际上不是“指涉的”，也就是说，认为上面的例子是用来指实体的是错误的。理解“爱丁堡自1999年以来一直是苏格兰的首都”的正确方法是说

苏格兰自1999年以来就有了首都，那就是爱丁堡。

这个句子不能变成一个疯狂的句子。问题解决了!奎因的观点是，自然语言是混乱的，或至少是复杂的，因为它是为了方便实际使用而设计的，但哲学家和逻辑学家应该通过正确的方式理解它们，从而使它们变得清晰。参考透明度是一种工具，用于带来这种意义的清晰度。

What does all this have to do with programming? Not very much, actually. As we said, referential transparency is a tool to be used in understanding language, i.e., in assigning meaning. Christopher Strachey, who founded the field of programming language semantics, used it in his study of meaning. His foundational paper "Fundamental concepts in programming languages" is available on the web. It is a beautiful paper and everybody can read and understand it. So, please do so. You will be much enlightened. He introduces the term "referential transparency" in this paragraph:

One of the most useful properties of expressions is that called by Quine referential transparency. In essence this means that if we wish to find the value of an expression which contains a sub-expression, the only thing we need to know about the sub-expression is its value. Any other features of the sub-expression, such as its internal structure, the number and nature of its components, the order in which they are evaluated or the colour of the ink in which they are written, are irrelevant to the value of the main expression.

The use of "in essence" suggests that Strachey is paraphrasing it in order to explain it in simple terms. Functional programmers seem to understand this paragraph in their own way. There are 9 other occurrences of "referential transparency" in the paper, but they don't seem to bother about any of the others. In fact, the whole paper of Strachey is devoted to explaining the meaning of imperative programming languages. But, today, functional programmers claim that imperative programming languages are not referentially transparent. Strachey would be turning in his grave.

We can salvage the situation. We said that natural language is "messy, or at least complicated" because it is made to be convenient for practical use. Programming languages are the same way. They are "messy, or at least complicated" because they are made to be convenient for practical use. That does not mean that they need to confuse us. They just have to be understood the right way, using a meta language that is referentially transparent so that we have clarity of meaning. In the paper I cited, Strachey does exactly that. He explains the meaning of imperative programming languages by breaking them down into elementary concepts, never losing clarity anywhere. An important part of his analysis is to point out that expressions in programming languages have two kinds of "values", called l-values and r-values. Before Strachey's paper, this was not understood and confusion reigned supreme. Today, the definition of C mentions it routinely and every C programmer understands the distinction. (Whether the programmers in other languages understand it equally well is hard to say.)

Both Quine and Strachey were concerned with the meaning of language constructions that involve some form of context-dependence. For example, our example "Edinburgh has been the capital of Scotland since 1999" signifies the fact that "capital of Scotland" depends on the time at which it is being considered. Such context-dependence is a reality, both in natural languages and programming languages. Even in functional programming, free and bound variables are to be interpreted with respect to the context in which they appear in. Context dependence of any kind blocks referential transparency in some way or the other. If you try to understand the meaning of terms without regard to the contexts they depend on, you would again end up with confusion. Quine was concerned with the meaning of modal logic. He held that modal logic was referentially opaque and it should be cleaned up by translating it into a referentially transparent framework (e.g., by regarding necessity as provability). He largely lost this debate. Logicians and philosophers alike found Kripke's possible world semantics to be perfectly adequate. Similar situation also reigns with imperative programming. State-dependence explained by Strachey and store-dependence explained by Reynolds (in a manner similar to Kripke's possible world semantics) are perfectly adequate. Functional programmers don't know much of this research. Their ideas on referential transparency are to be taken with a large grain of salt.

[Additional note: The examples above illustrate that a simple phrase such as "capital of Scotland" has multiple levels of meaning. At one level, we might be talking about the capital at the current time. At another level, we might talking about all possible capitals that Scotland might have had through the course of time. We can "zoom into" a particular context and "zoom out" to span all contexts quite easily in normal practice. The efficiency of natural language makes use of our ability to do so. Imperative programming languages are efficient in very much the same way. We can use a variable x on the right hand side of an assignment (the r-value) to talk about its value in a particular state. Or, we might talk about its l-value which spans all states. People are rarely confused by such things. However, they may or may not be able to precisely explain all the layers of meaning inherent in language constructs. All such layers of meaning are not necessarily 'obvious' and it is a matter of science to study them properly. However, the inarticulacy of ordinary people to explain such layered meanings doesn't imply that they are confused about them.]

下面的一个单独的“后记”将这个讨论与函数式编程和命令式编程的关注点联系起来。