{"id":175,"date":"2008-11-12T22:24:37","date_gmt":"2008-11-12T21:24:37","guid":{"rendered":"http:\/\/www.nobugs.org\/blog\/?p=175"},"modified":"2008-12-07T00:09:00","modified_gmt":"2008-12-06T23:09:00","slug":"why-do-they-call-it-referentially-transparent","status":"publish","type":"post","link":"https:\/\/www.nobugs.org\/blog\/archives\/2008\/11\/12\/why-do-they-call-it-referentially-transparent\/","title":{"rendered":"Why do they call it: Referentially transparent"},"content":{"rendered":"

I like understanding why things have the names that they do. Two stories immediately spring to mind: the first is from Michael Faraday, which I wrote about a while ago<\/a> whilst the second is from Richard Feynman<\/a> (at 6 mins).<\/p>\n

But back to the topic at hand! I know pretty well what the phrase “referentially transparent<\/strong>” means, at least in relation to computer programs. It’s used to describe languages like Haskell where its always safe to replace a function like “foo()” by its return value (eg. 4) without changing the meaning of the program. This is not something which you can do in general for languages like Java or C++ because functions, such as rand(), can potentially return different values each time you call them. Also, if foo() contains printf statement, then the expressions “foo()” and “4” are different beasts, even if foo() does always return 4.<\/p>\n

So that’s the concept in question. But why give it a name like “referentially transparent”? What’s “transparent” about it? And what “references” are we talking about? (and how many metajokes could one make here?)<\/p>\n

The term “referentially transparent” seems to come from a certain Mr Quine<\/a> philosophising about natural languages. Let’s do an example, but first we need to look at two things.<\/p>\n

Firstly, take a sentence like “Andrew is bigger than a mouse” and erase the word “Andrew” to leave a hole – ie. “??? is bigger than a mouse”. Philosophers call this a CONTEXT. By filling the blanks in the CONTEXT you get back to a sentence.<\/p>\n

Secondly, think about the city of Edinburgh. It’s a thing<\/em>. There are many ways to refer to it: “Edinburgh”, “Scotlands capital”, “the city at 56N 3W”.<\/p>\n

Now we’re ready to understand where the phrase “referentially transparent” comes from.<\/p>\n

The first word (“referentially”) is talking about the fact that “the Edinburgh thing” has many names. There are many ways to REFERENCE that thing.<\/p>\n

You should read “transparent” to mean “doesn’t make a difference”. If Microsoft rewrite Excel so that it runs faster but the UI is identical then you’d say the change was “transparent” to the users. It’s “transparent” in the sense of “you can’t see it”. It DOESN’T mean “you can see all the innards”, like a transparent PC case.<\/p>\n

Mr Quine says that a CONTEXT is “referentially transparent” if we can fill the hole with different names for the same underlying thing without affecting the meaning of the sentence.<\/p>\n

An example should make this clearer:<\/p>\n