[LukeShu]

As others have said, "purely functional data structure" is a jargon term with a precise meaning--that fact that this is true indicates that data structures can, in fact, be functional in this sense. To claim that "the word 'functional' is purely wrong" is, well, purely wrong; the data structure in Git being described is purely functional.

Beyond that, you seem to have shown misunderstanding of what that means (despite your claims of "I understand but I disagree"), as the other examples you give DAGs, dictionaries, trees, lists, and this comment thread, do not have the interesting properties of purely functional data structures that make them worth discussing in the context of Git. Namely, the immutability of existing entries.

The thesis of the article isn't that purely functional data structures are Good, or that functional programming is Good. Those are quality judgments that are not present in the article. The thesis is: Explaining Git in terms of the purely functional data structure it uses might be helpful to learning Git.

[erikb]

Yes, I didn't accept that part of the logic. Neither do I believe that immutability is something good, nor do I believe that git is very immutable, nor that git gains from immutability.

I believe mutability is a trade off. You can make objects more immutable by being less efficient in terms of storage size, search time, source code simplicity. In exchange for these negative attributes you get source code you can proof more easily. It's good when that attribute is needed, but not most of the time.

Git is only immutable on that meta level where we talk about the object key-value store. The implementation uses mutable files though, and even steps away from that immutable key-value store when the size gets too big and needs to store more efficiently.

And on the user level you can amend, edit, squash and delete commits easily, which is actually a strength of git not a weakness. People love it for being more mutable than SVN.

[Dylan16807]

> Git is only immutable on that meta level where we talk about the object key-value store. The implementation uses mutable files though, and even steps away from that immutable key-value store when the size gets too big and needs to store more efficiently.

Functional languages are only immutable on that meta level where we talk about objects in memory. The implementation uses mutable RAM though, and even garbage collects when the size gets too big and needs to store more efficiently.

Unless you want to narrow the term 'functional' so much it describes nothing at all, Git is a functional data structure.

[erikb]

It seems like you say that with the goal of proofing me wrong with an example. But what you say is

(a) an argument helping my thesis by showing that you can program functional languages quite well by using mutable recourses

(b) the biggest users of what you write there are also the biggest enemies of the status quo you describe. Functional programmers really hate that computers are so unlike what they love.

[Dylan16807]

> (a) an argument helping my thesis by showing that you can program functional languages quite well by using mutable recourses

You can implement a functional language with mutable resources, and you can implement a functional data structure like git with mutable resources. How does it help your thesis that git isn't a functional data structure?

> (b) the biggest users of what you write there are also the biggest enemies of the status quo you describe. Functional programmers really hate that computers are so unlike what they love.

Whatever, something that is purely functional all the way down to the wires is impossible.

Any definition of "functional" the rejects every single functional language is an objectively failed definition. A usable definition will go ahead and permit the git data structure to be called functional.

[vickychijwani]

> (a) an argument helping my thesis by showing that you can program functional languages quite well by using mutable recourses

Let me preface this by revealing that I'm neither a functional programmer, nor am I convinced that functional programming or immutability is any sort of silver bullet.

Your original thesis was that "a data structure cannot be functional," which you later defended by saying "git is only immutable on the meta level, its implementation uses mutable files", which, while being 100% true, is also true of the category of programming languages we call "functional" as they rely on mutable RAM. That is what the GP has been trying to explain to you, but your obvious hate for anything labelled "functional" makes you blind to all this.

Sorry mate, you just have to concede the point here. This is not a discussion about whether functional programming is any good, which is what you seem to be treating it as.

[pqh]

And yet, thankfully, the old versions of those commits are still present in the reflog for some time. Making it easy to recover from finger slips.