What if Git worked with programming languages?

https://github.com/GavinMendelGleason/syntactic_versioning

by LukeEF • 5 years ago

170 191 5 years ago

afavour• 5 years ago

I do kind of love the idea of Git using ASTs instead of source code. It makes a ton of sense.

Even just in the immediate term I wish I could make Git(hub) tabs/2 spaces/4 spaces/whatever agnostic. Seems crazy to me that in 2021 we still have to make opinionated choices across orgs about what to use... why can't we pull the code down, view it in whatever setup we want, then commit a normalized version?

[whispers] this is actually something tabs allow you to do natively by setting custom tab widths in text editors but I've given up trying to sell people on tabs at this point and just want to be able to do my own thing

enriquto2• 5 years ago

[whispers] don't give up! There's quite a bunch of us. Our day will come! Long live glorious tabs!

silon42• 5 years ago

I'm fine with using tabs, but my tab width will be set to 8... be sure to obey line length limits with that in mind.

convolvatron2• 5 years ago

having presentation by flexible and different than the underlying model is a great idea for code

but admit it, tabs are fragile and a pretty weak implementation

wutbrodo• 5 years ago

admit it, tabs are fragile and a pretty weak implementation

Could you elaborate? I don't have a personal opinion here and have only worked in orgs that require spaces, but I'm not familiar with the criticisms of tabs.

williamdclt• 5 years ago

It's not that you're going too far, it's that you're not going far enough!

It's not a Git question, it's a programming language question. There's no reason source code need to be stored as plain text[1]! Editors show it as text, we edit it as text, but why wouldn't it be _stored_ as an AST? Not only does formatting becomes an editor concern, but code could even be edited as a tree, as a graph, as whatever you want

[1] - well, actually there's plenty of reasons: chiefly because plaintext is very interoperable

jerf• 5 years ago

"but why wouldn't it be _stored_ as an AST?"

It profoundly is. You can't store "an AST". You can only store a serialization of it. The official language grammar is a serialization of the AST custom crafted for that language. It is as much an "AST" as any other serialization would be; all such alternative representations would all produce isomorphic memory representations if parsed from a proper library.

At a high level it may sound useful to try to then provide a cross-language AST representation, but it's one of those things that sounds great at a high level but as soon as you actually tried to implement it for, say, Python and C++, you'd rapidly discover that in practice there's not as much opportunity for "generic AST operations" as you may think.

The problem isn't that it isn't "stored as an AST" but that $YOUR_LANGUAGE apparently doesn't have good libraries or mechanisms for getting at it. Go, for instance, ships with the relevant bits of the compiler exposed, and as a result there are tons of tools that operate on Go code as ASTs and not textually, because it's readily available and supported by the core language team. I use this only as an example I know personally, there are other languages that have similar sorts of support as well.

Kinrany• 5 years ago

Storing everything in plain text is better. But there's no reason source code needs to be edited as plain text.

fwip• 5 years ago

You might be interested in https://unisonweb.org/

cratermoon4• 5 years ago

There's no reason source code need to be stored as plain text

The same can be said for lots of different documents, and it's been true for programs like Word for a long time. See also [1]T. H. Nelson, “Complex information processing: a file structure for the complex, the changing and the indeterminate,” in Proceedings of the 1965 20th national conference, New York, NY, USA, Aug. 1965, pp. 84–100. doi: 10.1145/800197.806036.

pid-1• 5 years ago

That's some cool discussion!

omegalulw• 5 years ago

Don't forget bugs. Bugs in git is nightmare scenario. There are so many complexities with just plaintext, there's no way AST diffs get broad support. Also, AST needn't always be simpler than plaintext, e.g. C++ macros.

BiteCode_dev• 5 years ago

Yes, but only if it falls back to text diff as soon as there is the smallest doubt it can't provide a good AST diff.

thrwyoilarticle2• 5 years ago

You can also write git hooks to turn their spaces into your tabs & vice versa.

OJFord2• 5 years ago

That's not a good solution - every commit with an author (well technically committer I suppose) whose opinion differs to the last will be horrendous.

fstrthnscnd3• 5 years ago

Tabs do work as long as they aren't fixed width (I don't know what you mean by "custom").

For instance, in many languages, one will sometimes have to split a function call to many lines, and in most languages function names aren't of fixed length, thus in order to get a correct alignment for parameters, the tab width at that point will have to match the function name length.

    #include<stdio.h>

    int main(int argc, char* argv[]) {
        printf("%s %s %s %s\n",
               __FILE__,
               __LINE__,
               __DATE__,
               __TIME__);

        return 0;
    }

I agree with your idea of storing a normalized version of the code in the repo: it wouldn't then matter whether that version contains characters to align the code properly, it would just be inserted by the editor/linter as needed. The difficulty is that sometimes linting isn't enough, and some manual formatting is needed. Or perhaps the formatting rules are under specified?

Another issue with AST diffing is when languages allow some form of syntactic sugar as preprocessing: the compiler might just see the simplified tree, not the one with the "sugary" forms. A tool capable of parsing such languages should also be able to handle these extensions.

Asraelite3• 5 years ago

the tab width at that point will have to match the function name length.

This is a non-issue. Use tabs for indentation and spaces for alignment.

Anon_troll• 5 years ago

The whitespace and formatting are not significant to the compiler, but they can provide a lot of information to the reader of the code.

You can often see where the writer put the most effort and thought by just seeing how they wrote it. This can help analyzing a codebase considerably.

If everything is normalized, you lose those valuable cues.

geofft2• 5 years ago

One of the practical issues here is, if your code fails to compile in CI with an error like

    /home/ci/src/foo.c:123:45: error: use of undeclared identifier 'a'

    /home/ci/src/bar.py:50: syntax error in type comment

or crashes in production with an error like

    java.lang.NullPointerException
        at com.example.Baz.doThings(Baz.java:1337)

you really want to be able to find line 123 column 45, line 50, or line 1337 in your editor, and have that be the same line as what your CI compiled and deployed.

On its own, tabs vs. spaces only affects columns, and you can probably figure things out without columns (although it's a shame to lose it). But different tab sizes affect how long your lines are, and line wrapping is a thing that people care about at least as much as tabs vs. spaces (people with different size monitor or fonts will easily see too-long or too-short lines on their display; if your spaces are equivalent to the tab stop, the distinction is literally invisible). And once you start rewrapping lines, everyone's line numbers are different.

I think it's possible to solve this by using some sort of AST-based index into the file and teaching IDEs to let you seek based on that, but it's suddenly a more complex problem.

ratww2• 5 years ago

This is already a very common problem with a solution: transpiled JS already needs source maps to display errors correctly.

thefreeman• 5 years ago

If you append `?w=1` to the diff view URL on a pull request it makes it whitespace agnostic just FYI

pbiggar• 5 years ago

fwiw, this is what we do in Dark [1]. We store (serialized) ASTs, then then we pretty print them in the editor. This converts the AST into tokens that you see on your screen, complete with configurable* indentation, line-length, etc. Code would be displayed according to your config* and the same code displayed differently to a different developer looking at the same code.

[1] https://darklang.com

* I haven't actually enabled users to configure this, but it's just some variables called 'indent' and `lineLength` in the code

iso8859-1• 5 years ago

This is on the http://lamdu.org roadmap.

ghoward• 5 years ago

I'm actually working on a VCS based on this idea and on tracking changes to binary files based on their structure as well. (It turns out that the same techniques work for both.)

AMA and please give me feedback!

tombert• 5 years ago

I would definitely support a Lisp-centric Git.

Whenever I do Clojure, something that can get difficult when working with multiple people is how the parentheses/brackets/braces stack up, especially when everyone seems to have different opinions on how that works. As a result, if you're not careful, when there's a merge conflict you can have a ton of extra parentheses, which can be irritating to debug.

Obviously this is at some level an issue inherent to Lisps (and to be clear, I love Lisps, and these small headaches are worth it), but I think problems like that could be reduced if our source controls were aware of the ASTs.

timgilbert• 5 years ago

Yeah, I've long thought a diff tool that works on s-exprs instead of lines would be invaluable for Lisp programming. It doesn't seem like it would be too hard to write, either, although getting GitHub etc to use it seems like it would be its own challenge...

fulafel• 5 years ago

Git can use an external tool for merging, so there could be eg a Clojure merge plugin even now. Apparently there are some commercial merge tools that Java programmers use for this like SemanticMerge. After all you hit the similar curly braces merge problems with other languages.

aardvark179• 5 years ago

I’ve done quite a lot of work on version management on structured data (in my case this was for a version managed GIS database) and it’s not an easy problem, and is likely even harder with something like an AST that is generated from a text file and so does not preserve the identity of nodes. I’m not saying that it’s impossible, but it is more work and requires more tooling around it than people think, and it keeps coming up here and other places as a, “really good idea.”

cormacrelf• 5 years ago

Counterpoint: a quick google reveals diffsitter: https://github.com/afnanenayet/diffsitter

The output could be a lot more compact, it could do better at adding context (in the same way https://github.com/romgrk/nvim-treesitter-context does, etc), but if you're interested in this it's really within reach, go help out.

I wonder if you can use it for automerge yet.

nerdponx2• 5 years ago

Storing AST instead of source code is one of the goals of the very interesting Unison programming language: https://www.unisonweb.org/

Part of what's nice about Git (and plain text in general) is that it's the lowest common denominator for a lot of things. This is why traditional Unix tools are built oriented around streams of bytes. Text is a low level carrier protocol; you can encode almost anything in it, but you need to agree on some kind of format.

The good part is that you can use very very generic tools on almost arbitrary pieces of data. The bad part is that you might have to do a lot of parsing and re-parsing of the same data, and you have to contend with the dangers of underspecified formats.

Git follows the Unix tradition in this regard. As a result, it is nearly universal in what it can store. You can use it to store pretty much anything, but you are now at the lowest common denominator of support for any particular data format.

Git-for-ASTs will no longer have this universality property, but will gain a lot more power in the covered domain. This is a design tradeoff.

One thing that's nice about Git is that you can specify arbitrary diff drivers with the "attributes" system. So even if the Git database is storing plain text, your diff driver can parse your source code into ASTs and present AST diffs to you when you run `git diff`. Perhaps more impressive, you can configure custom merge drivers, so you can (theoretically) implement semantic merging of ASTs right inside Git.

There are probably some fundamental limitations of this system, because the underlying data is still stored as blobs of bytes. But you can get pretty far as long as you don't mind parsing and re-parsing the same text over and over.

ssivark2• 5 years ago

Has this approach been tried? (Unison or otherwise…)

nerdponx2• 5 years ago

I believe Unison is the only attempt to do this at a programming language/environment level.

For Git diffs, there is Diffsitter, which uses Tree Sitter to generate semantic diffs of code files: https://github.com/afnanenayet/diffsitter

I have not used it, but it is high on my todo list.

Karellen• 5 years ago

`git` generally doesn't work with lines of text. Mostly it works with opaque file blobs and directory trees.

`git diff` and `git merge` work with lines of text by default - but they don't have to. You can supply your own `diff` and `merge` tools with the `difftool.*` and `mergetool.*` config options, try them out with `git-difftool` and `git-mergetool` commands, and set the default with the `git.diff` and `git.merge` config options.

If someone wanted to create AST-based diff and merge tools for a given language, they could be plugged right into the existing `git` infrastructure and it would work with them absolutely fine.

bspammer• 5 years ago

This feature is useful in so many different places. I use it to diff small encrypted files in my repo - just add `gpg -d` as a diff configuration and now I can use git log, diff etc in a meaningful way with binary files.

I've heard of people using it with pdfs as well - a pdf to html converter lets you get a good idea of what changed in the document.

colonwqbang• 5 years ago

Yes, I think this article is coming at it from the wrong end. Git is hardly the problem here, nor is it going to provide the solution.

The problem seems to be that we are lacking the format and the toolchain to manipulate it, and that is not the fault of git.

What is the state of the art in this area? Does somebody know of a viable format and toolchain, or any interesting projects looking to build them?

tyleo• 5 years ago

I believe that semantic merge does something like this: https://www.semanticmerge.com/

paul_h• 5 years ago

Came to link to that :)

kapep• 5 years ago

If someone wanted to create AST-based diff and merge tools for a given language, they could be plugged right into the existing `git` infrastructure and it would work with them absolutely fine.

There's a lot tooling in the Eclipse modelling ecosystem which could be easily used for this. Storing XML-based models in git is no problem and there's tooling for diffing and merging models via a GUI or programmatically. Combined with the fact that xtext DSLs use EMF models to represent ASTs, it wouldn't be too hard to glue together an AST-based a diff/merge tool for an xtext DSL.

dTal• 5 years ago

What if generating a diff is nontrivial? Say you rename an identifier. That might be a single command in an IDE. A sufficiently high-level "diff" format could easily capture that intent. But working backwards from hundreds of touched lines across many files to deduce that single semantic edit is not trivial. Git assumes that arbitrary diffs can be deduced from "before" and "after" files, but this isn't the case - it may be that you'd rather generate the new file from the diff!

rileymat22• 5 years ago

`git` generally doesn't work with lines of text. Mostly it works with opaque file blobs and directory trees.

I am not sure this is true.

In the past it gave me problems with line ending normalization between windows/mac/linux, in and out. In those cases it definitely had a lines of text view of things.

ajanuary• 5 years ago

Git is delegating to the diff to work out how to merge the blobs. It's the diff that is having trouble with the line endings.

Ajedi32• 5 years ago

It is generally true, but yes; automatic line ending conversion is an exception. You can turn it off with `git config --global core.autocrlf false`, though be aware that can cause issues if you have developers on different operating systems creating and committing files with different line endings.

indentit• 5 years ago

I guess this is what diffsitter[1] is for.

[1] https://news.ycombinator.com/item?id=27875333

kmeisthax• 5 years ago

Indeed. The Composer merge driver is critical for being able to work with modern PHP frameworks without tearing your hear out on every merge.

Merge drivers are Git's most powerful and least known feature, and I really wish they were more common.

Smaug1232• 5 years ago

I'm surprised they didn't mention Unison (https://www.unisonweb.org/), whose big idea is an immutable content-addressable store of ASTs. I really hope it changes everything.

renox2• 5 years ago

Except that Unison created its own language which makes pretty sure that they are doomed to fail.. I don't know if there is a technical reason for the new language or if it's NIH syndrome.

Smaug1232• 5 years ago

It's normal to experiment with new concepts in a new language built around that concept. Then eventually, if it's a good idea, other languages get the hint.

mangecoeur• 5 years ago

Interesting they mentioned Jupyter Notebooks but not NBDime https://github.com/jupyter/nbdime which is a Jupyter plugin specifically to address this problem. Without it, diffing notebooks is not feasible.

auscompgeek• 5 years ago

Note that you can specify a custom merge driver for different file types using a combination of gitattributes and git-config: https://git-scm.com/book/en/v2/Customizing-Git-Git-Attribute...

atonalfreerider• 5 years ago

Self-promote: Primitive does AST diffing and represents the changes graphically

primitive.io

jrm4• 5 years ago

What if Programming Languages worked with Lines of Text?

vxNsr• 5 years ago

Is it just me or is he describing an IDE with source control?

cies• 5 years ago

Structure editors haven't really taken off yet despite several historical and contemporary attempts.

This is a nice contemporary one:

https://github.com/projectional-haskell/structured-haskell-m...

Lisps also have all kinds of options available in Emacs, but it is more special to see this outside of the land of s-expressions.

raxxorrax• 5 years ago

Theoretically it might work, but I don't think I am too fond of the idea. I used git pull to completely waste my source and it would have been nice for git to have more intelligence here, but in the end I think some of its success lies in its simplicity.

SVN isn't too bad and not too much of a difference to git if you use a central repository anyway. The main neat thing was to just have one hidden folder, not in every subdirectory.

Git would also need the ability to transform from AST to source for every language. A bit unrealistic and there is no benefit to it. Could also do that with Assemblies and some meta info for the decompiler.

ufo• 5 years ago

I'm trying to remember the citation, but I remember seeing a presentation once from someone who studied this and they said that the thing that worked best was a hybrid approach: use structured diff at the top level of the program (modules / methods) but use line-based for statements and expressions. According to them, the structured diff can give unintuitive results if applied at the lowest syntactic levels.

jakeinspace• 5 years ago

I think the only useful way to implement AST-level diff/merge for non-trivial codebases would require the compiler to provide the parsed AST, since per-file ASTs would lack a lot of context. You could also ask the user to provide a separate file or files that describe the code topology, but why bother when the compiler can spit out an AST itself? A diff tool which targets a few of the bigger build systems (CMake, Maven, Gradle) and compilers might work, and could worry about small build environments after gaining momentum.

kitplummer• 5 years ago

I'm just a bit more "generally" curious. Is `git` being the _only_ DVCS a good thing? Not to say that `hg` or `darcs` don't exist, just that the hub on top of git has pushed us in a singular direction.

I would like to see, at least academically, something more.

tomphoolery• 5 years ago

The choice of DVCS tooling is ancillary to the success of GitHub. People learned Git so they could use GitHub, not the other way around. At least, this is the way I remember it.

If someone comes along and builds the best forge software ever, but uses Mercurial instead of Git, I'll bet a lot of people would switch technologies at some point. Until then, I'd say most people use GitHub because GitHub works for them, and they use Git because that's how you interact with GitHub. They don't care about the ivory tower benefits of their particular DVCS tooling, all they care about is easily collaborating with their teammates.

It would definitely be great if you could have a GitHub-like experience using Mercurial or Darcs, but so far I haven't seen anything close to that.

aayjaychan• 5 years ago

Does GitLab count as a GitHub-like experience? There is a fork of GitLab called Heptapod that supports Mercurial.

https://heptapod.net/

mabbo2• 5 years ago

Reading this article, I feel as though the author doesn't deeply understand git.

git works on blobs of data, not files, and not lines of text. It doesn't just happen to also work on binary files- that's all it works on.

Now, if the author is suggesting that git-diff ought to have a language specific mode that parses changed files as ASTs to compare, now I'm interested. Let's do that. I'll help!

But git does not need to change how it works for that to happen. Git does not even need git-diff to exist to serve it's main purpose.

dboreham2• 5 years ago

Pretty sure OP does understand, and is proposing what you deduced.

Incorporate some semantic understanding of the version controlled data into the VCS. Currently this work is subcontracted to humans.

mabbo2• 5 years ago

Maybe I'm misunderstanding. It's just lines like this:

The text-orientated design of git reflects...
The current version of git is also able to find differences in binary files.
if we were storing information as ASTs, rather than lines of text

These all, to me, show a gap in the authors understanding of how git works. And that's okay- git is often easier to use than is to understand.

But if they had a better understanding, they could make their point far better. And without understanding, they won't be able to implement this idea.

mbauman• 5 years ago

You can already choose different `diff` programs to use for particular filetypes. E.g., nbdime for Jupiter notebooks:

https://nbdime.readthedocs.io/en/latest/vcs.html#git-integra...

hardwaregeek3• 5 years ago

I dunno I feel like you're focusing on a detail that's not particularly relevant. The author's main thrust is precisely what you described about parsing changed files as ASTs.

nemetroid2• 5 years ago

It isn't relevant to the author's vision of content-aware diffing, but it is relevant to the author's complaints about how Git's (alleged) text-based-ness makes Git awkward to use with Jupyter notebooks. Has the author tried searching the web for "git diff jupyter"?

tux32• 5 years ago

Note that git does work with diffs a lot.

Rebases and cherry-picks work by applying diffs, not by copying blobs. Auto-merging also needs to look at file content as text, you can't auto-merge a binary file with git.

It's an often repeated fact that if you look inside Git, it doesn't work with diffs, it works with blobs. But if you look closer, it's often diffs again!

arghwhat4• 5 years ago

With cherry-picks (and thus rebase), you ask git to turn a commit into a patch, so it does just that.

I would mostly consider auto merges (which I guess are bolted on) as the main area where git itself uses diffs during resolution and even then only as a suggested resolution (you get warned and need to confirm it when validating the merge).

So no, it's blobs all the way down. Darcs and Pijul are patch based though.

screye• 5 years ago

The git extension on VSCode is already pretty good at doing diffs on jupyter notebooks.

I distinctly remember this not being a core feature of stock git and needing Jupytext to enable version control on notebooks. So, I feel like this sort of language specific stuff is already happening, but not in any unified product.

munificent• 5 years ago

The author is likely using "git" to mean "the entire typical git user experience that git users spend time looking at".

And, from that perspective, Git-the-UX definitely does work on line-oriented files.

munk-a• 5 years ago

There's also a historical angle here that's important to inspect - Git was designed to specifically be content agnostic. There are some predecessors in the SCM space (like VSS) that are specifically language aware and allow the checking out of line ranges (pinning them so that no one else will make a conflicting change specifically) and even entire functions - these systems can cause a lot of grief while failing to protect the logic they're specifically trying to protect. As the warts on SVN got more and more visible I think the general assumption was that the replacement SCM would come out of this code aware space - but it didn't and in retrospect we all dodged a huge bullet when that happened.

I absolutely adore tooling around git that makes diffs more visible - one thing I absolutely gush over is anything that can detect and highlight function reordering... however, the core process of merging and rebasing and all that jazz - I don't think we're going to find anything automated that I'll ever trust when I'm not working on a ridiculously clean codebase - minor changes can have echo effects and when two people are coding in the same general area they need to be aware of what the other person is trying to do.

tenaciousDaniel• 5 years ago

I'm in the process of building a programming language for UI designers, and realized that diffing the AST (or some other kind of object notation) would be far more useful and understandable. I'll probably be digging into this exact problem within the next year or so.

ggleason• 5 years ago

My two cents: https://github.com/GavinMendelGleason/syntactic_versioning/b...

ClassAndBurn• 5 years ago

Git is designed to require human oversight. This is usually a feature, but in recent years has become a bug with things like GitOps.

It's important to remember that Git is a terrible database because of its lack of semantic structure. All conflicts require a human who does have to context. This is why almost no one builds a system that uses Git as a two way interface. And when they do, its via Github Pull Requests (which go to humans) and not Git itself.

In all, this makes it a wonderful general purpose shared filesystem. And that's about it.

Jensson• 5 years ago

I don't see how this could ever work on evolving languages, different GIT versions would produce different commits and read commits differently based on the latest C++ standard. This would potentially lead to version control bugs where different GIT versions creates different results from the same commit, that is horrible, version control needs to be 100% bug free in that regard.

The only reasonable application would be to use a language AST parser to better identify relevant text diffs, but the commits still needs to be stored as text.

dboreham2• 5 years ago

This doesn't really make sense, because in order to have those code changes compile correctly, there must be a corresponding commit to CI config that changes the complier version or compiler switches for the new language version. The "semantic-diff-er" can also be driven by that commit such that it uses the correct language version.

verdverm• 5 years ago

It's non trivial to support multiple versions of the same language on a host system. You have to account for dev machines and workflows as well.

Docker can help with this, but often devs don't want to run a container to build their code. It's a hard habit to change.

Now, consider how difficult it would be to get the differ to understand where to find compiler versions.

shepherdjerred• 5 years ago

Commits could be stored as is, the difference would be that diffs are clearer when presented to a human.

pkghost• 5 years ago

How is this different from any other problem that is already solved by version pegging?

jcrites• 5 years ago

I’ve had loosely similar ideas before. The basic idea is to make the compiler tool chain aware of diffs, and help scrutinize and implement them. Refactoring suggestions could be included with the diff.

For example, say you’re dependending on a module and it renamed a class/method/trait/macro/constant/whatever. A synchronous method has become a sync or vice versa.

The diff could include programmatic instructions for consumers to apply to their code bases switching them over to the new method. This could be as simple as semantically changing the name used, or in the case of changing sync to a sync it could add `await` in the appropriate spot.

There’s no limit to how complex the rewrite rules could be. You could totally reorganize the parameters to a function and ship that refactoring, or even add a parameter along with the default code necessary to provide it.

Unlike the author I don’t think code in Git will likely ever move beyond source, plus the refactoring instructions needed to update a change from a dependency — perhaps a macro-like syntax.

Too much text manipulation is required from source control for me to conceive of it being anything but human programming text in a future I can imagine. Machines can already parse it; there doesn’t seem to be a compelling reason to store some other kind of structure.

Refactoring wouldn’t need to be any special Git extension, just a file accompanying the commit with instructions for the language tool chain.

Your IDE or CLI could walk you through interactively everywhere it’s getting applied, or you could apply all and review the result in your app that consumes the module.

This would also open up security risks from accepting diffs from dependencies and applying their refractors, but unless modules are sandboxed quite well that’s a risk you take with updating dependencies anyway. And you can always scrutinize the refactoring-diff manually after it runs before accepting it.

The industry would probably standardize onto the notion that a change that requires running automated room factoring from a dependency across your codebase is a major version change; in other words a breaking or backwards incompatible change, just one that’s much easier to upgrade to.

Languages with macros or other programmatic transformers would be well suited to this concept I think.

Maybe Rust macros could be enhanced for the purpose to pattern match over an existing codebase somehow: not just the macro invocation point, but anywhere, e.g., a given trait or function is used; and then the output of the macro would not feed into the next stage of the compiler step but would instead result in rewriting code on disk to produce a diff that you examine and apply to your code.

A capability like this would make it much easier to manage large aggregate code bases consisting of many dependencies. OSS package maintainers or infrastructure providers at companies could ship nominally backwards -incompatible changes that are still actually compatible when you run the macro transformer that updates the code that uses them.

For a simple example, imagine that `foo()` was previously a function and the implementation chooses to add some optional parameters or those with default values and change it into a macro `foo!()`. The accompanying transformer would semantically identify references to `foo()` and make the necessary updates. You could rename global constants or traits or other code elements this way.

Consider the way in which Google Guava has had to evolve over time. A number of its features have become part of the Java language, and thus the classes deprecated and removed gradually. With a compiler facility like what I am describing, users of Guava could run the transformer to migrate older could bases that use methods like Guava’s `Preconditions.checkNotNull(Object, message))` to use Java’s now-standard `Objects. requireNonNull (T obj, String message)`. Because the maintainers of Guava wanted to keep it modern, current, avoid redundancy, and designed in the best way that they knew how, they made a number of breaking changes for which the project lead later apologized [1]. Most changes wouldn’t have been be painful if accompanied by automated refactoring.

You could allow the transformer to produce code that still needs work from humans to finalize and compile. In that case it could change as much as it’s able and leave instructions at each call site.

At my company we have bots that submit proposed code changes to our codebase that need to be taken over as author by a human, reviewed, sometimes lightly edited, and shipped, and they work quite well. One bit finds unused code and submits diffs to remove it if it’s been in the code repository long enough. Another detects when launch experiments have been at 100% all on one treatment for a long period of time (meaning the feature has launched) and submits code removing the experiment check. The latter sometimes require removes surrounding code that would subsequently become unused from removing the experiment check.

These have provided meaningful value in helping keep the codebase tidy and I look forward to more automation like this in the future, including diff-aware compilers and refactoring tools.

[1] https://www.reddit.com/r/java/comments/mr03mi/comment/guk848...

CodeIsTheEnd• 5 years ago

I don't understand why GitHub hasn't solved the issue of diffs starting with a '}' (or ')' or 'end'). Just slide the diff over while it starts with a closing token! I suppose it's an artifact of the diffing algorithm, but aren't there better diffing algorithms, even built-in within git?

This is by far the most obvious example of "git doesn't understand programming languages", but it also seems like the most straightforward to fix.

nemetroid2• 5 years ago

Git supports a few different diff algorithms. GitHub only seems to support the standard Myers algorithm, though: https://github.com/isaacs/github/issues/455

mynegation• 5 years ago

It is because diff is syntax agnostic. You might be able to get away with this hack in some cases but that complicates algorithm and will break in some other cases (how about nested brackets? Multiple brackets on one line?). Once you want to handle this properly you need syntax aware diff algorithm and some resources are linked in this discussion.

hardwaregeek3• 5 years ago

I've wanted this for a while, but I will say there's some caveats. Sometimes I want to commit just as a "it's the end of the day, I want to leave, here's a code dump". I suppose you could have multiple tiers of code saving.

I've also wondered about whether you could do code analysis with time as a dimension. If you can analyze the evolution of the code and pull old implementations, what can you do? Autocomplete is a good example, as it can pull previous patterns you've used. Maybe some way to tell the programmer "hold up, you've made this mistake before, don't do it again"? I'm not sure.

inetknght• 5 years ago

I want to commit just as a "it's the end of the day, I want to leave, here's a code dump"

1. git checkout -b eod-$(date -Id)

2. commit

3. leave

4. return

5. git checkout -

6. git merge - --no-commit

hardwaregeek3• 5 years ago

Yes, but if we're talking about some hypothetical tool that requires a valid AST, there might be a situation where I don't have a valid AST and want to save the code. Similarly, I had a job where we used pre-commit hooks that ran a linting script. I had to override the hook to commit which was slightly annoying at times.

gumby• 5 years ago

Shared (concurrent) code editors might work better if their CRDT/OT model worked at that level.

Not that I really want to edit code in a shared environment (editing documents that way is bad enough), but just musing…

kazinator• 5 years ago

The blogger does not understand Git, fundamentally.

Git does does not work with text. It stores snapshots of artifacts.

The diffs that you see when you use the various commands like git log -p are recovered from the snapshots, when those artifacts happen to be text files.

Git absolutely works with texts when you connect it with external representations and tooling, such as when you "git format-patch" and then "git am" to import that; and the rebasing workflows obviously have textual merging with conflict resolution. Still, that seems like something that could be externalized. A language-specific three-way-diff tool can handle a merge by parsing all three pieces and working with ASTs. It's something that could be developed later, yet still work with your old commits.

There is this: https://git-scm.com/docs/git-mergetool

No idea how well it works.

skybrian• 5 years ago

If you’re interested in this sort of thing you might want to look at Dolt (for sharing databases in a git-like way) and Pijul, which records diffs explicitly, rather than calculating them on the fly.

I wonder if there might be a clever way to encode source code in a Dolt database? Maybe each function should be a record?

LukeEF• 5 years ago

Author is CTO of TerminusDB (https://terminusdb.com/), which is a more graphy version of Dolt! Check it out.

jpitz• 5 years ago

Didn't the VisualAge IDEs do this with their built-in version control? This was 20 years ago, and I seem to remember that the version control was at the method level, not file level.

olodus• 5 years ago

Ever since I learned about Git merge strategies and wrote a very basic one myself, I've been wanting to write one that syntaxticly understands a bit of the test framework code we use at work. It is super annoying when you copy a test because you want to vary a very specific case and gig gets all confused about what code is and isn't the same.

(yeah I know I should break out the copied part but who always has time for that)

mumblemumble2• 5 years ago

I would maybe be interested in Git allowing you to plug in your own diff generators for different file types.

But I would not want Git itself trying to understand the contents of files. That seems to me to be an idea that lives on a misconception of the "things programmers believe about names" variety. Not every file in source control is source code. Not every programming language's grammar maps to an abstract syntax tree. In some files, such as makefiles, the difference between tabs and spaces is semantically significant. Some languages (such as Fortran and Racket) have variable syntax. And so on and so forth.

So I think that we really don't want the source control system itself trying to get too smart about the contents of files. That will inevitably make the source control system less compatible with the various kinds of things you might want to put into source control. And it will also make the source control system a lot more complicated than it would otherwise be, in return for a largely theoretical payoff.

But if we want to delegate the work of generating diffs off to other people, so that Git can allow for syntax or semantics-aware diffing without having to personally wade into that quagmire (and perhaps also allowing language communities to support multiple source control systems, a bit like how it works with LSP), that might be an interesting thing to experiment with.

saurik• 5 years ago

I would maybe be interested in Git allowing you to plug in your own diff generators for different file types.

This is already supported.

lux• 5 years ago

A common example is UnityYAMLMerge for merging the Unity game engine's generated files.

https://docs.unity3d.com/Manual/SmartMerge.html

Configuring it to work with Git and others is a little ways down the page, but would apply the same for other diff tools.

franga2000• 5 years ago

I looked this up and for anyone wondering, it's called "diff/merge drivers", but there are only a handful of them out there. Some highlights from a few minutes of searching:

- MS Office: https://github.com/lcnittl/DMFO - SQLite: https://github.com/cannadayr/git-sqlite - Jupyter notebooks: https://nbdime.readthedocs.io/en/latest/vcs.html#git-integra...

One big caveat of this is that since git doesn't really store just a stack of diffs, despite the fact it presents itself as such to the user, a custom merge driver will not make your .git grow any less than it would normally.

shoo2• 5 years ago

I investigated the option of using a custom git merge driver for a project where we were planning to version control a bunch of data files using git.

Here's a proof of concept python merge driver I bashed together at the time to auto-merge JSON objects: https://github.com/fcostin/jsonmerge_git_merge_driver

This never went anywhere near production, but it was very easy to put together something basic.

One complication with using a custom merge driver, as discussed by https://github.com/Praqma/git-merge-driver , is that they need to be configured inside the `.git/config` of the repo, which itself is not version controlled. So there's an additional config management overhead to rolling that out to everyone in a machine. Additionally, if outsourcing hosting for git repos, it may not be supported to install and configure a custom merge driver for merges conducted by the hosting platform (e.g. merges created by github.com pull request workflow).

One idea I had at the time was using external schema files (e.g. JSON schema for JSON files) to help guide/constrain the result of the merge. I never implemented it, but it should be possible. If the schemas were also version controlled in the same git repo that stores the data, you'd need to figure out which one (and which version) to load when resolving a merge conflict of a data file. There doesn't seem to be a well-supported robust way for a merge driver script to discover the source and destination branches, but there are some potentially fragile ways of doing it that work some of the time.

pwdisswordfish8• 5 years ago

Next you will be telling me that git is already a blockchain!

madmax96• 5 years ago

I disagree. Many engineers want to refactor across a sequence of small PRs, for example. Small PRs are a good thing, because they’re easier to understand. But today, Git makes this painful. Also, understanding how the meaning of code changes over time can help reduce bugs.

The solution will have to be pluggable. But I think it is possible, and there are sane things to do (e.g. fall back to vanilla git) when there are missing plugs.

ffwacom• 5 years ago

Not every programming language's grammar maps to an abstract syntax tree

Are there some examples of this?

simcop23873• 5 years ago

Perl, this is because you can't actually properly parse Perl without also running Perl code at the same time.

mumblemumble2• 5 years ago

Forth. You could certainly define a formal grammar for it and construct an AST, but it would be trivial and not very useful.

ironmagma• 5 years ago

Not only that, but imagine you realize there is a bug in the parsing tool. Now you have to go back and re-parse the code, or otherwise just deal with a bad history forever. Suddenly you’re storing text again.

tomxor• 5 years ago

The fact that git works on lines of text [...] we could be looking at the alterations to the abstract syntax tree.

Fundamentally git does not operate on text, it operates on files (content addressed SCM not a ledger of text diffs); diffs are generated upon request between arbitrary merkel trees. So there is no need to implicate git in such a tool, it can be independent:

       GIT_EXTERNAL_DIFF
           When the environment variable GIT_EXTERNAL_DIFF is set, the program
           named by it is called to generate diffs, and Git does not use its
           builtin diff machinery. For a path that is added, removed, or
           modified, GIT_EXTERNAL_DIFF is called with 7 parameters:

               path old-file old-hex old-mode new-file new-hex new-mode

lamontcg• 5 years ago

This has been posted before

bialpio• 5 years ago

This made me think of Unison: https://www.unisonweb.org/

Discussion: https://news.ycombinator.com/item?id=27652677

maweki• 5 years ago

Working on the AST is quite an interesting idea, until your comments aren't in the AST and you want to commit a syntax error of work in progress.

Not to mention changing ASTs (while maintaining concrete syntax) in different versions of the language.

ozim• 5 years ago

I feel this is just an example of "worse is better" and whole proposition as interesting but totally not practical and I would not like for GIT to go anywhere near that idea.

alkonaut• 5 years ago

I’d give anything just to get a few basic merge modes. For example “this file can treat two one line additions as unordered”.

So any shared append-only file (a change log, an enumeration,…) doesn’t automatically conflict.

Syntax aware diffing would be great too, but I’d take something much simpler. For syntax aware stuff I’d love something that could tell semantic changes from noise.

aidenn0• 5 years ago

The problem with a tool that depends on structured data is that it only works with structured data.

Of course the problem with a tool built for unstructured data is that it's dumber than it need be, and when you do treat the data as structured, it's ad-hoc and often buggy.

When talking about how "powerful" a tool is, there's always this tension between structured and unstructured.

g0510512• 5 years ago

Back in the early 2000's Visual Age for Java allowed you to version individual methods. Since Visual Age for Java was derived from Visual Age for Smalltalk (and was actually written in Smalltalk) I suppose it inherited the capability from there.

igouy• 5 years ago

More specifically from ENVY/Developer

https://www.google.com/books/edition/Mastering_ENVY_Develope...

which was available for the three main commercial Smalltalk platforms: VisualWorks, IBM Smalltalk, Digitalk.

g0510512• 5 years ago

I never knew that! Thanks.

elif• 5 years ago

It sounds inspirational and revolutionary but the longer I think about it, the less utopian it feels. The idea of forgetting about pieces of code or how things are linked.

Files also provide an opportunity to purposefully communicate organizational intent.

shoo2• 5 years ago

There's a good blog post about auto-merging JSON/XML structured data files (for game content) on the bitsquid blog from 2010:

having content conflicts is no fun either. A level designer wants to work in the level editor, not manage strange content conflicts in barely understandable XML-files. The level designer should never have to mess with WinMerging the engine's file formats.
And conflicts shouldn't be necessary. Most content conflicts are not actual conflicts. It is not that often that two people have moved the exact same object or changed the exact same settings parameter. Rather, the conflicts occur because a line-based merge tool tries to merge hierarchical data (XML or JSON) and messes up the structure.
In those rare cases when there is an actual conflict, the content people don't want to resolve it in WinMerge. If two level designers have moved the same object, we don't really help them address the issue by bringing up a dialog box with a ton of XML mumbo-jumbo. Instead, it is much better to just pick one of the two locations and go ahead with merging the file. Then, the level designers can fix any problems that might have occurred in the level editor -- the right tool for the job.

-- http://bitsquid.blogspot.com/2010/06/avoiding-content-locks-...

WalterBright• 5 years ago

I'm fine with the line oriented nature of git. I wouldn't want to be confronted with problems due to the constantly shifting nature of languages vs whatever plugin exists for them on git. Even C is constantly changing.

zomglings• 5 years ago

I maintain a free/open source project that does exactly what the author asks for: https://github.com/bugout-dev/locust.

Our tool uses git as the foundation of its functionality. It superimposes git diffs on top of ASTs.

It is insanely powerful.

For example, we use it to power semantic code search and current support Python, Javascript, and Java. We generate a JSON object describing the AST differences between initial and terminal commits on GitHub PRs. A full text search on the JSON objects performs surprisingly well when we want to answer questions like, "When did we add dateutils as a dependency?" or "When did we last change the /journals handler on the API?"

The Python integration currently sees the most use but if you are interested in other languages, we would be happy to support it.

Do drop me a DM if you want help getting started with Locust.

What if Git worked with programming languages?

Related Stories