Phabricator for patches and code review

Austin Seipp austin at
Sat Jun 7 07:28:45 UTC 2014

I don't think Arcanist forces any particular workflow after working
with it a bit. Generally, all you have to do is checkout a branch,
make some commits on that branch, and run 'arc diff'. Make more
commits on that branch, run 'arc diff' again. When it's ready I can
merge it however I want. This workflow should be very sensible to most
people, and it's easier than uploading diffs to Trac IMO.

Phabricator will not host the repository. Existing committers will
have the exact same access they always did. The only thing that would
change is people submitting patches can use Phabricator instead of
uploading diffs manually.

In addition, existing committers can also use Phabricator's
post-review tools to track commits they might find relevant after
people author them. This is a very useful feature in my experience,
and GitHub has no equivalent to this.

On Sat, Jun 7, 2014 at 2:18 AM, Arash Rouhani
<rarash at> wrote:
> Could not have agreed more with Manuel.
> I would also like to point out that one of the missions of the arcanist tool
> is to support all version control systems. That have made sense for FaceBook
> Inc, who went from Subversion to Git to Mercurial. GHC team only use git
> now. I think the consequence is that the arcanist command line tool becomes
> quite weak*, for example I were not able to push a given gitrevision, you
> have to go through `arc diff` which only pushes the commit that HEAD is at.
> For sure github is the lead thing most everyone is using and already know
> how to use. As for side-by-side diffs on github, there is a browser
> extension for it. But yes, the Phabricator has a better review tool. :)
> Cheers,
> Arash
> * Based on my experience with it from my summer internship at FaceBook 2013.
> On 2014-06-07 07:21, Manuel M T Chakravarty wrote:
>> So, why not put everything on GutHub and use pull requests and so on?
>> SimonM writes that Phabricator is better than GitHub. I’m happy to believe
>> that, but he also writes that using it requires installing local software
>> and quite a bit of work. Moreover, I like to add that lots of people already
>> know how to use GitHub and probably few know Phabricator.
>> So, we are talking about having a somewhat better tool in return for three
>> very significant disadvantages: (1) local installation, (2) work to set up
>> and maintain Phabricator, and (3) effort by many people to learn to use it.
>> We also have a constant lack of sufficient men power. So, why spend effort
>> on building our own infrastructure, which will only increase the hurdle for
>> contributors (as they have to deal with an unknown system)? Let’s outsource
>> the effort to GitHub.
>> Manuel
>> Simon Peyton Jones <simonpj at>:
>>> At the moment GHC's main sources aren't on github, which means that that
>>> (in my highly imperfect understanding) people can't submit pull requests or
>>> use their code review mechanisms.  Moreover, most people don't have commit
>>> rights on the main GHC server, so if someone wants to offer a patch they can
>>> really only do so in textual form attached to Trac.  People with commit
>>> rights can make a branch, but there's a danger that over a decade we'll
>>> accumulate zillions of dead branches which people forgot to delete.  I think
>>> on github the branch is in a different repo, belonging to the patch author.
>>> So we really don't have a good work flow for creating, reviewing,
>>> modifying, and finally apply patches.  I am no expert on these matters. If
>>> Phabricator would help with that I'm all for it.  But perhaps there are
>>> other alternatives?  Or is Phab the lead thing.  Will it stay around?
>>> Also before going too far I'd really like someone to document the
>>> workflow carefully, and make sure it works from Windows equally well.
>>> I'm not too stressed out about losing the review trail of a patch.  Much
>>> of it will be commenting on stuff that no longer appears in the final patch.
>>> Anything that's important should appear in a Note in the source code; even
>>> the commit messages are invisible until you really start digging.
>>> Simon
>>> | -----Original Message-----
>>> | From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of
>>> Austin
>>> | Seipp
>>> | Sent: 06 June 2014 05:06
>>> | To: ghc-devs at
>>> | Subject: RFC: Phabricator for patches and code review
>>> |
>>> | Hello all,
>>> |
>>> | Recently, while doing server maintenance, several of the administrators
>>> | for set up an instance of Phabricator[1], located at
>>> |
>>> |
>>> | For those who aren't aware, Phabricator (or "Phab") is a suite of tools
>>> | for software development. Think of it like a polished, semi-private
>>> | GitHub with a lot of applications and tools for all kinds of needs.
>>> | We've been using it to do issue tracking for maintenance
>>> and
>>> | like it a lot so far.
>>> |
>>> | One very nice aspect of Phabricator though is it has a very nice code
>>> | review tool, called 'Differential', that is very useful. For people who
>>> | have used a tool like Review Board, it's similar. Furthermore, it has a
>>> | very convenient userland tool called 'Arcanist' which makes it easy for
>>> | newcomers to post a review and get it merged when it's ready all from
>>> | the command line.
>>> |
>>> | I'd like to see if people are interested in using Phab _strictly_ for
>>> | code review of GHC patches. It is a dedicated tool specifically for
>>> | this, and I think it works much better than Trac or inline GitHub
>>> | comments.
>>> |
>>> | Also, Phab can also support post-commit reviews. So if I touch
>>> something
>>> | in the runtime system and just push, perhaps Simon or Edward would like
>>> | to look, and they can be alerted right when I do this, and then yell if
>>> | I did something stupid.
>>> |
>>> | Before I go much further, I'd like to ask: is there *any* interest in
>>> | this? Or are people satisifed with Trac? The primary motivations are
>>> | roughly, in no particular order:
>>> |
>>> |  1) Code review is good for everyone, a good way for people to learn
>>> the
>>> | code and ask questions, and useful to give feedback to newcomers.
>>> | And even experienced GHC hackers can learn things from reading code, as
>>> | we all do regularly, or find things that need cleanup.
>>> |
>>> |  2) Phabricator in particular makes it very easy to submit patches for
>>> | review. To submit a patch, I just run the command 'arc diff' and it
>>> Does
>>> | The Right Thing. It also makes it easy to ensure people are
>>> | *alerted* when a patch might be relevant to them.
>>> |
>>> |  3) They can be uploaded and created from the command line, and merged
>>> | easily afterwords the same way. This is particularly useful for
>>> | newcomers, and for me. :)
>>> |
>>> |  4) Differential is dedicated to code review, and much better at it
>>> than
>>> | just reading patches on Trac IMO.
>>> |
>>> |  5) It supports both post-commit code review, as well as pre-commit
>>> | review. Post commit would be especially useful for us too, I think.
>>> |
>>> | Point #2 and #3 are mostly relevant for me, because I mostly handle
>>> | incoming patches. But I think in general it would be nice, and make it
>>> a
>>> | lot easier for newcomers to submit patches, and us to look over them.
>>> |
>>> | Here's an example of a Differential code review:
>>> |
>>> |
>>> |
>>> | This is a demo using my 'wip/ermsb' patch. You'll need to create an
>>> | account to login, but it shouldn't be much trouble, you can login
>>> | several ways. I'll fix the login requirement soon. Feel free to read
>>> the
>>> | code, comment on it, and play around. It's more of a demonstration, but
>>> | real code review would be welcome too. :)
>>> |
>>> | If people are interested in doing this, I can add notes to the wiki
>>> | pages for newcomers, and I'll send another email about Phab so people
>>> | can understand it a little better. But I want to ask first.
>>> |
>>> | There is an argument that our team is so small, code review has
>>> | unnecessary burdens. But I think Phab could help a lot with tracking
>>> | outside patches and getting good reviews for incoming patches, and
>>> it'll
>>> | make it easier for newcomers. And experienced pros can probably learn a
>>> | thing as well.
>>> |
>>> | Again, to be clear, I don't propose we migrate anything to Phabricator
>>> | from, say, Trac. There's no real pressure to do so and it would be tons
>>> | of work. I only propose we use it for code review, which is perfectly
>>> | fine, and how other projects like LLVM do code review (they use
>>> | Bugzilla).
>>> |
>>> | I also don't think the usage of Phabricator should be mandatory (unless
>>> | we decide that later because we like it), but I would like to see
>>> people
>>> | use it if possible.
>>> |
>>> | [1]
>>> |
>>> | --
>>> | Regards,
>>> |
>>> | Austin Seipp, Haskell Consultant
>>> | Well-Typed LLP,
>>> | _______________________________________________
>>> | ghc-devs mailing list
>>> | ghc-devs at
>>> |
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Austin Seipp, Haskell Consultant
Well-Typed LLP,

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