- Setting up and working with our development tools
- Installing and setting up Nix
- Note on pre-commit hooks
- What to do once inside the
nix developshell - How to build the code during development
- How to setup
haskell-language-server - How to build the project’s artifacts with Nix
- How to develop without Nix
- How to add a new Haskell package
- How to update our Haskell dependencies
- How to update our pinned Nix dependencies
- How to upgrade GHC
- How to build the code with profiling
- Updating dependencies
- How to work with a local copy of source dependencies
- Working conventions
- Making and reviewing changes
- Supporting systems
- Project roles and responsibilities
This project relies on Nix to provision the complete toolchain needed to build all repository artifacts.
Thanks to Nix we ensure that everyone has consistent versions of the tools that we use.
Please use Nix since problems due to mismatched versions of tools are particularly annoying to fix!
If you really cannot use Nix and still want to contribute, then skip to the relevant section.
This repository uses Nix to provide the development and build environment.
For instructions on how to install and configure Nix (including how to enable access to our binary caches), refer to this document.
If you already have Nix installed and configured, you may enter the development shell by running nix develop.
If you are committing code outside nix develop, you will get this error:
pre-commit not found. Did you forget to activate your virtualenv?
In that case, you may either pass the flag --no-verify to git commit, or pip install pre-commit.
The pre-commit checks will be run by CI anyway, so any formatting errors will be caught when submitting your PR.
Your prompt will change to [something] and you will be presented with a menu of available commands.
Please read that menu carefully.
The nix develop environment has the correct GHC with all the external Haskell dependencies of the project.
From here you can build the project packages directly with cabal.
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Note
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You may need to run cabal update so that cabal knows about the index state we have pinned.
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Run cabal build all from the root to build all modules.
See the cabal project file for a list of other packages that you can build with cabal.
The nix develop environment has a haskell-language-server-wrapper binary for the right version of GHC.
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Important
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this binary is called haskell-language-server-wrapper, rather than haskell-language-server, which is what some of the editor integrations expect.
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We don’t have a hie.yaml, the implicit cradle support in HLS seems to work fine these days.
Haskell components are provisioned by Nix through IOGX via Haskell.nix
Once inside the nix develop shell, you can run the list-flake-outputs script to list all buildable Nix derivations.
You can build some of the Haskell packages without Nix, but this is not recommended and we don’t guarantee that these prerequisites are sufficient.
If you use Nix, these tools are provided for you via nix develop, and you do not need to install them yourself.
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If you want to build our Haskell packages with
cabal, then install it.
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Warning
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You can also use
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You need to do a few things when adding a new package, in the following order:
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Add the cabal file for the new package.
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Add the package to
cabal.project. -
Check that you can build the package with nix as well (see How to build with Nix) or wait for CI to check this for you.
Our Haskell packages come from two package repositories: - Hackage - CHaP (which is essentially another Hackage)
The "index state" of each repository is pinned to a particular time in cabal.project.
This tells Cabal to treat the repository "as if" it was the specified time, ensuring reproducibility.
If you want to use a package version from repository X which was added after the pinned index state time, you need to bump the index state for X.
This is not a big deal, since all it does is change what packages cabal considers to be available when doing solving, but it will change what package versions cabal picks for the plan, and so will likely result in significant recompilation, and potentially some breakage.
That typically just means that we need to fix the breakage (and add a lower-bound on the problematic package), or add an upper-bound on the problematic package.
Note that cabal itself keeps track of what index states it knows about, so when you bump the pinned index state you may need call cabal update in order for cabal to be happy.
The Nix code which builds our packages also cares about the index state.
This is represented by some pinned inputs in our flake (see here for more details)
You can update these by running:
- nix flake update hackage for Hackage
- nix flake update CHaP for CHaP
After updating these, you should also consider updating haskell.nix with nix flake lock --update-input haskell-nix (although that is not mandatory).
We can use Cabal’s source-repository-package mechanism to pull in un-released package versions.
However, we should try and avoid this.
In particular, we should not release our packages while we depend on a source-repository-package.
If we are stuck in a situation where we need a long-running fork of a package, we should release it to CHaP instead (see the CHaP README for more).
If you do add a source-repository-package, you need to update the sha256 mapping in nix/project.nix.
For the moment you have to do this by hand, using the following command to get the sha: nix-prefetch-git --quiet <repo-url> <rev> | jq .sha256, or by just getting it wrong and trying to build it, in which case Nix will give you the right value.
We pin versions of some git repositories that are used by Nix, for example nixpkgs.
For documentation see the Nix flake inputs documentation and the Nix flake lock command.
Specifically, you will probably want to say nix flake lock --update-input <input-name>.
Do not use nix flake update, as that will update all the inputs, which we typically don’t want to do.
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Warning
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Do not delete the |
Refer to the mkHaskellProject
https://github.com/input-output-hk/iogx/blob/main/doc/api.md#mkhaskellproject function in IOGX to learn how to use a different compiler.
Afterwards, you need to restart your Nix shell.
Then, you need to compile all of the Cabal projects in the repository, and fix any compilation warnings and errors.
Finally, you can submit a PR and CI system will also rebuild GHC.
Once it’s done, it will cache the compiled packages, so that they can be reused when users open a Nix shell.
TODO: Currently not available, coming soon
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Warning
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The shell with profiling dependencies is not currently cached, so this will result in you rebuilding all of our dependencies with profiling on your machine. This will take a long time. |
Once you have a shell with profiling libraries for our dependencies, add profiling: true to cabal.project.local, which will tell cabal that you want profiling (in particular, that will cause it to build our libraries with profiling).
Alternatively, you can pass the --enable-profiling option to cabal on an ad-hoc basis, but adding the option to cabal.project.local will make it apply to everything, which is probably what you want when you’re doing profiling work.
At this point you need to configure which cost centres you want GHC to insert.
The GHC user guide explains this very well.
A typical way of doing this is to add -fprof-auto to either the ghc-options in the .cabal file for the project, or in an OPTIONS_GHC pragma in the module you care about.
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Warning
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Do not set the |
Then you can use the RTS -p option to dump a profile e.g. cabal run plc … — +RTS -p.
There are various tools for visualizing the resulting profile, e.g. https://hackage.haskell.org/package/ghc-prof-flamegraph.
Dependencies are managed through Nix via the IOGX flake. To update
IOGX means to update hackage.nix and CHaP.
==== … from Hackage
Updating package dependencies from Hackage should work like normal in a
Haskell project. The most important thing to note is that we pin the
index-state of the Hackage package index in cabal.project. This
means that cabal will always see Hackage “as if” it was that time, ensuring
reproducibility. But it also means that if you need a package version that
was released after that time, you need to bump the index-state (and
to run cabal update locally). Please also note that index-state is a
property of your working environment (the cabal project) not of the
packages. This means that downstream consumers have no idea of your
index-state setting and you need to make sure your packages work
correctly also without index-state.
Because of how we use Nix to manage our Haskell build, whenever you do this
you will also need to pull in the Nix equivalent of the newer
index-state. Refer to `this link
https://github.com/input-output-hk/iogx#312-inputs`in IOGX’s documentation
to learn how to update hackage.nix.
You can do this by running nix flake lock --update-input hackage.
Many Cardano packages are not on Hackage and are instead in the Cardano
package repository
https://github.com/input-output-hk/cardano-haskell-packages, see the
README for (lots) more information.
Getting new packages from there works much like getting them from Hackage.
The differences are that it can have an independent index-state.
Refer to `this link
https://github.com/input-output-hk/iogx#312-inputs`in IOGX’s documentation
to learn how to update CHaP.
You can do this by running nix flake lock --update-input CHaP.
Sometimes we need to use an unreleased version of one of our dependencies, either to fix an issue in a package that is not under our control, or to experiment with a pre-release version of one of our own packages.
You can use a source-repository-package stanza to pull in the
unreleased version.
Please note that consumers of our packages will not pull these unreleased
versions when compiling our packages so consider using source-repository-package
only as a temporary solution.
For packages that we do not control, we can end up in a situation where we have a fork that looks like it will be long-lived or permanent (e.g. the maintainer is unresponsive, or the change has been merged but not released).
In that case, release a patched version to the Cardano package repository
https://github.com/input-output-hk/cardano-haskell-packages, which
allows us to remove the source-repository-package stanza.
Sometimes you may want to make a change that spans both cardano-node-emulator and some of its dependencies.
The obvious workflow is to make changes in the dependency’s repository, update the pin in cardano-node-emulator to point to the new commit, test, repeat.
But this is very tedious and it’s much nicer to work with a local checkout where cabal can incrementally rebuild the whole thing.
Plutus-apps repository has a nice example of such workflow.
We are a relatively large team working on sometimes quite abstruse problems. As such, it’s important that future people who work on the project know how things work, and just as importantly, why. These future people may even be yourself - we forget things very quickly!
When writing, try to put yourself in the position of someone coming to this code for the first time. What do they need to do to understand it and do their job? Write it down!
Code review is a good lens for this: if you have to explain something to a reviewer, then it is probably not clear in the code and should have a note.
This applies both to the code itself (structure, naming, etc.) and also to comments. How to write useful comments is a large topic which we don’t attempt to cover here, but Antirez is good. If in doubt: write more!
One special kind of comment is worth drawing attention to. We adopt a convention (stolen from GHC) of writing fairly substantial notes in our code with a particular structure. These correspond to what Antirez calls "design comments", with some conventions about cross-referencing them.
The structure is:
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The Note should be in a multiline comment (i.e.
{- -}) -
The first line of the Note should be
Note [Name of note] -
Refer to a Note from where it is relevant with a comment saying
See Note [Name of note]
For example:
{- Note [How to write a note]
A note should look a bit like this.
Go wild, write lots of stuff!
Here's a small diagram:
A ----> B >> C
And of course, you should see Note [Another note].
-}
Notes are a great place to put substantial discussion that you need to refer to from multiple places. For example, if you used an encoding trick to fit more data into an output format, you could write a Note describing the trick (and justifying its usage!), and then refer to it from the encoder and the decoder.
If a new feature or code refactor requires you to make an "architecturally significant" decision, then you should probably write an ADR.
See the readthedocs page for more details.
We use fourmolu for Haskell code formatting, and cabal-fmt for cabal files.
They are run automatically as pre-commit hooks, but CI will run them again and expect that to be a no-op, so if you somehow don’t apply them your PR will not go green.
To run fourmolu or cabal-fmt manually over your tree, type pre-commit run fourmolu or pre-commit run cabal-fmt respectively.
They are provided by the nix develop environment.
=== Compiler warnings
The CI builds Haskell code with -Werror, so will fail if there are any compiler warnings.
So fix your own warnings!
If the warnings are stupid, we can turn them off, e.g. sometimes it makes sense to add -Wno-orphans to a file where we know it’s safe.
Please make informative commit messages! It makes it much easier to work out why things are the way they are when you’re debugging things later.
A commit message is communication, so as usual, put yourself in the position of the reader: what does a reviewer, or someone reading the commit message later need to do their job? Write it down! It is even better to include this information in the code itself, but sometimes it doesn’t belong there (e.g. ticket info).
Also, include any relevant meta-information, such as ticket numbers. If a commit completely addresses a ticket, you can put that in the headline if you want, but it’s fine to just put it in the body.
There is plenty to say on this topic, but broadly the guidelines in this post are good.
A pull request is a change to the codebase, but it is also an artifact which goes through a change acceptance process. There are a bunch of things which we can do to make this process smooth which may have nothing to do with the code itself.
The key bottleneck in getting a PR merged is code review. Code review is great (see below), but it can slow you down if you don’t take the time to make it easy.
The amount of time it’s worth spending doing this is probably much more than you think.
There are two protected branches, main and next-node.
PRs should generally target the main branch, unless the change is only applicable to next-node.
When developing a non-trivial new feature, usually the best way to get the code reviewed is to break the implementation down to a chain of small diffs, each representing a meaningful, logical and reviewable step. Unfortunately GitHub doesn’t have good support for this. You basically have three options:
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Open the first PR against
main, the second PR against the first PR’s branch, and so on. Merging a stack of PRs created this way intomaincan be non-trivial. -
Wait until one PR is merged before opening the next PR.
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Use a single PR for the whole feature that contains multiple small commits. The problem is that Github doesn’t support approving, rejecting or merging individual commits in a PR. You can look at each individual commit, but it’s not necessarily useful or even appropriate - many PRs have quite messy commits, and commits are sometimes overwritten via force push.
The first two options are often referred to as ["trunk-based development"](https://trunkbaseddevelopment.com/), while the third "long-lived feature branches". There is no single best option for all cases, although in general we encourage adopting trunk-based development styles. Long-lived feature branches with too many commits are harmful because
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they are difficult to review - the PR can be quite large, and it is hard to review it incrementally;
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it can be difficult to resolve merge conflicts;
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they make it more likely that other people need to depend on your unmerged changes.
It is fine to have partially implemented features or not well-tested features in main.
You can simply not turn them on until they are ready, or guard them with conditional flags.
But this is not a hard rule and should be determined on a case-by-case basis. Sometimes for a small or medium-sized piece of work, you may not want to break it into multiple PRs, and wait till each PR is merged before creating the next one. You’d rather put all your code out quickly in a single PR for review. And that’s fine. Or maybe it’s a piece of performance improvement work, and you don’t know whether or not it actually improves the performance, until you finish implementing and testing the whole thing.
Whichever option you choose, please keep each of your PR to a single topic. Do not mix business logic with such things as reformatting and refactoring in a single PR.
A pull request is communication, so as usual, put yourself in the position of the reader: what does your audience (the reviewer) need to know to do their job? This information is easy for you to access, but hard for them to figure out, so write it down!
However, better to put information in the code, commit messages or changelog if possible: these persist but PR descriptions do not. It’s okay to repeat information from such places, or simply to point to it. For one-commit PRs, Github will automatically populate the PR description with the commit message, so if you’ve written a good commit message you’re done! Sometimes there is "change-related" information that doesn’t belong in a commit message but is useful ("Kris I think this will fix the issue you had yesterday").
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Review the diff of your own PR at the last minute before hitting "create". It’s amazing how many obvious things you spot here, and it stops the reviewer having to point them all out.
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It’s fine to make WIP PRs if you just want to show your code to someone else or have the CI check it. Use the Github "draft" feature for this.
Force pushing to main or next-node is never allowed.
There is no exception to this rule.
Rebasing and force pushing to other branches you own is fine, even when you have an open PR on the branch. Indeed, if you need to update your branch with changes from main, rebasing is typically better than merging.
Some projects do not allow force pushing to any remote branch. This is not a popular policy and we do not adopt it, because
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This means you must only ever use the "merge commit" merge method (or occasionally, fast forward merge, which GitHub doesn’t support).
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This means you aren’t even allowed to clean up commits in your own PR, and must eventually merge everything into
main. It discourages people from pushing commits frequently when developing. We should instead encourage cleaning up commits in PRs, at least before merging. -
The argument that this will cause massive pain for those who merge other people’s PR branch into their branch is questionable. This should be rare to begin with, if we adopt trunk-based development in general, instead of long-lived feature branches. And even if you do need to depend on other people’s unmerged work, you can instead rebase your branch on theirs, and if their branch changes, just rebase again.
Rebasing and force pushing can be used to your advantage, for example:
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Add low-effort or WIP commits to fix review comments, and then squash them away before merging the PR.
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If you have already had a PR review, don’t rebase away the old commits until the PR is ready to merge, so that the reviewer only has to look at the "new" commits.
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Rewrite the commits to make the story clearer where possible.
It is advisable to always prefer git push --force-with-lease instead of git push --force to ensure that no work gets accidentally deleted.
All pull-requests should be approved by at least one other person. We don’t enforce this, though: a PR fixing a typo is fine to self-merge, beyond that use your judgement.
As an author, code review is an opportunity for you to get feedback from clear eyes. As a reviewer, code review is an opportunity for you to help your colleagues and learn about what they are doing. Make the best use of it you can!
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Pick the right reviewer(s). If you don’t know who to pick, ask!
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Respect your reviewers' time. Their time is as valuable as yours, and it’s typically more efficient for you to spend time explaining or clarifying something in advance than for them to puzzle it out or pose a question.
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If someone had to ask about your code, it wasn’t clear enough so change it or add a comment.
Read this blog post for more good tips: https://mtlynch.io/code-review-love/
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Respond to review requests as quickly as you can. If you can’t review it all, say what you can and come back to it. Waiting for review is often a blocker for other people, so prioritize it.
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If you don’t understand something, ask. You are as clever as any person who will read this in the future, if it confuses you it’s confusing.
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Do spend the time to understand the code. This will help you make more useful comments, help you review future changes more easily, and help you if you ever need to work on it yourself.
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More reviewing is usually helpful. If you think a PR is interesting, you can review it even if nobody asked you to, you will probably have things to contribute and you’ll learn something.
Read these blog posts for more good tips: - https://mtlynch.io/human-code-reviews-1/ - https://mtlynch.io/human-code-reviews-2/
This repo allows two merge methods: squash and merge, and rebase and merge. Use the one you deem appropriate. As said before, sometimes people use a single PR with multiple commits for their work; other times they create multiple small PRs. The best merge method is different for different cases.
The "rebase and merge" method should be used with caution.
If you use this method, your PR must have a clean commit history: every commit should have a meaningful message, and should be buildable.
You don’t want to have commits like "fix a typo", "this may work" or "wip, done for the day" in main with a linear history.
And if some of these commits are non-buildable, it can create problems for "git bisect".
Merging a PR can break main, if the PR branch has diverged from main, even if CI on the PR is green.
This happens because the PRs conflict in a way that isn’t obvious to git, e.g. one adds a usage of a function and the other removes that function.
The problems with a broken main include inconveniencing other developers, and causing problems for "git bisect".
There are ways to guarantee main never breaks, such as GitHub’s [merge queue](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue).
We don’t use the merge queue because
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A broken
mainhas historically been quite infrequent. -
The merge queue increases the time it takes to merge a PR, which causes productivity loss if you are waiting to create the next PR after merging the current one (which happens often).
However, if your PR branch has diverged too much from main, it is recommended that you rebase or merge main into the PR branch before merging.
And whenever you notice a broken main, please fix it ASAP.
The same holds for next-node if your PR targets next-node.
Each cabal package has its own changelog. We use scriv — a changelog management tool — to avoid conflicts.
The broad heuristic is to put yourself in the position of the consumer of the piece of software in question and ask if you would want to know about this change. If the answer is yes, then write a quick changelog entry.
It’s important to reflect the changes that affect the API and break backwards compatibility.
The basic idea is that you write a changelog "fragment" in the changelog.d directory.
When we do a release, these will be collected into the main CHANGELOG.md.
Usually we don’t edit CHANGELOG.md directly.
You can make a changelog fragment using scriv create in the package directory, but you can also just create the fragment directly with an editor.
A fragment is a markdown file beginning with a header giving the category of change.
We have a few sources of CI checks at the moment:
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Hydra
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ReadTheDocs
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Github Actions
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Buildkite
The CI will report statuses on your PRs with links to the logs in case of failure. Pull requests cannot be merged without at least the Hydra CI check being green.
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Note
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This isn’t strictly true: repository admins (notably @koslambrou) can force-merge PRs without the checks being green If you really need this, ask. |
Hydra is the "standard" CI builder for Nix-based projects. It builds everything in the project, including all the tests, documentation, etc.
Hydra builds jobs based on the hydraJobs flake output.
Hydra can be a bit flaky, unfortunately: - If evaluation fails saying "out of memory" or "unexpected EOF reading line", then this is likely a transient failure. These will be automatically retried, but if you’re in a hurry @koslambrou has permissions to force a new evaluation. - If a build fails spuriously, this is a problem: please report it to whoever is responsible for that build and we should try and iron it out. Nondeterministic failures are very annoying. @koslambrou also has permissions to restart failed builds.
The documentation site is built on ReadTheDocs. It will build a preview for each PR which is linked from the PR status. It’s useful to take a look if you’re changing any of the documentation.
Enter the development shell using nix develop.
If you get a segfault, run GC_DONT_GC=1 nix develop instead.
Then you can run serve-docs to host a local instance at http://0.0.0.0:8002 (Haddock is at http://0.0.0.0:8002/haddock).
These perform some of the same checks as Hydra, but Github Actions is often more available, so they return faster and act as a "smoke check".
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The regular contributors to the Haskell code, all of whom can review and merge PRs are:
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@koslambrou
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@berewt
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@eyeinsky
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@kayvank
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@ak3n
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The maintainer of the documentation is @berewt or @koslambrou.
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If you have a technical dispute that you need help resolving, you can ask @koslambrou.
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For problems with the developer environment setup, builds, or CI, you can ask @kayvank or @koslambrou.
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For any QA related issues, you can mainly query @james-iohk, or any of the other regular contributors.
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The releasing of the software is handled by @ak3n, but if you have a specific problem you may also ask @koslambrou.