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Contributors Guide

Thank you for considering contributing to the maintenance or development of Stack, or otherwise supporting users of Stack! We hope that the following information will encourage and assist you. We start with some advice about Stack's goals and governance, and approach to supporting users.

Stack's goals

Stack's current goals are:

  • To provide easy to use tooling for Haskell development
  • To provide complete support for at least the following three development environments: Linux, macOS, and Windows
  • To address the needs of industrial users, open source maintainers, and other people
  • To focus on the 'curated package set' use case
  • To prioritize reproducible build plans

The goals above are not set in stone. However, any major changes to them should involve significant public discussion and a public vote by the Stack maintainer team.

Stack's governance

People involved in maintaining or developing Stack with rights to make commits to the repository can be classified into two groups: 'committers' and 'maintainers'.

Stack's committers

We encourages a wide range of people to be granted rights to make commits to the repository.

People are encouraged to take initiative to make non-controversial changes, such as documentation improvements, bug fixes, performance improvements, and feature enhancements.

Maintainers should be included in discussions of controversial changes and tricky code changes.

Our general approach is "it's easier to ask forgiveness than permission". If there is ever a bad change, it can always be rolled back.

Stack's maintainers

Stack's maintainers are long-term contributors to the project. Michael Snoyman (@snoyberg) was the founder of Stack, and its initial maintainer - and he has added others. Michael's current interests and priorities mean that he is no longer actively involved in adding new features to Stack.

Maintainers are recognized for their contributions including:

  • Direct code contribution
  • Review of pull requests
  • Interactions on the GitHub issue tracker
  • Documentation management
  • External support - for example, hosting or training

The maintainer team make certain decisions when that is necessary, specifically:

  • How to proceed, if there is disagreement on how to do so on a specific topic
  • Whether to add or remove (see further below) a maintainer

Generally, maintainers are only removed due to non-participation or actions unhealthy to the project. Removal due to non-participation is not a punishment, simply a recognition that maintainership is for active participants only.

We hope that removal due to unhealthy actions will never be necessary, but would include protection for cases of:

  • Disruptive behavior in public channels related to Stack
  • Impairing the codebase through bad commits/merges

Like committers, maintainers are broadly encouraged to make autonomous decisions. Each maintainer is empowered to make a unilateral decision. However, maintainers should favor getting consensus first if:

  • They are uncertain what is the best course of action
  • They anticipate that other maintainers or users of Stack will disagree on the decision

Stack's support

A large part of the general discussion around Stack is on support-related topics, and that is reflected in the current issue tracker content. Assistance in responding to such matters is greatly appreciated.

While support-related matters can be posted here as an 'issue', we encourage the use of other forums, in particular Haskell's Discourse. We also recommend Haskell's Discourse for general discussions about Stack's current or desired features. Stack is also discussed on Reddit's Haskell community.

We encourage use of those other forums because support-related discussions can clog up the issue tracker and make it more difficult to maintain the project. People needing support may also get a faster and fuller response on other forums.

Additions to the issue tracker are better suited to concrete feature proposals, bug reports, and other code base discussions (for example, refactorings).

Bug Reports

Please open an issue and use the provided template to include all necessary details.

The more detailed your report, the faster it can be resolved and will ensure it is resolved in the right way. Once your bug has been resolved, the responsible person will tag the issue as Needs confirmation and assign the issue back to you. Once you have tested and confirmed that the issue is resolved, close the issue. If you are not a member of the project, you will be asked for confirmation and we will close it.


Consistent with its goal of being easy to use, Stack aims to maintain a high quality of in-tool and online documentation.

The in-tool documentation includes the output when the --help flag is specified and the content of Stack's warning and error messages.

When drafting documentation it is helpful to have in mind the intended reader and what they are assumed to know, and not know, already. In that regard, documentation should aim to meet, at least, the needs of a person who is about to begin to study computing as an undergraduate but who has not previously coded using Haskell. That person may be familiar with one popular operating system but may not be familiar with others.

The files which make up Stack's online documentation are located in the doc directory of the repository. They are formatted in the Markdown syntax, with some extensions.

Those files are rendered on by Read the Docs using MkDocs and the Material for MkDocs theme. The stable branch of the repository provides the 'stable' version of the online documentation. The master branch provides the 'latest' version of the documentation.

The 'stable' version of the online documentation is intended to be applicable to the latest released version of Stack. If you would like to help with that documentation, please submit a pull request with your changes/additions based off the stable branch.

The Markdown files are organised into the navigation menu (the table of contents) in the file mkdocs.yml, the configuration file for MkDocs. The description of a file in the menu can differ from the file's name. The navigation menu allows files to be organised in a hierarchy. Currently, up to three levels are used. The top level is:

  • Welcome!: The introduction to Stack. This page aims to be no longer than necessary but also to not assume much existing knowledge on the part of the reader. It provides a 'quick start' guide to getting and using Stack.
  • How to get & use Stack: This includes Stack's user's guide, answers to frequently asked questions, and more thorough explanations of aspects of Stack. The user's guide is divided into two parts. The first part is 'introductory', and has the style of a tutorial. The second part is 'advanced', and has more of a reference style.
  • How Stack works (advanced): Many users will not need to consult this advanced documentation.
  • Stack's code (advanced): Other information useful to people contributing to, or maintaining, Stack's code, documentation, and other files.
  • Signing key: How Stack's released executables are signed.
  • Glossary: A glossary of terms used throughout Stack's in-tool and online documentation. We aim to describe the same things in the same way in different places.
  • Version history: The log of changes to Stack between versions.

The specific versions of the online documentation (eg v: v2.9.1) are generated from the content of files at the point in the repository's history specified by the corresponding release tag. Consequently, that content is fixed once released.

If the names of Markdown files do not change between versions, then people can use the flyout on the online documentation to move between different versions of the same page. For that reason, the names of new Markdown files should be chosen with care and existing Markdown files should not be deleted or renamed without due consideration of the consequences.

The Markdown syntax supported by MkDocs and the Material for MkDocs theme can differ from the GitHub Flavored Markdown (GFM) supported for content on Please refer to the MkDocs documentation and the Material for MkDocs reference to ensure your pull request will achieve the desired rendering.

The extensions to the basic Markdown syntax used are set out in mkdocs.yml and include:

  • admonitions
  • code blocks, with syntax highlighting provided by Pygments
  • content tabs, which can be nested
  • icons and emojis

The files in the doc directory of the repository include two symbolic links (symlinks), and Users of Git on Windows should be aware of its approach to symbolic links. See the Git for Windows Wiki. If git config --show-scope --show-origin core.symlinks is false in a local repository on Windows, then the files will be checked out as small plain files that contain the link text See the Git documentation.

The online documentation can be previewed using the mkdocs tool, as described in Getting Started with MkDocs. The prerequisites are:

  • Python; and
  • the required Python packages set out in doc/requirements.txt. They can be installed using Python's package manager pip with:

    pip install --requirement doc/requirements.txt

Once the required version of mkdocs is installed, command mkdocs serve in the same directory as the mkdocs.yml file to start a web server. The command will, eventually, output the URL at which the documentation is being served.

Command mkdocs build to build the documentation.

With the correct prerequisites (see further below), users of the make tool in the Stack-supplied MSYS2 environment can automate some of these steps from Stack's project directory with:

  • preview: stack exec -- make docs-serve; and
  • build: stack exec -- make _site/index.html.

However, Windows and the Stack-supplied MSYS2 environment do not come with Python or make by default. Further, Python on Windows does not use the python3 command (used on Unix-like operating systems) to invoke Python. Further still, in the MSYS2 environment, development versions of packages libxml2 and libxslt are necessary dependencies. Consequently, the automation requires the following command to install requirements into the the MSYS2 environment:

stack exec -- pacman --sync python make libxml2-devel libxslt-devel


If the automation fails before the mkdocs tool etc is installed, the directory .python-doc-virtualenv created by the automation will need to be deleted before the automation will work again.

For most users, the automation will be less convenient than simply using the mkdocs serve command directly.

With python3 and make available on the PATH, users of the make tool can automate some of these steps from Stack's project directory with:

  • preview: make docs-serve; and
  • build: make _site/index.html.

Error messages

Stack catches exceptions thrown by its dependencies or by Stack itself in Stack.main. In addition to exceptions that halt Stack's execution, Stack logs certain other matters as 'errors'.

To support the Haskell Foundation's Haskell Error Index initiative, all Stack error messages generated by Stack itself should have a unique initial line:

Error: [S-nnnn]

where nnnn is a four-digit number in the range 1000 to 9999.

If you create a new Stack error, select a number using a random number generator (see, for example, RANDOM.ORG) and check that number is not already in use in Stack's code. If it is, pick another until the number is unique.

All exceptions generated by Stack itself are implemented using data constructors of closed sum types. Typically, there is one such type for each module that exports functions that throw exceptions. This type and the related instance definitions are usually located at the top of the relevant module.

Stack supports two types of exceptions: 'pretty' exceptions that are instances of class RIO.PrettyPrint.Pretty, which provides pretty :: e -> StyleDoc, and thrown as expressions of type RIO.PrettyPrint.PrettyException.PrettyException; and other 'plain' exceptions that are simply instances of class Control.Exception.Exception and, hence, instances of class Show. These types and classes are re-exported by Stack.Prelude.

Stack throws exceptions in parts of the code that should, in principle, be unreachable. The functions Stack.Prelude.bugReport and Stack.Prelude.bugPrettyReport are used to give the messages a consistent format. The names of the data constructors for those exceptions usually end in Bug.

In a few cases, Stack may throw an exception in 'pure' code. The function RIO.impureThrow :: Exception e => e -> a, re-exported by Stack.Prelude, is used for that purpose.


If you would like to contribute code to fix a bug, add a new feature, or otherwise improve stack, pull requests are most welcome. It's a good idea to submit an issue to discuss the change before plowing into writing code.

If you'd like to help out but aren't sure what to work on, look for issues with the awaiting pull request label. Issues that are suitable for newcomers to the codebase have the newcomer friendly label. Best to post a comment to the issue before you start work, in case anyone has already started.

Please include a ChangeLog entry and documentation updates with your pull request.

Backwards Compatability

The Stack package provides a library and an executable (stack) that depends on the library. The library is intended for use only by the executable.

Consequently, the Stack package does not need to, and does not, strive for the compatibility with a range of versions of GHC that a library package (such as pantry) would seek.

Stack aims to depend on well-known packages. The specific versions on which it depends at any time are specified by package.yaml and stack.yaml. It does not aim to be compatible with more than one version of the Cabal package at any time. At the time of writing (May 2024) the package versions are primarily ones in Stackage snapshot LTS Haskell 22.21 (for GHC 9.6.5), together with extra-deps to depend on the latest version of pantry and tar.

A Stack executable makes use of Cabal (the library) through a small 'Setup' executable that it compiles from Haskell source code. The executable compiles that code with a dependency on the version of Cabal that ships with the specified GHC compiler. Each release of Stack will normally aim to support all versions of GHC and the Cabal package in Stackage snapshots published within seven years of the release. For example, snapshot LTS Haskell 10.0, published on 19 December 2017, was the first LTS Haskell snapshot to provide GHC 8.2.2 which comes with base- and Cabal- Normally, until, at least, 19 December 2024, Stack releases would aim to support the immediate predecessor, GHC 8.0.2 and base-, Cabal- and Haddock 2.17.4. However, the next version of Stack will drop support for versions of Cabal before 2.2. Cabal- was released with GHC 8.4.1 on 8 March 2018.

When a version of the Stack executable actually ceases to support a version of GHC and Cabal, that should be recorded in Stack's ChangeLog.

Code Quality

The Stack project uses yamllint as a YAML file quality tool and HLint as a code quality tool.

Linting of YAML files

The yamllint configuration extends the tools default and is set out in .yamllint.yaml. In particular, indentation is set at 2 spaces and - in sequences is treated as part of the indentation.

Linting of Haskell source code

The HLint configurations is set out in .hlint.yaml.

Stack contributors need not follow dogmatically the suggested HLint hints but are encouraged to debate their usefulness. If you find a HLint hint is not useful and detracts from readability of code, consider marking it in the configuration file to be ignored. Please refer to the HLint manual for configuration syntax.

Quoting @mgsloan:

We are optimizing for code clarity, not code concision or what HLint thinks.

You can install HLint with Stack. You might want to install it in the global project in case you run into dependency conflicts. HLint can report hints in your favourite text editor. Refer to the HLint repository for more details.

To install, command:

stack install hlint

Once installed, you can check your changes with command:

stack exec -- sh ./etc/scripts/

Code syntax

Stack makes use of GHC's GHC2021 collection of language extensions, which is set using the language key in the package.yaml file.

Stack makes use of single-constructor types where the constructor has a large number of fields. Some of those fields have similar types, and so on. Given that, Stack makes use of OverloadedRecordDot, introduced in GHC 9.2.1. It also makes use of NoFieldSelectors, also introduced in GHC 9.2.1, and, where necessary, DuplicateRecordFields. Together, these language extensions enable the removal from the names of fields of the prefixes that were used historically to indicate the type and make field names unique. This is because the names of fields no longer need to be unique in situations where the intended field is unambiguous. This allows for a terser syntax without loss of expressiveness. For example:

let cliTargets = (boptsCLITargets . bcoBuildOptsCLI) bco

can become:

let cliTargets = bco.buildOptsCLI.targets

The intended field is unambiguous in almost all cases. In the case of a few record updates it is ambiguous. The name of the field needs to be qualified in those cases. For example:

import qualified  Stack.Types.Build as ConfigCache ( ConfigCache (..) )
let ignoreComponents :: ConfigCache -> ConfigCache
    ignoreComponents cc = cc { ConfigCache.components = Set.empty }

Code Style

A single code style is not applied consistently to Stack's code and Stack is not Procrustean about matters of style. Rules of thumb, however, are:

  • keep pull requests that simply reformat code separate from those that make other changes to code; and
  • when making changes to code other than reformatting, follow the existing style of the function(s) or module(s) in question.

That said, the following may help:

  • Stack's code generally avoids the use of C preprocessor (CPP) directives. Windows and non-Windows code is separated in separate source code directories and distinguished in Stack's Cabal file. Stack.Constants.osIsWindows :: Bool is provided. Multi-line strings are generally formatted on the assumption that GHC's CPP language pragma is not being used.
  • Language pragmas usually start with NoImplictPrelude, where applicable, and then all others are listed alphabetically. The closing #-} are aligned, for purely aesthetic reasons.
  • Stack is compiled with GHC's -Wall enabled, which includes -Wtabs (no tabs in source code). Most modules are based on two spaces (with one space for a where) for indentation but older and larger modules are still based on four spaces.
  • Stack's code and documentation tends to be based on lines of no more than 80 characters or, if longer, no longer than necessary.
  • Stack uses export lists.
  • Stack's imports are listed alphabetically, including Stack.Prelude, where applicable. The module names are left aligned, with space left for qualified where it is absent.
  • Stack's code is sufficiently stable that explict import lists can sensibly be used. The exception is the import of Stack.Prelude. Not all modules have comprehensive explicit import lists.
  • Short explicit import lists follow the module name. Longer lists start on the line below the module name. Spaces are used to separate listed items from their enclosing parentheses.
  • As noted above, the types used to implement Stack's exceptions and the related instance definitions are usually located at the top of the relevant module.
  • In function type signatures, the :: is kept on the same line as the function's name. This format is Haskell syntax highlighter-friendly.
  • If where is used, the declarations follow on a separate line.


The Stack code has both unit tests and integration tests.

Working with Unit Tests

Unit tests can be found in the tests/unit directory. Tests are written using the Hspec framework. In order to run the full test suite, you can simply command:

stack test

The --file-watch is a very useful option to get quick feedback. However, running the entire test suite after each file change will slow you down. You'll need to specify which test suite (unit test or integration) and pass arguments to specify which module you'd specifically like to run to get quick feedback. A description of this follows below.

If you would like to run the unit tests on their own, you can command:

stack test stack:stack-unit-test

Running an individual module works with a command like this:

stack test stack:stack-unit-test --ta "-m <PATTERN>"

Where <PATTERN> is the name of the module without Spec.hs.

You may also load tests into GHCi and run them with these command:

stack ghci stack:stack-unit-test --only-main
# GHCi starting up output ...
> :main -m "<PATTERN>"

Where again, <PATTERN> is the name of the module without Spec.hs.

Working with Integration Tests

Integration tests can be found in the tests/integration folder.

Running the integration tests is a little involved, you'll need to command:

stack build --flag stack:integration-tests stack --exec stack-integration-test

Running an individual module works with a command like this:

stack build --flag stack:integration-tests stack --exec "stack-integration-test -m <PATTERN>"

Where <PATTERN> is the name of the folder listed in the test/integration/tests/ directory.

You may also achieve this through GHCi with this command:

stack ghci stack:stack-integration-test
# GHCi starting up output ...
> :main -m "<PATTERN>"

Where again, <PATTERN> is the name of the folder listed in the test/integration/tests/ directory.

You can disable a few integration tests through the -n option :

stack build --flag stack:integration-tests stack --exec "stack-integration-test -n <PATTERN1> -n <PATTERN2>"

To disable folders named after <PATTERN1> and <PATTERN2> It's especially useful when some tests are taking a while to complete.

Continuous integration (CI)

We use GitHub Actions to do CI on Stack. The configuration of the workflows is in the YAML files in .github/workflows. The current active workflows are:

Linting - lint.yml

This workflow will run if:

  • there is a pull request
  • commits are pushed to these branches: master, stable and rc/**

The workflow has one job (style). It runs on ubuntu only and applies yamllint and Hlint.

Test suite - unit-tests.yml

This workflow will run if:

  • there is a pull request
  • commits are pushed to these branches: master, stable and rc/**.
  • requested

The workflow has two jobs: pedantic and unit-tests.

The pedantic job runs on ubuntu only and builds Stack with the --pedantic flag.

The unit-tests job runs on a matrix of operating systems and Stack project-level YAML configuration files (stack.yaml, by default). It builds and tests Stack with the following flags: --haddock --no-haddock-deps.

Its approach to creating a cache depends on the operating system. Its 'Cache dependencies on Unix-like OS' step caches the Stack root on Unix-like operating systems. Its 'Cache dependencies on Windows' step caches the same information on Windows, but takes into account that a relevant directory is located outside of the Stack root.

Integration-based - integration-tests.yml

This workflow will run if:

  • there is a pull request
  • commits are pushed to these branches: master, stable and rc/**
  • any tag is created
  • requested

The workflow has three jobs: integration-tests, linux-arm64 and github-release.

The integration-tests job runs on a matrix of operating systems (ubuntu, windows and macos) and makes use of the release.hs script at etc/scripts. Its approach to creating a cache is the same as for unit-tests.yml, described above.

Its 'Install deps and run checks' step uses release.hs check.

Its 'Build bindist' step uses release.hs build.

Its 'Upload bindist' step uploads artifacts using the name of the runner's operating system (Linux, Windows or macOS) as the name for the artifacts.

The linux-arm64 job runs on a self-hosted runner for Linux and ARM64. It makes use of Docker and a Docker file at etc/dockerfiles/arm64.Dockerfile.

Its 'Build bindist' step makes use of a compiled version of release.hs script at etc/scripts to command release build.

Its 'Upload bindist' step uploads artifacts using Linux-ARM64 as the name for the artifacts.

The github-release job needs integration-tests and linux-arm64. It only takes effect if the trigger for the workflow was the creation of a tag.

Its four steps Download Linux/Windows/macOS/Linux-ARM64 artifact download the named artifacts to path _release.

Its step 'Hash and sign assets' makes use of a 'secret' environment variable RELEASE_SIGNING_KEY established by the owner of the Stack repository. The variable contains the private key for the GPG key with ID 0x575159689BEFB442. That key is imported into GPG and then used by GPG to create a detached signature for each file.

Stan tool - stan.yml

Stan is a Haskell static analysis tool. As of stan-, it supports GHC >= 9.6.3 and Stack is built with GHC 9.6.5. The tool is configured by the contents of the .stan.toml file.

This workflow will run if:

  • there is a pull request
  • requested

Haskell Language Server

You may be using Visual Studio Code (VS Code) with its Haskell extension, which is powered by the Haskell Language Server (HLS).

Stack can be built with Stack (which is recommended) or with Cabal (the tool).

If you use Stack to build Stack, command stack ghci in the root directory of the Stack project should work as expected, if you have first commanded stack build once. stack build causes Cabal (the library) to create the automatically generated module Stack_build.

If ghc is not on your PATH, then Haskell Language Server may report the following error about Stack.Constants.ghcShowOptionsOutput:

• Exception when trying to run compile-time code:
    ghc: readCreateProcess: does not exist (No such file or directory)
  Code: (TH.runIO (readProcess "ghc" ["--show-options"] "")
           >>= TH.lift . lines)
• In the untyped splice:
    $(TH.runIO (readProcess "ghc" ["--show-options"] "") >>= TH.lift
        . lines)

ghc should be on the PATH if you run VS Code itself in the Stack environment:

stack exec -- code .

The following cradle (hie.yaml) should suffice to configure Haskell Language Server (HLS) explicitly for ./Setup.hs and each of the buildable components in Stack's Cabal file:

  - path: "./Setup.hs"
          arguments: []
  - path: "./"
        - path: "./src"
          component: "stack:lib"
        - path: "./app"
          component: "stack:exe:stack"
        - path: "./tests/integration"
          component: "stack:exe:stack-integration-test"
        - path: "./tests/unit"
          component: "stack:test:stack-unit-test"

If you use Cabal (the tool) to build Stack, command cabal repl in the root directory of the Stack project should work as expected, if you have GHC and (on Windows) MSYS2 on the PATH. Stack's custom ./Setup.hs causes cabal repl to cause Cabal (the library) to create the automatically generated module Stack_build.

If ghc is not on your PATH, then Haskell Language Server may report the following error about Stack.Constants.ghcShowOptionsOutput:

• Exception when trying to run compile-time code:
    ghc: readCreateProcess: does not exist (No such file or directory)
  Code: (TH.runIO (readProcess "ghc" ["--show-options"] "")
           >>= TH.lift . lines)
• In the untyped splice:
    $(TH.runIO (readProcess "ghc" ["--show-options"] "") >>= TH.lift
        . lines)

ghc and (on Windows) MSYS2 should be on the PATH if you run commands (including cabal) in the Stack environment:

stack exec --no-ghc-package-path -- cabal repl


stack exec --no-ghc-package-path -- code .

Use of GHC's environment variable GHC_PACKAGE_PATH is not compatible with Cabal (the tool). That is why the --no-ghc-package-path flag must be specified with stack exec when relying on Cabal (the tool).

The following cradle (hie.yaml) should suffice to configure Haskell Language Server (HLS) explicitly for ./Setup.hs and each of the buildable components in Stack's Cabal file:

  - path: "./Setup.hs"
          arguments: []
  - path: "./"
        - path: "./src"
          component: "lib:stack"
        - path: "./app"
          component: "exe:stack"
        - path: "./tests/integration"
          component: "exe:stack-integration-test"
        - path: "./tests/unit"
          component: "test:stack-unit-test"

A cradle is not committed to Stack's repository because it imposes a choice of tool used for building.

Dev Containers

A Development Container (or Dev Container for short) allows you to use a container as a full‑featured development environment.

You can run Dev Containers locally/remotely (with VS Code), or create a Codespace for a branch in a repository to develop online.

Stack's default Dev Container is intended for use with its default project‑level configuration (stack.yaml). But there are also Dev Containers for the experimental project‑level configurations.

For further information, see the documentation for Dev Containers.

Slack channel

If you're making deep changes and real-time communication with the Stack team would be helpful, we have a #stack-collaborators Slack channel in the Haskell Foundation workspace. To join the workspace, follow this link.

Matrix room

There is also a Haskell Stack room at address on Matrix.