So that this doesn't become repetitive: for the reasons behind the answers below, see the Architecture page. The goal of the answers here is to be as helpful and concise as possible.

What version of GHC is used when I run something like stack ghci?

The version of GHC, as well as which packages can be installed, are specified by the resolver. This may be something like lts-8.12, which is from the Long Term Support (LTS) Haskell project. The user guide discusses resolvers in more detail.

Which resolver is used is determined by finding the relevant stack.yaml configuration file for the directory you're running the command from. This essentially works by:

  1. Check for a STACK_YAML environment variable or the --stack-yaml command line argument
  2. If none present, check for a stack.yaml file in the current directory or any parents
  3. If no stack.yaml was found, use the implicit global

The implicit global is a shared project used whenever you're outside of another project. It's a sort of "mutable shared state" that you should be aware of when working with Stack. The most recent request when working with the implicit global is how to move to a more recent LTS snapshot. You can do this by running the following from outside of a project:

stack config set resolver lts

Where is stack installed and will it interfere with ghc (etc) I already have installed?

Stack itself is installed in normal system locations based on the mechanism you used (see the Install and upgrade page). Stack installs the Stackage libraries in ~/.stack and any project libraries or extra dependencies in a .stack-work directory within each project's directory. None of this should affect any existing Haskell tools at all.

What is the relationship between stack and cabal?

  • Cabal-the-library is used by stack to build your Haskell code.
  • cabal-install (the executable) is used by stack for its dependency solver functionality.
  • A .cabal file is provided for each package, and defines all package-level metadata just like it does in the cabal-install world: modules, executables, test suites, etc. No change at all on this front.
  • A stack.yaml file references 1 or more packages, and provides information on where dependencies come from.
  • stack build currently initializes a stack.yaml from the existing .cabal file. Project initialization is something that is still being discussed and there may be more options here for new projects in the future (see issue 253)

I need to use a different version of a package than what is provided by the LTS Haskell snapshot I'm using, what should I do?

You can make tweaks to a snapshot by modifying the extra-deps configuration value in your stack.yaml file, e.g.:

resolver: lts-2.9
- '.'
- text-

I need to use a package (or version of a package) that is not available on hackage, what should I do?

Add it to the packages list in your project's stack.yaml, specifying the package's source code location relative to the directory where your stack.yaml file lives, e.g.

resolver: lts-2.10
- '.'
- third-party/proprietary-dep
- github-version-of/conduit
- patched/diagrams
extra-deps: []

The above example specifies that it should include the package at the root directory ('.'), that the proprietary-dep package is found in the project's third-party folder, that the conduit package is found in the project's github-version-of folder, and that the diagrams package is found in the project's patched folder. This autodetects changes and reinstalls the package.

To install packages directly from a Git repository, use e.g.:

resolver: lts-2.10
- location:
    git: https://github.com/githubuser/reponame.git
    commit: somecommitID

Note that the - '.' line has been omitted, so the package in the root directory will not be used.

What is the meaning of the arguments given to stack build, test, etc?

Those are the targets of the build, and can have one of three formats:

  • A package name (e.g., my-package) will mean that the my-package package must be built
  • A package identifier (e.g., my-package-1.2.3), which includes a specific version. This is useful for passing to stack install for getting a specific version from upstream
  • A directory (e.g., ./my-package) for including a local directory's package, including any packages in subdirectories

I need to modify an upstream package, how should I do it?

Typically, you will want to get the source for the package and then add it to your packages list in stack.yaml. (See the previous question.) stack unpack is one approach for getting the source. Another would be to add the upstream package as a submodule to your project.

Am I required to use a Stackage snapshot to use stack?

No, not at all. If you prefer dependency solving to curation, you can continue with that workflow. Instead of describing the details of how that works here, it's probably easiest to just say: run stack init --solver and look at the generated stack.yaml.

How do I use this with sandboxes?

Explicit sandboxing on the part of the user is not required by stack. All builds are automatically isolated into separate package databases without any user interaction. This ensures that you won't accidentally corrupt your installed packages with actions taken in other projects.

Can I run cabal commands inside stack exec?

With a recent enough version of cabal-install (>= 1.22), you can. For older versions, due to haskell/cabal#1800, this does not work. Note that even with recent versions, for some commands you may need this extra level of indirection:

$ stack exec -- cabal exec -- cabal <command>

However, virtually all cabal commands have an equivalent in stack, so this should not be necessary. In particular, cabal users may be accustomed to the cabal run command. In stack:

$ stack build && stack exec <program-name>

Or, if you want to install the binaries in a shared location:

$ stack install
$ <program-name>

assuming your $PATH has been set appropriately.

Using custom preprocessors

If you have a custom preprocessor, for example, Ruby, you may have a file like:


module B where

<% (1..5).each do |i| %>
test<%= i %> :: Int
test<%= i %> = <%= i %>
<% end %>

To ensure that Stack picks up changes to this file for rebuilds, add the following line to your .cabal file:

extra-source-files:   B.erb

I already have GHC installed, can I still use stack?

Yes. In its default configuration, stack will simply ignore any system GHC installation and use a sandboxed GHC that it has installed itself (typically via the stack setup command). You can find these sandboxed GHC installations in ~/.stack/programs/$platform/ghc-$version/.

If you would like stack to use your system GHC installation, use the --system-ghc flag or run stack config set system-ghc --global true to make stack check your PATH for a suitable GHC by default.

Note that stack can only use a system GHC installation if its version is compatible with the configuration of the current project, particularly the resolver setting.

Note that GHC installation doesn't work for all OSes, so in some cases you will need to use system-ghc and install GHC yourself.

How does stack determine what GHC to use?

In its default configuration, stack determines from the current project which GHC version, architecture etc. it needs. It then looks in ~/.stack/programs/$platform/ghc-$version/ for a compatible GHC, requesting to install one via stack setup if none is found.

If you are using the --system-ghc flag or have configured system-ghc: true either in the project stack.yaml or the global ~/.stack/config.yaml, stack will use the first GHC that it finds on your PATH, falling back on its sandboxed installations only if the found GHC doesn't comply with the various requirements (version, architecture) that your project needs.

See this issue for a detailed discussion of stack's behavior when system-ghc is enabled.

How do I upgrade to GHC 7.10.2 with stack?

If you already have a prior version of GHC use stack --resolver ghc-7.10 setup --reinstall. If you don't have any GHC installed, you can skip the --reinstall.

How do I get extra build tools?

stack will automatically install build tools required by your packages or their dependencies, in particular alex and happy.

NOTE: This works when using lts or nightly resolvers, not with ghc or custom resolvers. You can manually install build tools by running, e.g., stack build alex happy.

How does stack choose which snapshot to use when creating a new config file?

It checks the two most recent LTS Haskell major versions and the most recent Stackage Nightly for a snapshot that is compatible with all of the version bounds in your .cabal file, favoring the most recent LTS. For more information, see the snapshot auto-detection section in the architecture document.

I'd like to use my installed packages in a different directory. How do I tell stack where to find my packages?

Set the STACK_YAML environment variable to point to the stack.yaml config file for your project. Then you can run stack exec, stack ghc, etc., from any directory and still use your packages.

My tests are failing. What should I do?

Like all other targets, stack test runs test suites in parallel by default. This can cause problems with test suites that depend on global resources such as a database or binding to a fixed port number. A quick hack is to force stack to run all test suites in sequence, using stack test --jobs=1. For test suites to run in parallel developers should ensure that their test suites do not depend on global resources (e.g. by asking the OS for a random port to bind to) and where unavoidable, add a lock in order to serialize access to shared resources.

Can I get bash autocompletion?

Yes, see the shell-autocompletion documentation

How do I update my package index?

Users of cabal are used to running cabal update regularly. You can do the same with stack by running stack update. But generally, it's not necessary: if the package index is missing, or if a snapshot refers to package/version that isn't available, stack will automatically update and then try again. If you run into a situation where stack doesn't automatically do the update for you, please report it as a bug.

Isn't it dangerous to automatically update the index? Can't that corrupt build plans?

No, stack is very explicit about which packages it's going to build for you. There are three sources of information to tell it which packages to install: the selected snapshot, the extra-deps configuration value, and your local packages. The only way to get stack to change its build plan is to modify one of those three. Updating the index will have no impact on stack's behavior.

I have a custom package index I'd like to use, how do I do so?

You can configure this in your stack.yaml. See YAML configuration.

How can I make sure my project builds against multiple ghc versions?

You can create multiple yaml files for your project, one for each build plan. For example, you might set up your project directory like so:

  stack.yaml --> symlink to stack-7.8.yaml

When you run stack build, you can set the STACK_YAML environment variable to indicate which build plan to use.

$ stack build                             # builds using the default stack.yaml
$ STACK_YAML=stack-7.10.yaml stack build  # builds using the given yaml file

I heard you can use this with Docker?

Yes, stack supports using Docker with images that contain preinstalled Stackage packages and the tools. See Docker integration for details.

How do I use this with Travis CI?

See the Travis CI instructions

What is licensing restrictions on Windows?

Currently on Windows GHC produces binaries linked statically with GNU Multiple Precision Arithmetic Library (GMP), which is used by integer-gmp library to provide big integer implementation for Haskell. Contrary to the majority of Haskell code licensed under permissive BSD3 license, GMP library is licensed under LGPL, which means resulting binaries have to be provided with source code or object files. That may or may not be acceptable for your situation. Current workaround is to use GHC built with alternative big integer implementation called integer-simple, which is free from LGPL limitations as it's pure Haskell and does not use GMP. Unfortunately it has yet to be available out of the box with stack. See issue #399 for the ongoing effort and information on workarounds.

How to get a working executable on Windows?

When executing a binary after building with stack build (e.g. for target "foo"), the command foo.exe might complain about missing runtime libraries (whereas stack exec foo works).

Windows is not able to find the necessary C++ libraries from the standard prompt because they're not in the PATH environment variable. stack exec works because it's modifying PATH to include extra things.

Those libraries are shipped with GHC (and, theoretically in some cases, MSYS). The easiest way to find them is stack exec which. E.g.

>stack exec which libstdc++-6.dll

A quick workaround is adding this path to the PATH environment variable or copying the files somewhere Windows finds them (cf. https://msdn.microsoft.com/de-de/library/7d83bc18.aspx).

Cf. issue #425.

Can I change stack's default temporary directory?

Stack downloads and extracts files to $STACK_ROOT/programs on most platforms, which defaults to ~/.stack/programs. On Windows $LOCALAPPDATA\Programs\stack is used. If there is not enough free space in this directory, Stack may fail. For instance, stack setup with a GHC installation requires roughly 1GB free. If this is an issue, you can set local-programs-path in your ~/.stack/config.yaml to a directory on a file system with more free space.

If you use Stack with Nix integration, be aware that Nix uses a TMPDIR variable, and if it is not set Nix sets it to some subdirectory of /run, which on most Linuxes is a Ramdir. Nix will run the builds in TMPDIR, therefore if you don't have enough RAM you will get errors about disk space. If this happens to you, please manually set TMPDIR before launching Stack to some directory on the disk.

Why doesn't stack rebuild my project when I specify --ghc-options on the command line?

Because GHC options often only affect optimization levels and warning behavior, stack doesn't recompile when it detects an option change by default. This behavior can be changed though by setting the rebuild-ghc-options option to true.

To force recompilation manually, use the --force-dirty flag. If this still doesn't lead to a rebuild, add the -fforce-recomp flag to your --ghc-options.

Why doesn't stack apply my --ghc-options to my dependencies?

By default, stack applies command line GHC options only to local packages (these are all the packages that are specified in the packages section of your stack.yaml). For an explanation of this choice see this discussion on the issue tracker.

If you still want to set specific GHC options for a dependency, use the ghc-options option in your stack.yaml or global ~/.stack/config.yaml.

To change the set of packages that command line GHC options apply to, use the apply-ghc-options option.

stack setup on a windows system only tells me to add certain paths to the PATH variable instead of doing it

If you are using a powershell session, it is easy to automate even that step:

$env:Path = ( stack setup | %{ $_ -replace '[^ ]+ ', ''} ), $env:Path -join ";"

How do I reset / remove Stack (such as to to do a completely fresh build)?

The first thing to remove is project-specific .stack-work directory within the project's directory. Next, remove ~/.stack directory overall. You may have errors if you remove the latter but leave the former. Removing Stack itself will relate to how it was installed, and if you used GHC installed outside of Stack, that would need to be removed separately.

How does stack handle parallel builds? What exactly does it run in parallel?

See issue #644 for more details.

I get strange ld errors about recompiling with "-fPIC"

Some users (myself included!) have come across a linker errors (example below) that seem to be dependent on the local environment, i.e. the package may compile on a different machine. The issue has been reported to be non-deterministic in some cases. I've had success using the docker functionality to build the project on a machine that would not compile it otherwise.

tmp- build
Building tmp-
Preprocessing executable 'tmp' for tmp-
Linking dist-stack/x86_64-linux/Cabal- ...
/usr/bin/ld: dist-stack/x86_64-linux/Cabal- relocation R_X86_64_32S against `stg_bh_upd_frame_info' can not be used when making a shared object; recompile with -fPIC
dist-stack/x86_64-linux/Cabal- error adding symbols: Bad value
collect2: error: ld returned 1 exit status

--  While building package tmp- using:
      /home/philip/.stack/programs/x86_64-linux/ghc-7.10.1/bin/runghc-7.10.1 -package=Cabal- -clear-package-db -global-package-db /home/philip/tmp/Setup.hs --builddir=dist-stack/x86_64-linux/Cabal- build
    Process exited with code: ExitFailure 1

The issue may be related to the use of hardening flags in some cases, specifically those related to producing position independent executables (PIE). This is tracked upstream in the following ticket. Some distributions add such hardening flags by default which may be the cause of some instances of the problem. Therefore, a possible workaround might be to turn off PIE related flags.

In Arch Linux, the support for this is provided by the hardening-wrapper package. Some possible workarounds:

  • Selectively disabling its PIE forcing by setting HARDENING_PIE=0 in /etc/hardening-wrapper.conf.
  • Uninstalling the hardening-wrapper package and logging out then into your account again.

If you manage to work around this in other distributions, please include instructions here.

Where does the output from --ghc-options=-ddump-splices (and other -ddump* options) go?

These are written to *.dump-* files inside the package's .stack-work directory. Specifically, they will be available at PKG-DIR/$(stack path --dist-dir)/build/SOURCE-PATH, where SOURCE-PATH is the path to the source file, relative to the location of the *.cabal file. When building named components such as test-suites, SOURCE-PATH will also include COMPONENT/COMPONENT-tmp, where COMPONENT is the name of the component.

Why is DYLD_LIBRARY_PATH ignored?

If you are on Mac OS X 10.11 ("El Capitan") or later, there is an upstream GHC issue which prevents the DYLD_LIBRARY_PATH environment variable from being passed to GHC when System Integrity Protection (a.k.a. "rootless") is enabled. There are two known workarounds:

  1. Known to work in all cases: disable System Integrity Protection. WARNING: Disabling SIP will severely reduce the security of your system, so only do this if absolutely necessary!
  2. Experimental: modify GHC's shell script wrappers to use a shell outside the protected directories.

Why do I get a /usr/bin/ar: permission denied error?

If you are on OS X 10.11 ("El Capitan") or later, GHC 7.8.4 is incompatible with System Integrity Protection (a.k.a. "rootless"). GHC 7.10.2 includes a fix, so this only affects users of GHC 7.8.4. If you cannot upgrade to GHC 7.10.2, you can work around it by disabling System Integrity Protection. WARNING: Disabling SIP will severely reduce the security of your system, so only do this if absolutely necessary!

Why is the -- argument separator ignored in Windows PowerShell

Some versions of Windows PowerShell don't pass the -- to programs. The workaround is to quote the "--", e.g.:

stack exec "--" cabal --version

This is known to be a problem on Windows 7, but seems to be fixed on Windows 10.

Does stack also install the system/C libraries that some Cabal packages depend on?

No, this is currently out of the scope of stack's target set of features. Instead of attempting to automate the installation of 3rd party dependencies, we have the following approaches for handling system dependencies:

  • Nix and docker help make your build and execution environment deterministic and predictable. This way, you can install system dependencies into a container, and share this container with all developers.

  • If you have installed some libraries into a non-standard location, extra-lib-dirs / extra-include-dirs to specify it.

In the future, stack might give OS specific suggestions for how to install system libraries.

How can I make stack aware of my custom SSL certificates?


In principle, you can use the following command to add a certificate to your system certificate keychain:

sudo security add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain <certificate>

Some users have reported issues with this approach, see #907 for more information.

Other *NIX OSs

Use the SYSTEM_CERTIFICATE_PATH environment variable to point at the directory where you keep your SSL certificates.

How do I get verbose output from GHC when I build with cabal?

Unfortunately stack build does not have an obvious equivalent to cabal build -vN which shows verbose output from GHC when building. The easiest workaround is to add ghc-options: -vN to the .cabal file or pass it via stack build --ghc-options="-v".

Does Stack support the Hpack specification?

Yes. You can run stack init as usual and Stack will create a matching stack.yaml.