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.

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 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

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. stack will default to using whatever GHC is on your PATH. If that GHC is a compatible version with the snapshot you’re using, it will simply use it. Otherwise, it will prompt you to run stack setup. Note that stack setup installs GHC into ~/.stack/programs/$platform/ghc-$version/ and not a global location.

Note that GHC installation doesn’t work for all OSes, so in some cases the first option will need to install GHC yourself.

How does stack determine what GHC to use?

It uses the first GHC that it finds on the PATH. If that GHC does not comply with the various requirements (version, architecture) that your project needs, it will prompt you to run stack setup to get it. stack is fully aware of all GHCs that it has installed itself.

See this issue for a detailed discussion.

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 section in the GUIDE

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 makes use of a temporary directory for some commands (/tmp by default on linux). If there is not enough free space in this directory, stack may fail (see issue #429 ). For instance stack setup with a GHC installation requires roughly 1GB free.

A custom temporary directory can be forced:

  • on Linux by setting the environment variable TMPDIR (eg $ TMPDIR=path-to-tmp stack setup)
  • on Windows by setting one of the environment variable (given in priority order), TMP, TEMP, USERPROFILE

stack sometimes rebuilds based on flag changes when I wouldn’t expect it to. How come?

stack tries to give you reproducibility whenever possible. In some cases, this means that you get a recompile when one may not seem necessary. The most common example is running something like this in a multi-package project:

stack build --ghc-options -O0 && stack build --ghc-options -O0 one-of-the-packages

This may end up recompiling local dependencies of one-of-the-packages without optimizations on. Whether stack should or shouldn’t do this depends on the needs of the user at the time, and unfortunately we can’t make a solution that will make everyone happy in all cases. If you’re curious for details, there’s a long discussion about it on the issue tracker.

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. There is no known workaround (if you come across one please include details), however 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

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.

Why is DYLD_LIBRARY_PATH ignored?

If you are on Mac OS X 10.11 (“El Capitan”) or later, System Integrity Protection (a.k.a. “rootless”) prevents the DYLD_LIBRARY_PATH environment variable from being passed to sub-processes. The only workaround we are aware of is disabling System Integrity Protection:

  1. Reboot into recovery mode (hold down Cmd-R at boot)
  2. Open a terminal (select Terminal from the Utilities menu)
  3. Run csrutil disable; reboot

Note that this reduces the security of your system.

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

On OS X 10.11 (“El Capitan”) and later, this is caused by System Integrity Protection (a.k.a. “rootless”). GHC 7.10.2 includes a fix, so this only effects 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

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.