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# FAQ

So that this doesn't become repetitive: for the reasons behind the answers
below, see the [Build overview](build_overview.md) 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 _snapshot_. This may be something like `lts-22.7`, which is from
[Stackage](https://www.stackage.org/). The [user's guide](GUIDE.md) discusses
the snapshot in more detail.

The snapshot is determined by finding the relevant project-level configuration
file (`stack.yaml`) 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` file 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.

A frequent request when working with the implicit global is how to move to a
more recent LTS snapshot. You can do this using the following command from
outside of a project:

~~~text
stack config set resolver lts
~~~

## Where is Stack installed and will it interfere with the 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](install_and_upgrade.md) page). Stack
installs files in the Stack root and other files 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 (the tool)?

* 'Cabal' can refer to Cabal (the library) or to Cabal (the tool). Cabal (the
  library) is used by Stack to build your Haskell code. Cabal (the tool) is
  provided by the `cabal-install` package.
* A Cabal file is provided for each package, named `<package_name>.cabal`. It
  defines all package-level metadata, just like it does in the world of Cabal
  (the tool): modules, executables, test suites, etc. No change at all on this
  front.
* A `stack.yaml` file references one or more packages, and provides information
  on where dependencies come from.
* The `stack init` command initializes a `stack.yaml` file from an existing
  Cabal file.
* Stack uses Cabal (the library) via an executable. For `build-type: Simple`
  (the most common case), Stack builds that executable using the version of
  Cabal which came with the compiler. Stack caches such executables, in the
  Stack root under directory `setup-exe-cache`.
* In rare or complex cases, a different version of Cabal to the one that came
  with the compiler may be needed. `build-type: Custom` and a `setup-custom`
  stanza in the Cabal file, and a `Setup.hs` file in the package directory, can
  be specified. The `stack.yaml` file can then specify the version of Cabal
  that Stack will use to build the executable (named `setup`) from `Setup.hs`.
  Stack will use Cabal via `setup`.

For detail on the differences between a `stack.yaml` file and a Cabal file, see
[stack.yaml vs a Cabal file](stack_yaml_vs_cabal_package_file.md).

## 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.:

~~~yaml
snapshot: lts-22.7
packages:
- .
extra-deps:
- text-2.0.2@rev:1
~~~

## 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
[`extra-deps`](yaml_configuration.md#extra-deps) list in your project's
`stack.yaml` file, specifying the package's source code location relative to the
directory where your `stack.yaml` file lives, e.g.

~~~yaml
snapshot: lts-22.7
packages:
- .
extra-deps:
- third-party/proprietary-dep
- github-version-of/conduit
- patched/diagrams
~~~

The above example specifies that the `proprietary-dep` package is found in the
project's `third-party` directory, that the `conduit` package is found in the
project's `github-version-of` directory, and that the `diagrams` package is
found in the project's `patched` directory. This autodetects changes and
reinstalls the package.

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

~~~yaml
extra-deps:
  - 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 the `stack.yaml` file. (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.

## 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 (the tool) (1.22 or later), you can. For
earlier versions this does not work, due to Cabal issue
[#1800](https://github.com/haskell/cabal/issues/1800). Note that
even with recent versions, for some commands you may need the following extra
level of indirection. Command:

~~~text
stack exec -- cabal exec -- cabal <command>
~~~

However, virtually all `cabal` commands have an equivalent in Stack, so this
should not be necessary. In particular, users of Cabal (the tool) may be
accustomed to the `cabal run` command. With Stack, command:

~~~text
stack build
stack exec <program-name>
~~~

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

~~~text
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:

***B.erb***

~~~haskell
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 lines to your `stack.yaml` file:

~~~yaml
custom-preprocessor-extensions:
- erb

require-stack-version: ">= 2.6.0"
~~~

And for backwards compatability with older versions of Stack, also add the
following line to your Cabal file:

    extra-source-files:   B.erb

You could also use the
[`--custom-preprocessor-extensions`](yaml_configuration.md#custom-preprocessor-extensions)
flag.

## 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. You can find
these sandboxed GHC installations in the `ghc-*` directories in the
`stack path --programs` directory.

If you would like Stack to use your system GHC installation, use the
[`--system-ghc`](yaml_configuration.md#system-ghc) flag or run
`stack config set system-ghc --global true` to make Stack check your PATH for a
suitable GHC by default.

Stack can only use a system GHC installation if its version is compatible with
the configuration of the current project, particularly the snapshot specified by
the [`snapshot`](yaml_configuration.md#snapshot) or
[`resolver`](yaml_configuration.md#resolver) key.

GHC installation doesn't work for all operating systems, 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 the `ghc-<version>`
subdirectory of the `stack path --programs` directory for a compatible GHC,
requesting to install one via `stack setup` if none is found.

If you are using the [`--system-ghc`](yaml_configuration.md/#system-ghc) flag or
have configured `system-ghc: true` either in the project `stack.yaml` or the
global `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 issue [#420](https://github.com/commercialhaskell/stack/issues/420) for a
detailed discussion of Stack's behavior when `system-ghc` is enabled.

## How do I get extra build tools?

Stack will automatically install build tools required by your packages or their
dependencies, in particular [Alex](https://hackage.haskell.org/package/alex) and
[Happy](https://hackage.haskell.org/package/happy).

!!! note

    This works when using LTS or nightly snapshots, not with GHC or custom
    snapshots. 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 configuration 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`
configuration 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 operating system 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](shell_autocompletion.md) documentation.

## How do I update my package index?

Users of Cabal (the tool) 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 project-level configuration file (`stack.yaml`).
See [YAML configuration](yaml_configuration.md).

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

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

~~~text
myproject/
  stack-ghc-9.0.2.yaml
  stack-ghc-9.2.4.yaml
  stack.yaml --> symlink to stack-ghc-9.2.4.yaml
  myproject.cabal
  src/
    ...
~~~

When you run `stack build`, you can set the `STACK_YAML` environment variable to
indicate which build plan to use. On Unix-like operating systems command:

~~~bash
stack build  # builds using the default stack.yaml
STACK_YAML=stack-ghc-7.10.yaml
stack build  # builds using the given yaml file
~~~

On Windows (with PowerShell) command:

~~~ps
$Env:STACK_YAML='stack-ghc-9.0.2.yaml'
stack build
~~~

## 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](docker_integration.md) for
details.

## How do I use this with Travis CI?

See the [Travis CI instructions](travis_ci.md)

## How do I use this with Azure CI?

See the [Azure CI instructions](azure_ci.md)

## What is licensing restrictions on Windows?

Currently, on Windows, GHC produces binaries linked statically with
[GNU Multiple Precision Arithmetic Library](https://gmplib.org/) (GMP), which is
used by [integer-gmp](https://hackage.haskell.org/package/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](http://www.gnu.org/licenses/gpl-faq.html#LGPLStaticVsDynamic).
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](https://github.com/commercialhaskell/stack/issues/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, MSYS2).
The easiest way to find them is `stack exec which`. For example, command:

~~~text
stack exec -- which libstdc++-6.dll
/c/Users/Michael/AppData/Local/Programs/stack/i386-windows/ghc-7.8.4/mingw/bin/libstdc++-6.dll
~~~

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

See issue [#425](https://github.com/commercialhaskell/stack/issues/425).

Another issue that may arise with building on Windows is as follows. The default
location of Stack's programs folder is `%LOCALAPPDATA\Programs\stack`. If there
is a space character in the `%LOCALAPPDATA%` path this may, in some
circumstances, cause problems with building packages that make use of the GNU
project's `autoconf` package and `configure` shell script files. It may be
necessary to override the default location of Stack's programs folder. See the
[local-programs-path](yaml_configuration.md#local-programs-path) option for more
information.

See issue [#4726](https://github.com/commercialhaskell/stack/issues/4726).

## 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](yaml_configuration.md#rebuild-ghc-options) 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` file). For an explanation of this choice see this discussion on
issue
[#827](https://github.com/commercialhaskell/stack/issues/827#issuecomment-133263678).

If you still want to set specific GHC options for a dependency, use the
[`ghc-options`](yaml_configuration.md#ghc-options) option in your
YAML configuration file.

To change the set of packages that command line GHC options apply to, use the [`apply-ghc-options`](yaml_configuration.md#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

With PowerShell, it is easy to automate even that step. Command:

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

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

The first thing to remove is project-specific `.stack-work` directory within
the project's directory. Next, remove the Stack root 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](https://github.com/commercialhaskell/stack/issues/644) for more
details.

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

(Updated in January 2019)

This is related to more recent versions of Linux distributions that have GCC
with PIE enabled by default. The continuously-updated distros like Arch, in
particular, had been in flux with this change and the upgrading
libtinfo6/ncurses6, and there were some workarounds attempted in Stack that
ended up causing trouble as these distros evolved.

GHC added official support for this setup in 8.0.2, so if you are using an
older version your best bet is to upgrade. You may be able to work around it
for older versions by editing `~/.stack/programs/x86_64-osx/ghc-VER/lib/ghc-
VER/settings` (replace `VER` with the GHC version) and adding `-no-pie` (or
`--no-pie` in the case of Gentoo, at least as of December 2017) to the __C
compiler link flags__.

If `stack setup` complains that there is no `linuxNN-*-nopie` bindist available,
try adding `ghc-build: *` (replacing the `*` with the actual value that
precedes `-nopie`, which may be empty) to your `~/.stack/config.yaml` (this
will no longer be necessary for stack >= 1.7).

If you are experiencing this with GHC >= 8.0.2, try running
`stack setup --reinstall` if you've upgraded your Linux distribution or you set
up GHC before late December 2017.

If GHC doesn't recognize your C compiler as being able to use `-no-pie`, this
can happen even with GCC and Clang, it might be necessary to enable this feature
manually. To do this, just change
`("C compiler supports -no-pie", "NO"),` to
`("C compiler supports -no-pie", "YES"),`
in the file `~/.stack/programs/x86_64-osx/ghc-VER/lib/ghc-VER/settings`.

If you are still having trouble after trying the above, check the following for
more possible workarounds:

  * Previous version of this [FAQ entry](https://docs.haskellstack.org/en/v1.6.3/faq/#i-get-strange-ld-errors-about-recompiling-with-fpic)
  * Related issues:
    [#3518](https://github.com/commercialhaskell/stack/issues/3518),
    [#2712](https://github.com/commercialhaskell/stack/issues/2712),
    [#3630](https://github.com/commercialhaskell/stack/issues/3630),
    [#3648](https://github.com/commercialhaskell/stack/issues/3648)

## 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.

## <a name="dyld-library-path-ignored"></a>Why is DYLD_LIBRARY_PATH ignored?

If you are on Mac OS X 10.11 ("El Capitan") or later, there is a GHC issue
[#11617](https://ghc.haskell.org/trac/ghc/ticket/11617) which prevents the
`DYLD_LIBRARY_PATH` environment variable from being passed to GHC (see issue
[#1161](https://github.com/commercialhaskell/stack/issues/1161)) 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](http://osxdaily.com/2015/10/05/disable-rootless-system-integrity-protection-mac-os-x/).
    **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 (see issue
    [#1161](https://github.com/commercialhaskell/stack/issues/1161#issuecomment-186690904)).

## <a name="usr-bin-ar-permission-denied"></a>Why do I get a `/usr/bin/ar: permission denied` error?

## Why is the `--` argument separator ignored in Windows PowerShell

Some versions of Windows PowerShell don't pass the `--` to programs (see issue
[#813](https://github.com/commercialhaskell/stack/issues/813)). The workaround
is to quote the `"--"`. For example, command:

~~~ps
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, use the
  [`extra-lib-dirs`](yaml_configuration.md#extra-lib-dirs) option or the
  [`extra-include-dirs`](yaml_configuration.md#extra-include-dirs) option to
  specify it.

In the future, Stack might give operating system-specific suggestions for how to
install system libraries.

## How can I make Stack aware of my custom SSL certificates?

### macOS

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

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

Some users have reported issues with this approach, see issue
[#907](https://github.com/commercialhaskell/stack/issues/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?

Add `ghc-options: -vN` to the Cabal file or pass it via
`stack build --ghc-options="-v"`.

## Does Stack support the Hpack specification?

Yes:

* If a package directory contains an [Hpack](https://github.com/sol/hpack)
  `package.yaml` file, then Stack will use it to generate a Cabal file when
  building the package.
* You can run `stack init` to initialize a `stack.yaml` file regardless of
  whether your packages are declared with Cabal files or with Hpack
  `package.yaml` files.
* You can use the `with-hpack` YAML configuration or command line option to
  specify an Hpack executable to use instead of Stack's in-built Hpack
  functionality.

## How do I resolve linker errors when running `stack setup` or `stack build` on macOS?

This is likely to be caused by having both a LLVM installation and default Apple
Clang compiler on the PATH. The symptom of this issue is a linker error "bad
relocation (Invalid pointer diff)". The compiler picks up inconsistent versions
of binaries and the mysterious error occurs.

The workaround is to remove LLVM binaries from the PATH.

## How do I suppress `'-nopie'` warnings with `stack build` on macOS?

~~~bash
clang: warning: argument unused during compilation: '-nopie'
 [-Wunused-command-line-argument]
~~~

This warning is shown when compiler support of `-no-pie` is expected but
unavailable. It's possible to bypass the warning for a specific version of GHC
by modifying a global setting:

~~~bash
# ~/.stack/programs/x86_64-osx/ghc-8.2.2/lib/ghc-8.2.2/settings
-- ("C compiler supports -no-pie", "YES"),
++ ("C compiler supports -no-pie", "NO"),
~~~

**Note that we're fixing `ghc-8.2.2` in this case; repeat for other versions as necessary.**
You should apply this fix for the version of GHC that matches your snapshot.

Issue [#4009](https://github.com/commercialhaskell/stack/issues/4009) goes into
further detail.

## How do I install GHC in Stack when it fails with the error: Missing ghc bindist for "linux64-ncurses6"?

Example Error:

~~~text
No setup information found for ghc-8.6.4 on your platform.
This probably means a GHC bindist has not yet been added for OS key 'linux64-ncurses6'.
Supported versions: ghc-7.10.3, ghc-8.0.1, ghc-8.0.2, ghc-8.2.1, ghc-8.2.2
~~~

Most Linux distributions have standardized on providing libtinfo.so.6 (either
directly or as a symlink to libncursesw.so.6). As such, there aren't GHC 8.6.*
bindists that link to libncursesw.so.6 available.

So creating a symlink to libncursesw.so.6 as libtinfo.so.6 can prevent this
error (root privileges might be required). Command:

~~~bash
ln -s /usr/lib/libncursesw.so.6 /usr/lib/libtinfo.so.6
~~~
