User guide (introductory)¶
Stack is a modern, cross-platform build tool for Haskell code.
This introductory guide takes a new Stack user through the typical workflows. This guide will not teach Haskell or involve much code, and it requires no prior experience with the Haskell packaging system or other build tools. Terms used in the guide are defined in the glossary.
Some of Stack's features will not be needed regularly or by all users. See the advanced user's guide for information about those features.
Stack handles the management of your toolchain (including GHC — the Glasgow Haskell Compiler — and, for Windows users, MSYS2), building and registering libraries, building build tool dependencies, and more. While it can use existing tools on your system, Stack has the capacity to be your one-stop shop for all Haskell tooling you need. This guide will follow that Stack-centric approach.
What makes Stack special?¶
The primary Stack design point is reproducible builds. If you run
stack build today, you should get the same result running
tomorrow. There are some cases that can break that rule (changes in your
operating system configuration, for example), but, overall, Stack follows this
design philosophy closely. To make this a simple process, Stack uses curated
package sets called snapshots.
Stack has also been designed from the ground up to be user friendly, with an
intuitive, discoverable command line interface. For many users, simply
downloading Stack and reading
stack --help will be enough to get up and
running. This guide provides a more gradual tour for users who prefer that
To build your project, Stack uses a project-level configuration file, named
stack.yaml, in the root directory of your project as a sort of blueprint. That
file contains a reference to the snapshot (also known as a resolver) which
your package will be built against.
Finally, Stack is isolated: it will not make changes outside of specific
Stack directories. Stack-built files generally go in either the Stack root
./.stack-work directories local to each project. The
Stack root directory holds packages belonging to snapshots and
any Stack-installed versions of GHC. Stack will not tamper with any system
version of GHC or interfere with packages installed by other build tools, such
as Cabal (the tool).
Downloading and Installation¶
The documentation dedicated to downloading Stack has
the most up-to-date information for a variety of operating systems. Instead of
repeating that content here, please go check out that page and come back here
when you can successfully run
We also assume that the directory reported by
stack path --local-bin has been
added to the PATH.
Hello World Example¶
With Stack installed, let's create a new project from a template and walk through the most common Stack commands.
In this guide, an initial
$ represents the command line prompt. The prompt may
differ in the terminal on your operating system. Unless stated otherwise, the
working directory is the project's root directory.
stack new command¶
We'll start off with the
stack new command to create a new project, that
will contain a Haskell package of the same name. So let's pick a valid
package name first:
A package is identified by a globally-unique package name, which consists of one or more alphanumeric words separated by hyphens. To avoid ambiguity, each of these words should contain at least one letter.
(From the Cabal users guide)
We'll call our project
helloworld, and we'll use the
template. This template is used by default, but in our example we will refer to
it expressly. Other templates are available. For further information about
templates, see the
stack templates command
From the root directory for all our Haskell projects, we command:
For this first Stack command, there's quite a bit of initial setup it needs to do (such as downloading the list of packages available upstream), so you'll see a lot of output. Over the course of this guide a lot of the content will begin to make more sense.
After creating the project directory, and obtaining and populating the project
template, Stack will initialise its own project-level configuration. For further
information about setting paramaters to populate templates, see the YAML
configuration documentation. For further
information about initialisation, see the
stack init command
stack new and
commands have options and flags in common.
--bare flag to cause Stack to create the project in the current
working directory rather than in a new project directory.
Parameters to populate project templates can be set at the command line with
--param <key>:<value> (or
We now have a project in the
helloworld directory! We will change to that
directory, with command:
stack build command¶
Next, we'll run the most important Stack command,
Stack needs a version of GHC in order to build your project. Stack will discover that you are missing it and will install it for you.
You'll get intermediate download percentage statistics while the download is occurring. This command may take some time, depending on download speeds.
GHC will be installed to your Stack programs directory, so calling
the command line won't work. See the
stack ghc, and
stack runghc commands below for more information.
Once a version of GHC is installed, Stack will then build your project.
stack exec command¶
Looking closely at the output of the previous command, you can see that it built
both a library called
helloworld and an executable called
helloworld-exe.exe). We'll explain more in the next section, but, for
now, just notice that the executables are installed in a location in our
Now, Let's use the
stack exec command to run our executable (which just
stack exec works by providing the same reproducible environment that was used
to build your project to the command that you are running. Thus, it knew where
helloworld-exe even though it is hidden in the
stack path --bin-path to see the PATH in the Stack
On Windows, the Stack environment includes the
\usr\local\bin directories of the Stack-supplied MSYS2. If your
executable depends on files (for example, dynamic-link libraries) in those
directories and you want ro run it outside of the Stack environment, you
will need to ensure copies of those files are on the PATH.
stack test command¶
Finally, like all good software,
helloworld actually has a test suite.
Let's run it with the
stack test command:
Reading the output, you'll see that Stack first builds the test suite and then
automatically runs it for us. For both the
test command, already
built components are not built again. You can see this by using the
stack build and
stack test commands a second time:
Inner Workings of Stack¶
In this subsection, we'll dissect the
helloworld example in more detail.
Files in helloworld¶
Before studying Stack more, let's understand our project a bit better. The files in the directory include:
test/Spec.hs files are all Haskell
source files that compose the actual functionality of our project (we won't
dwell on them here).
.gitignore files have no effect
on the build.
helloworld.cabal file is updated automatically as part of the
stack build process and should not be modified.
The files of interest here are
Setup.hs file is a component of the Cabal build system which Stack uses.
It's technically not needed by Stack, but it is still considered good practice
in the Haskell world to include it. The file we're using is straight
Next, let's look at our
stack.yaml file, which gives our project-level
settings. Ignoring comments beginning
#, the contents will look something like
The value of the
resolver key tells Stack
how to build your package: which GHC version to use, versions of package
dependencies, and so on. Our value here says to use
LTS Haskell 22.11, which implies GHC 9.6.4
(which is why
stack build installs that version of GHC if it is not already
available to Stack). There are a number of values you can use for
which we'll cover later.
The value of the
packages key tells Stack which project packages, located
locally, to build. In our simple example, we have only a single project package,
located in the same directory, so '
.' suffices. However, Stack has powerful
support for multi-package projects, which we'll elaborate on as this guide
Another file important to the build is
package.yaml file describes the package in the
Hpack format. Stack has in-built Hpack
functionality and this is its preferred package format. The default behaviour is
to generate the Cabal file (here named
helloworld.cabal) from this
package.yaml file, and accordingly you should not modify the Cabal file.
It is also important to remember that Stack is built on top of the Cabal build
system. Therefore, an understanding of the moving parts in Cabal are necessary.
In Cabal, we have individual packages, each of which contains a single Cabal
<package_name>.cabal. The Cabal file can define one or more
components: a library, executables, test suites, and benchmarks. It also
specifies additional information such as library dependencies, default
language pragmas, and so on.
In this guide, we'll discuss the bare minimum necessary to understand how to
package.yaml file. You can see a full list of the available options
at the Hpack documentation. The
Cabal User Guide the definitive reference for the
Cabal file format.
The location of GHC¶
As we saw above, the
build command installed GHC for us. You can use the
stack path command for quite a bit of path information (which we'll play with
more later). We'll look at where GHC is installed:
As you can see from that path (and as emphasized earlier), the installation is placed to not interfere with any other GHC installation, whether system-wide or even different GHC versions installed by Stack.
Cleaning your project¶
You can clean up build artifacts for your project using the
stack clean and
stack purge commands.
stack clean command¶
stack clean deletes the local working directories containing compiler output.
By default, that means the contents of directories in
.stack-work directories within a project.
stack clean <specific-package> to delete the output for the package
stack purge command¶
stack purge deletes the local stack working directories, including extra-deps,
git dependencies and the compiler output (including logs). It does not delete
any snapshot packages, compilers or programs installed using
This essentially reverts the project to a completely fresh state, as if it had
never been built.
stack purge is just a shortcut for
stack clean --full
stack build command¶
build command is the heart and soul of Stack. It is the engine that powers
building your code, testing it, getting dependencies, and more. Quite a bit of
the remainder of this guide will cover more advanced
build functions and
features, such as building test and Haddocks at the same time, or constantly
rebuilding blocking on file changes.
build command twice with the same options and arguments should
generally do nothing (besides things like rerunning test suites), and
should, in general, produce a reproducible result between different runs.
Let's say we decide to modify our
helloworld source a bit to use a new
library, perhaps the ubiquitous
text package. In
src/Lib.hs, we can, for
When we try to build this, things don't go as expected:
# build failure output (abridged for clarity) ...
Could not load module ‘Data.Text.IO’
It is a member of the hidden package ‘text-220.127.116.11’.
Perhaps you need to add ‘text’ to the build-depends in your .cabal file.
Use -v (or `:set -v` in ghci) to see a list of the files searched for.
6 | import qualified Data.Text.IO as T
This means that the package containing the module in question is not available.
To tell Stack to use text, you need
to add it to your
package.yaml file — specifically in your
section, like this:
Now if we rerun
stack build, we should get a successful result. Command:
This output means that the
text package was downloaded, configured, built, and
locally installed. Once that was done, we moved on to building our project
helloworld). At no point did we need to ask Stack to build
dependencies — it does so automatically.
Let's have Stack add a few more dependencies to our project. First, we'll
include two new packages in the
dependencies section for our library in our
After adding these two dependencies, we can again run
stack build to have them
Finally, to find out which versions of these libraries Stack installed, we can
ask Stack to
ls dependencies. Command:
Let's try a more off-the-beaten-track package: the joke acme-missiles package. Our source code is simple:
Again, we add this new dependency to the
package.yaml file like this:
stack build shows us the following error message. Command:
It says that it was unable to construct the build plan.
This brings us to the next major topic in using Stack.
Curated package sets¶
Remember above when
stack new selected some
LTS snapshot for us?
That defined our build plan and available packages. When we tried using the
text package, it just worked, because it was part of the LTS package set.
We've specified the
acme-missiles package in the
package.yaml file (see
acme-missiles is not part of that LTS package set, so building
acme-missiles to the available packages, we'll use the
stack.yaml file. That key defines extra packages, not present in the
snapshot, that will be needed as dependencies. You can add this like so:
stack build will succeed.
With that out of the way, let's dig a little bit more into these package sets, also known as snapshots. We mentioned the LTS snapshots, and you can get quite a bit of information about it at https://www.stackage.org/lts, including:
- The appropriate value (
lts-22.7, as is currently the latest LTS)
- The GHC version used
- A full list of all packages available in this snapshot
- The ability to perform a Hoogle search on the packages in this snapshot
- A list of all modules in a snapshot,
which can be useful when trying to determine which package to add to your
You can also see a list of all available snapshots. You'll notice two flavors: LTS (for "Long Term Support") and Nightly. You can read more about them on the LTS Haskell GitHub page. If you're not sure which to use, start with LTS Haskell (which Stack will lean towards by default as well).
Snapshots and changing your compiler version¶
Let's explore package sets a bit further. Instead of
lts-22.7, let's change
stack.yaml file to use the
latest nightly. Right now, this is currently
2023-09-24 - please see the snapshot from the link above to get the latest.
stack build again will produce:
We can also change snapshots on the command line, which can be useful in a Continuous Integration (CI) setting, like on Travis. For example, command:
When passed on the command line, you also get some additional "short-cut"
versions of snapshots:
--snapshot nightly will use the newest Nightly snapshot
--snapshot lts will use the newest LTS, and
will use the newest LTS in the 22.x series. The reason these are only available
on the command line and not in your
stack.yaml file is that using them:
- Will slow down your build (since Stack then needs to download information on the latest available LTS each time it builds)
- Produces unreliable results (since a build run today may proceed differently tomorrow because of changes outside of your control)
Changing GHC versions¶
Finally, let's try using an older LTS snapshot. We'll use the newest 21.x snapshot with the command:
This succeeds, automatically installing the necessary GHC along the way. So, we see that different LTS versions use different GHC versions and Stack can handle that.
Other snapshot values¶
lts-X.Y values for the snapshot.
There are actually other options available, and the list will grow over time.
At the time of writing:
ghc-X.Y.Z, for requiring a specific GHC version but no additional packages
- Experimental custom snapshot support
The most up-to-date information can always be found in the stack.yaml documentation.
Alright, enough playing around with simple projects. Let's take an open source
package and try to build it. We'll be ambitious and use
yackage, a local package server
using Yesod. To get the code, we'll use the
stack unpack command from the root directory for all our Haskell projects:
You can also unpack to the directory of your liking instead of the current one by issuing the command:
This will create a
yackage-0.8.1 directory inside
We will change to that directory, with the command:
stack init command¶
This new directory does not have a
stack.yaml file, so we need to make one
first. We could do it by hand, but let's be lazy instead with the
stack init does quite a few things for you behind the scenes:
- Finds all of the Cabal files in your current directory and subdirectories
(unless you use
--ignore-subdirs) and determines the packages and versions they require
- Finds the best combination of snapshot and package flags that allows everything to compile with minimum external dependencies
- It tries to look for the best matching snapshot from latest LTS, latest nightly, other LTS versions in that order
Assuming it finds a match, it will write your
stack.yaml file, and everything
yackage package does not currently support Hpack, but you can also use
hpack-convert should you need to generate a
Sometimes multiple packages in your project may have conflicting requirements.
In that case
stack init will fail, so what do you do?
You could manually create
stack.yaml by omitting some packages to resolve the
conflict. Alternatively you can ask
stack init to do that for you by
--omit-packages flag on the command line. Let's see how that
To simulate a conflict we will use
yackage and we will
yackage.cabal to another directory and change the name of the file
and package to
yackage-test. In this new package we will use
acme-missiles-0.2 instead. Let's see what happens when we command
stack.yaml, you will see that the excluded packages have been
commented out under the
packages field. In case wrong packages are excluded
you can uncomment the right one and comment the other one.
Packages may get excluded due to conflicting requirements among user packages or due to conflicting requirements between a user package and the snapshot compiler. If all of the packages have a conflict with the compiler then all of them may get commented out.
When packages are commented out you will see a warning every time you run a command which needs the configuration file. The warning can be disabled by editing the configuration file and removing it.
Using a specific snapshot¶
Sometimes you may want to use a specific snapshot for your project instead of
stack init picking one for you. You can do that by using
stack init --snapshot <snapshot>.
You can also init with a compiler snapshot if you do not want to use a
Stackage snapshot. That will result in all of your project's dependencies being
put under the
Installing the compiler¶
Stack will automatically install the compiler when you run
stack build but you
can manually specify the compiler by running
stack setup <GHC-VERSION>.
Miscellaneous and diagnostics¶
Add selected packages: If you want to use only selected packages from your project directory you can do so by explicitly specifying the package directories on the command line.
Duplicate package names: If multiple packages under the directory tree have
stack init will report those and automatically ignore one of them.
Ignore subdirectories: By default
stack init searches all the subdirectories
for Cabal files. If you do not want that then you can use
command line switch.
stack init will show warnings if there were issues in
reading a Cabal file. You may want to pay attention to the warnings as sometimes
they may result in incomprehensible errors later on during dependency solving.
Package naming: If the
Name field defined in a Cabal file does not match
with the Cabal file name then
stack init will refuse to continue.
User warnings: When packages are excluded or external dependencies added Stack will show warnings every time the configuration file is loaded. You can suppress the warnings by editing the configuration file and removing the warnings from it. You may see something like this:
Warning: Some packages were found to be incompatible with the resolver and have been left commented out in the packages section.
Warning: Specified resolver could not satisfy all dependencies. Some external packages have been added as dependencies.
You can suppress this message by removing it from stack.yaml
Time to take a short break from hands-on examples and discuss a little architecture. Stack has the concept of multiple databases.
A database consists of a GHC package database (which contains the compiled
version of a library), executables, and a few other things as well. To give you
an idea, the contents of the parent directory of the
stack path --local-pkg-db
directory are the directories:
Databases in Stack are layered. For example, the database listing we just gave is called a local database (also known as a mutable database). That is layered on top of a snapshot database (also known as a write-only database). The snapshot database contains the libraries and executables that are considered to be immutable. Finally, GHC itself ships with a number of libraries and executables, also considered to be immutable, which forms the global database.
To get a quick idea of this, we can look at the output of the
stack exec -- ghc-pkg list command in our
<stack path --global-pkg-db directory>
<stack path --snapshot-pkg-db directory>
<stack path --local-pkg-db directory>
<stack path --global-pkg-db directory> refers to the directory output by
stack path --global-pkg-db, and so on.
acme-missiles ends up in the snapshot database. Any package
which comes from Hackage, an archive, or a repository is considered to be an
Anything which is considered mutable, or depends on something mutable, ends up in the local database. This includes your own code and any other packages located on a local file path.
The reason we have this structure is that:
- it lets multiple projects reuse the same binary builds of immutable packages,
- but doesn't allow different projects to "contaminate" each other by putting non-standard content into the shared snapshot database.
As you probably guessed, there can be multiple snapshot databases available. See
the contents of the
snapshots directory in the Stack root.
On Unix-like operating systems, each snapshot is in the last of a sequence of three subdirectories named after the platform, a 256-bit hash of the source map (how the package should be built -- including the compiler, options, and immutable dependencies), and the GHC version.
On Windows, each snapshot is in a subdirectory that is a shorter hash (eight characters) of the sequence of three directories used on Unix-like operating systems. This is done to avoid problems created by default limits on file path lengths on Windows systems.
These snapshot databases don't get layered on top of each other; they are each used separately.
In reality, you'll rarely — if ever — interact directly with these databases, but it's good to have a basic understanding of how they work so you can understand why rebuilding may occur at different points.
The build synonyms¶
Let's look at a subset of the
stack --help output:
Four of these commands are just synonyms for the
build command. They are
provided for convenience for common cases (e.g.,
stack test instead of
stack build --test) and so that commonly expected commands just work.
What's so special about these commands being synonyms? It allows us to make much more composable command lines. For example, we can have a command that builds executables, generates Haddock documentation (Haskell API-level docs), and builds and runs your test suites, with:
You can even get more inventive as you learn about other flags. For example, take the following command:
This command will:
- turn on all warnings and errors (the
- build your library and executables
- generate Haddocks (the
- build and run your test suite (the
- run the command
echo Yay, it succeededwhen that completes (the
- after building, watch for changes in the files used to build the project, and
kick off a new build when done (the
stack install command and
It's worth calling out the behavior of the
install command and
option, since this has confused a number of users (especially when compared to
behavior of other tools like Cabal (the tool)). The
install command does
precisely one thing in addition to the build command: it copies any generated
executables to the local binary directory. You may recognize the default value
for that path:
On Unix-like operating systems, command:
On Windows, command:
That's why the download page recommends adding that directory to your PATH. This
feature is convenient, because now you can simply run
executable-name in your
shell instead of having to run
stack exec executable-name from inside your
Since it's such a point of confusion, let me list a number of things Stack does
not do specially for the
- Stack will always build any necessary dependencies for your code. The install
command is not necessary to trigger this behavior. If you just want to build a
- Stack will not track which files it's copied to your local binary directory nor provide a way to automatically delete them. There are many great tools out there for managing installation of binaries, and Stack does not attempt to replace those.
Stack will not necessarily be creating a relocatable executable. If your executables hard-codes paths, copying the executable will not change those hard-coded paths.
At the time of writing, there's no way to change those kinds of paths with Stack, but see issue #848 about --prefix for future plans.
That's really all there is to the
install command: for the simplicity of what
it does, it occupies a much larger mental space than is warranted.
Targets, locals, and extra-deps¶
We haven't discussed this too much yet, but, in addition to having a number of
synonyms and taking a number of options on the command line, the
command also takes many arguments. These are parsed in different ways, and can
be used to achieve a high level of flexibility in telling Stack exactly what you
want to build.
We're not going to cover the full generality of these arguments here; instead, there's documentation covering the full build command syntax. Here, we'll just point out a few different types of arguments:
- You can specify a package name, e.g.
stack build vector.
- This will attempt to build the
vectorpackage, whether it's a local package, in your extra-deps, in your snapshot, or just available upstream. If it's just available upstream but not included in your locals, extra-deps, or snapshot, the newest version is automatically promoted to an extra-dep.
- This will attempt to build the
- You can also give a package identifier, which is a package name plus
stack build yesod-bin-1.4.14.
- This is almost identical to specifying a package name, except it will (1) choose the given version instead of latest, and (2) error out if the given version conflicts with the version of a project package.
- The most flexibility comes from specifying individual components, e.g.
stack build helloworld:test:helloworld-testsays "build the test suite component named helloworld-test from the helloworld package."
- In addition to this long form, you can also shorten it by skipping what
type of component it is, e.g.
stack build helloworld:helloworld-test, or even skip the package name entirely, e.g.
stack build :helloworld-test.
- In addition to this long form, you can also shorten it by skipping what type of component it is, e.g.
- Finally, you can specify individual directories to build to trigger building of any project packages included in those directories or subdirectories.
When you give no specific arguments on the command line (e.g.,
it's the same as specifying the names of all of your project packages. If you
just want to build the package for the directory you're currently in, you can
stack build ..
Components, --test, and --bench¶
Here's one final important yet subtle point. Consider our
it has a library component, an executable
helloworld-exe, and a test suite
helloworld-test. When you run
stack build helloworld, how does it know which
ones to build? By default, it will build the library (if any) and all of the
executables but ignore the test suites and benchmarks.
This is where the
--bench flags come into play. If you use them,
those components will also be included. So
stack build --test helloworld will
end up including the helloworld-test component as well.
You can bypass this implicit adding of components by being much more explicit,
and stating the components directly. For example, the following will not build
stack build :helloworld-test
helloworld> configure (lib + test)
helloworld> build (lib + test) with ghc-9.6.4
Preprocessing library for helloworld-0.1.0.0..
Building library for helloworld-0.1.0.0..
[1 of 2] Compiling Lib
[2 of 2] Compiling Paths_helloworld
Preprocessing test suite 'helloworld-test' for helloworld-0.1.0.0..
Building test suite 'helloworld-test' for helloworld-0.1.0.0..
[1 of 2] Compiling Main
[2 of 2] Compiling Paths_helloworld
[3 of 3] Linking .stack-work\dist\<hash>\build\helloworld-test\helloworld-test.exe
Installing library in ...\helloworld\.stack-work\install\...
Registering library for helloworld-0.1.0.0..
helloworld> test (suite: helloworld-test)
Test suite not yet implemented
helloworld> Test suite helloworld-test passed
Completed 2 action(s).
We first purged our project to clear old results so we know exactly what Stack is trying to do.
The last line shows that our command also runs the test suite it just built.
This may surprise some people who would expect tests to only be run when using
stack test, but this design decision is what allows the
stack build command
to be as composable as it is (as described previously). The same rule applies to
benchmarks. To spell it out completely:
--benchflags simply state which components of a package should be built, if no explicit set of components is given
- The default behavior for any test suite or benchmark component which has been built is to also run it
You can use the
--no-run-benchmarks flags to disable
running of these components. You can also use
--no-rerun-tests to prevent
running a test suite which has already passed and has not changed.
Stack doesn't build or run test suites and benchmarks for non-local
packages. This is done so that a command like
stack test doesn't need to
run 200 test suites!
Until now, everything we've done with Stack has used a single-package project. However, Stack's power truly shines when you're working on multi-package projects. All the functionality you'd expect to work just does: dependencies between packages are detected and respected, dependencies of all packages are just as one cohesive whole, and if anything fails to build, the build commands exits appropriately.
Let's demonstrate this with the
starting in the root directory for all our Haskell projects. Command:
stack unpack wai-app-static yackage
Unpacked wai-app-static (from Hackage) to .../multi/wai-app-static-18.104.22.168/
Unpacked yackage (from Hackage) to .../multi/yackage-0.8.1/
Looking for .cabal or package.yaml files to use to init the project.
Using cabal packages:
Cabal file warning in .../multi/yackage-0.8.1/yackage.cabal@47:40: version operators used. To use version operators the package needs to specify at least 'cabal-version: >= 1.8'.
Cabal file warning in .../multi/yackage-0.8.1/yackage.cabal@21:36: version operators used. To use version operators the package needs to specify at least 'cabal-version: >= 1.8'.
Selecting the best among 18 snapshots...
* Matches ...
Selected resolver: ...
Initialising configuration using resolver: ...
Total number of user packages considered: 2
Writing configuration to file: stack.yaml
stack build --haddock --test
# Goes off to build a whole bunch of packages
If you look at the
stack.yaml file, you'll see exactly what you'd expect:
Notice that multiple directories are listed in the
In addition to local directories, you can also refer to packages available in a Git repository or in a tarball over HTTP/HTTPS. This can be useful for using a modified version of a dependency that hasn't yet been released upstream.
When adding upstream packages directly to your project it is important to
distinguish project packages located locally from the upstream
dependency packages. Otherwise you may have trouble running
See stack.yaml documentation for more
Flags and GHC options¶
There are two common ways to alter how a package will install: with Cabal flags and with GHC options.
Cabal flag management¶
To change a Cabal flag setting, we can use the command line
--flag option. The
yackage package has an
upload flag that is enabled by default. We can
This means: when compiling the
yackage package, turn off the
-upload). Unlike other tools, Stack is explicit about which
package's flag you want to change. It does this for two reasons:
- There's no global meaning for Cabal flags, and therefore two packages can use the same flag name for completely different things.
- By following this approach, we can avoid unnecessarily recompiling snapshot packages that happen to use a flag that we're using.
You can also change flag values on the command line for extra-dep and snapshot packages. If you do this, that package will automatically be promoted to an extra-dep, since the build plan is different than what the plan snapshot definition would entail.
GHC options follow a similar logic as in managing Cabal flags, with a few nuances to adjust for common use cases. Let's consider the command:
This will set the
-Wall -Werror options for all local targets. Note that
this will not affect extra-dep and snapshot packages at all. This design
provides us with reproducible and fast builds.
(By the way: the above GHC options have a special convenience flag:
There's one extra nuance about command line GHC options: Since they only apply
to local targets, if you change your local targets, they will no longer apply
to other packages. Let's play around with an example from the
which includes the
warp packages, the latter depending on the
former. If we command again:
It will build all of the dependencies of
wai, and then build
wai with all
optimizations disabled. Now let's add in
warp as well. Command:
This builds the additional dependencies for
warp, and then builds
optimizations disabled. Importantly: it does not rebuild
configuration has not been altered. Now the surprising case. Command:
You may expect this to be a no-op: neither
warp has changed.
However, Stack will instead recompile
wai with optimizations enabled again,
and then rebuild
warp (with optimizations disabled) against this newly built
wai. The reason: reproducible builds. If we'd never built
before, trying to build
warp would necessitate building all of its
dependencies, and it would do so with default GHC options (optimizations
enabled). This dependency would include
wai. So when we command:
We want its behavior to be unaffected by any previous build steps we took. While this specific corner case does catch people by surprise, the overall goal of reproducible builds is - in the Stack maintainers' views - worth the confusion.
Final point: if you have GHC options that you'll be regularly passing to your
packages, you can add them to your
stack.yaml file. See the
documentation section on ghc-options
for more information.
That's it, the heavy content of this guide is done! Everything from here on out is simple explanations of commands. Congratulations!
stack path command¶
Generally, you don't need to worry about where Stack stores various files. But
some people like to know this stuff. That's when the
stack path command is
stack path --help explains the available options and, consequently,
the output of the command:
--stack-root Global Stack root directory
--global-config Global Stack configuration file
--project-root Project root (derived from stack.yaml file)
--config-location Configuration location (where the stack.yaml file is)
--bin-path PATH environment variable
--programs Install location for GHC and other core tools (see
'stack ls tools' command)
--compiler-exe Compiler binary (e.g. ghc)
--compiler-bin Directory containing the compiler binary (e.g. ghc)
--compiler-tools-bin Directory containing binaries specific to a
--local-bin Directory where Stack installs executables (e.g.
~/.local/bin (Unix-like OSs) or %APPDATA%\local\bin
--extra-include-dirs Extra include directories
--extra-library-dirs Extra library directories
--snapshot-pkg-db Snapshot package database
--local-pkg-db Local project package database
--global-pkg-db Global package database
--ghc-package-path GHC_PACKAGE_PATH environment variable
--snapshot-install-root Snapshot installation root
--local-install-root Local project installation root
--snapshot-doc-root Snapshot documentation root
--local-doc-root Local project documentation root
--local-hoogle-root Local project documentation root
--dist-dir Dist work directory, relative to package directory
--local-hpc-root Where HPC reports and tix files are stored
stack path accepts the flags above on the command line to state
which keys you're interested in. This can be convenient for scripting. As a
simple example, let's find out the sandboxed versions of GHC that Stack
While we're talking about paths, to wipe our Stack install completely, here's what typically needs to be removed:
- the Stack root folder (see
stack path --stack-root, before you uninstall);
- if different, the folder containing Stack's global YAML configuration file
stack path --global-config, before you uninstall);
- on Windows, the folder containing Stack's tools (see
stack path --programs, before you uninstall), which is located outside of the Stack root folder; and
stackexecutable file (see
which stack, on Unix-like operating systems, or
where.exe stack, on Windows).
You may also want to delete
.stack-work folders in any Haskell projects that
you have built using Stack. The
stack uninstall command provides information
about how to uninstall Stack.
stack exec command¶
We've already used
stack exec multiple times in this guide. As you've likely
already guessed, it allows you to run executables, but with a slightly modified
environment. In particular:
stack exec looks for executables on Stack's bin
paths, and sets a few additional environment variables (like adding those paths
to the PATH, and setting
GHC_PACKAGE_PATH, which tells GHC which package
databases to use).
If you want to see exactly what the modified environment looks like, try command:
The only issue is how to distinguish flags to be passed to Stack versus those
for the underlying program. Thanks to the
optparse-applicative library, Stack
follows the Unix convention of
-- to separate these. For example, command:
yields output like:
Run from outside a project, using implicit global project config
Using latest snapshot resolver: lts-22.7
Writing global (non-project-specific) config file to: /home/michael/.stack/global/stack.yaml
Note: You can change the snapshot via the resolver field there.
I installed the stm package via --package stm
Flags worth mentioning:
--package foocan be used to force a package to be installed before running the given command.
--no-ghc-package-pathcan be used to stop the
GHC_PACKAGE_PATHenvironment variable from being set. Some tools — notably Cabal (the tool) — do not behave well with that variable set.
You may also find it convenient to use
stack exec to launch a subshell
bash with your preferred shell) where your compiled executable is
available at the front of your PATH. Command:
stack ghci or
stack repl command¶
GHCi is the interactive GHC environment, a.k.a. the REPL. You could access it with command:
But that won't load up locally written modules for access. For that, use the
stack ghci or
stack repl commands, which are equivalent. To then load
modules from your project in GHCi, use the
:module command (
:m for short)
followed by the module name.
If you have added packages to your project please make sure to mark them as
extra-deps for faster and reliable usage of
stack ghci. Otherwise GHCi may
have trouble due to conflicts of compilation flags or having to
unnecessarily interpret too many modules. See Stack's project-level
configuration to learn how to
configure a package as an extra-dep.
For further information, see the REPL environment documentation.
stack ghc and
stack runghc commands¶
You'll sometimes want to just compile (or run) a single Haskell source file,
instead of creating an entire Cabal package for it. You can use
stack exec ghc
stack exec runghc for that. As simple helpers, we also provide the
stack ghc and
stack runghc commands, for these common cases.
Finding project configs, and the implicit global project¶
Whenever you run something with Stack, it needs a project-level configuration file. The algorithm Stack uses to find such a file is:
- Check for a
--stack-yamloption on the command line
- Check for a
- Check the current directory and all ancestor directories for a
The first two provide a convenient method for using an alternate configuration.
stack build --stack-yaml stack-ghc-9.2.3.yaml can be used by your
CI system to check your code against GHC 9.2.3. Setting the
environment variable can be convenient if you're going to be running commands
stack ghc in other directories, but you want to use the configuration you
defined in a specific project.
If Stack does not find a project level configuration file in any of the three specified locations, the implicit global logic kicks in. You've probably noticed that phrase a few times in the output from commands above. Implicit global is essentially a hack to allow Stack to be useful in a non-project setting. When no implicit global configuration file exists, Stack creates one for you with the latest LTS snapshot. This allows you to do things like:
- compile individual files easily with
- build executables without starting a project, e.g.
stack install pandoc
Keep in mind that there's nothing magical about this implicit global configuration. It has no effect on projects at all. Every package you install with it is put into isolated databases just like everywhere else. The only magic is that it's the catch-all project whenever you're running Stack somewhere else.
stack.yaml versus Cabal files¶
Now that we've covered a lot of Stack use cases, this quick summary of
stack.yaml versus Cabal files will hopefully make sense and be a good reminder
for future uses of Stack:
- A project can have multiple packages.
- Each project has a
- Each package has a Cabal file, named
- The Cabal file specifies which packages are dependencies.
stack.yamlfile specifies which packages are available to be used.
- The Cabal file specifies the components, modules, and build flags provided by a package
stack.yamlcan override the flag settings for individual packages
stack.yamlspecifies which packages to include
Comparison to other tools¶
Stack is not the only tool available for building Haskell code. Stack came into existence due to limitations at that time with some of the existing tools. If you are happily building Haskell code with other tools, you may not need Stack. If you're experiencing problems with other tools, give Stack a try instead.
If you're a new user who has no experience with other tools, we recommend Stack. The defaults match modern best practices in Haskell development, and there are fewer corner cases you need to be aware of. You can develop Haskell code with other tools, but you probably want to spend your time writing code, not convincing a tool to do what you want.
Underlying package format¶
Before turning to differences, we clarify an important similarity: Stack, Cabal (the tool), and presumably all other tools share the same underlying package format of Cabal (the library). This is a Good Thing: we can share the same set of upstream libraries, and collaboratively work on the same project with Stack, Cabal (the tool), and NixOS. In that sense, we're sharing the same ecosystem.
Curation vs dependency solving¶
Stack uses 'curation' (snapshots and Stack's project-level configuration file (
stack.yaml, by default) define precisely the set of packages available for a project). The Stack team firmly believes that the majority of users want to simply ignore dependency resolution nightmares and get a valid build plan from day one. That's why we've made 'curation' the focus of Stack.
Cabal (the tool) can use 'curation' too but its origins are in dependency solving.
Emphasis on reproducibility¶
Stack goes to great lengths to ensure that
stack buildtoday does the same thing tomorrow. With Stack, changing the build plan is always an explicit decision.
Cabal (the tool) does not go to the same lengths: build plans can be affected by the presence of pre-installed packages, and running
cabal updatecan cause a previously successful build to fail.
Automatic building of dependencies¶
Stack's automatically builds dependencies. So for example, in Stack,
stack testdoes the same job as:
(newer versions of Cabal (the tool) may make this command sequence shorter).
With Cabal (the tool), you need to use
cabal installto trigger dependency building. This is somewhat necessary as building dependencies can, in some cases, break existing installed packages.
Stack is isolated - provides 'sandboxed' behaviour - by default, via its databases. In other words: when you use Stack, there's no need for sandboxes, everything is (essentially) sandboxed by default.
With Cabal (the tool), the default behavior is a non-isolated build where working on two projects can cause the user package database to become corrupted. The Cabal solution to this is sandboxes.