1. A Hello World example¶

With Stack installed, let's create a new project and walk through the most common Stack commands.

In this guide, unless stated otherwise, the working directory is the project's root directory.

The `stack new` command¶

We'll start off with the stack new command to create a new project from a project template. We'll use the new-template project template. This template is used by default, but in our example we will refer to it expressly.

That template will create a project with a package of the same name. So, we need to pick a name for the project that is a valid package name. We'll call our project helloworld.

How do project packages relate to projects?

A project can have one or more packages. Each project package has its own root directory. In the case of a single-package project, the project directory and the package directory can be the same directory.

What is a valid package name?

A valid package name consists of one or more alphanumeric words separated by hyphens. Each word must contain at least one letter. That is, the word must not be interpreted as a number.

The names of packages are intended to be unique.

Are other project templates available?

Yes. For further information about project templates, command:

stack templates

From the root directory for all our Haskell projects, we command:

stack new helloworld new-template

For this first Stack command, Stack will do some setting up. For example, it will create the Stack root directory.

Other than any setting up, Stack will: * create the project directory; * download the project template; * attempt to populate the project template based on parameters; and * create and initialise Stack's project-level configuration file.

Unless the parameters have been configured, Stack will note that parameters were needed by the template but not provided. That can be ignored for now.

How can I configure project template paramaters?

For further information, see the templates non-project specific configuration option.

As noted in Stack's output, parameters to populate project templates can also be set at the command line by using the options of the stack new command.

Can I create a new project in the current working directory?

Yes. Pass the --bare flag to cause Stack to create the project in the current working directory rather than in a new project directory.

We now have a project in the helloworld directory! We will change to that directory, with command:

cd helloworld

The `stack build` command¶

Next, we'll run the most important Stack command, stack build. We command:

stack build

Stack needs a version of GHC and, on Windows, a version of MSYS2, 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.

Where is the Stack-supplied GHC located?

You can use the stack path command for path information. To identify where GHC is installed, command:

Unix-likeWindows (with PowerShell)

stack exec -- which ghc
/home/<user_name>/.stack/programs/x86_64-linux/ghc-9.6.5/bin/ghc

stack exec -- where.exe ghc
C:\Users\<user_name>\AppData\Local\Programs\stack\x86_64-windows\ghc-9.6.5\bin\ghc.exe

As you can see from that path, the installation is placed to not interfere with any other GHC installation, whether system-wide or different GHC versions installed by Stack.

Can I use that version of GHC by commanding ghc?

No. GHC will be installed to the Stack programs directory, which is likely not on the PATH, so commanding ghc will not work. However, that version of GHC can be used in the Stack environment. For more information, see the stack exec command, stack ghc command, and stack runghc command documentation.

Once a version of GHC and, on Windows, a version of MSYS2, is installed, Stack will then build your project. The end of the output should look similar to this:

Unix-likeWindows

...
helloworld> configure (lib + exe)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe) with ghc-9.6.6
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 executable 'helloworld-exe' for helloworld-0.1.0.0..
Building executable 'helloworld-exe' for helloworld-0.1.0.0..
[1 of 2] Compiling Main
[2 of 2] Compiling Paths_helloworld
[3 of 3] Linking .stack-work/dist/x86_64-linux-tinfo6/ghc-9.6.6/build/helloworld-exe/helloworld-exe
helloworld> copy/register
Installing library in .../helloworld/.stack-work/install/x86_64-linux-tinfo6/a2caceceda039eb4f791856f85a68f9582d4daf3d0527344693ff3d1fcd92ba4/9.6.6/lib/x86_64-linux-ghc-9.6.6/helloworld-0.1.0.0-KFyX8zLxDvzLZURq3JaCVX
Installing executable helloworld-exe in .../helloworld/.stack-work/install/x86_64-linux-tinfo6/a2caceceda039eb4f791856f85a68f9582d4daf3d0527344693ff3d1fcd92ba4/9.6.6/bin
Registering library for helloworld-0.1.0.0..

...
helloworld> configure (lib + exe)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe) with ghc-9.6.6
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 executable 'helloworld-exe' for helloworld-0.1.0.0..
Building executable 'helloworld-exe' for helloworld-0.1.0.0..
[1 of 2] Compiling Main
[2 of 2] Compiling Paths_helloworld
[3 of 3] Linking .stack-work\dist\effaccc7\build\helloworld-exe\helloworld-exe.exe
helloworld> copy/register
Installing library in ...\helloworld\.stack-work\install\c8c71a24\lib\x86_64-windows-ghc-9.6.6\helloworld-0.1.0.0-KFyX8zLxDvzLZURq3JaCVX
Installing executable helloworld-exe in ...\helloworld\.stack-work\install\c8c71a24\bin
Registering library for helloworld-0.1.0.0..

On Windows, Stack uses hashes of certain information to keep paths short.

Stack aims not to rebuild unnecessarily. If we command stack build a second time, nothing happens.

The `stack exec` command¶

The output of the previous command has three main steps. You can see, from the first two steps, that a library (lib) and an executable (exe) are being built and that the final step involved the installation of an executable named helloworld-exe (on Windows, the file is helloworld-exe.exe) (extract):

helloworld> configure (lib + exe)
...
helloworld> build (lib + exe) with ghc-9.6.6
...
helloworld> copy/register
...
Installing executable helloworld-exe in .../helloworld/.stack-work/.../bin

The executable is installed in a location in the project's .stack-work directory.

Now, let's use the stack exec command to run our executable. We command:

stack exec helloworld-exe

and the output is just:

someFunc

Why is the output just someFunc?

The code in the new-template project template is very simple. The package has a Haskell module Lib:

module Lib
    ( someFunc
    ) where

someFunc :: IO ()
someFunc = putStrLn "someFunc"

and a Haskell module Main:

module Main (main) where

import Lib

main :: IO ()
main = someFunc

putStrLn "someFunc" is an action that, when executed, outputs the string someFunc to the standard output channel.

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 to find helloworld-exe even though it is not on the PATH outside of that environment.

How I can find the PATH used in the Stack environment?

Command stack path --bin-path to see the PATH in the Stack environment.

Info

On Windows, the Stack environment includes the \mingw64\bin, \usr\bin and \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.

The `stack test` command¶

Finally, like all good software, helloworld actually has a test suite.

Let's run it with the stack test command. We command:

stack test

The start of the output should look similar to this:

Unix-likeWindows

helloworld-0.1.0.0: unregistering (components added: test:helloworld-test)
helloworld> configure (lib + exe + test)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe + test) with ghc-9.6.6
Preprocessing library for helloworld-0.1.0.0..
Building library for helloworld-0.1.0.0..
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/x86_64-linux-tinfo6/ghc-9.6.6/build/helloworld-test/helloworld-test
Preprocessing executable 'helloworld-exe' for helloworld-0.1.0.0..
Building executable 'helloworld-exe' for helloworld-0.1.0.0..
helloworld> copy/register
Installing library in .../helloworld/.stack-work/install/x86_64-linux-tinfo6/a2caceceda039eb4f791856f85a68f9582d4daf3d0527344693ff3d1fcd92ba4/9.6.6/lib/x86_64-linux-ghc-9.6.6/helloworld-0.1.0.0-KFyX8zLxDvzLZURq3JaCVX
Installing executable helloworld-exe in .../helloworld/.stack-work/install/x86_64-linux-tinfo6/a2caceceda039eb4f791856f85a68f9582d4daf3d0527344693ff3d1fcd92ba4/9.6.6/bin
Registering library for helloworld-0.1.0.0..

helloworld-0.1.0.0: unregistering (components added: test:helloworld-test)
helloworld> configure (lib + exe + test)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe + test) with ghc-9.6.6
Preprocessing library for helloworld-0.1.0.0..
Building library for helloworld-0.1.0.0..
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\effaccc7\build\helloworld-test\helloworld-test.exe
Preprocessing executable 'helloworld-exe' for helloworld-0.1.0.0..
Building executable 'helloworld-exe' for helloworld-0.1.0.0..
helloworld> copy/register
Installing library in ...\helloworld\.stack-work\install\0aa166fa\lib\x86_64-windows-ghc-9.6.6\helloworld-0.1.0.0-KFyX8zLxDvzLZURq3JaCVX
Installing executable helloworld-exe in ...\helloworld\.stack-work\install\0aa166fa\bin
Registering library for helloworld-0.1.0.0..

Again, Stack does not rebuild unnecessarily. Only the test suite component is compiled and linked.

The output should then conclude:

helloworld> test (suite: helloworld-test)

Test suite not yet implemented



helloworld> Test suite helloworld-test passed
Completed 2 action(s).

Having build the test suite executable, Stack then automatically runs it.

Inner workings of Stack¶

Let's look at the helloworld example in more detail to understand better how Stack works.

The files in the project include:

app/Main.hs
src/Lib.hs
test/Spec.hs
ChangeLog.md
README.md
LICENSE
.gitignore
package.yaml
helloworld.cabal
Setup.hs
stack.yaml

The app/Main.hs, src/Lib.hs, and test/Spec.hs files are all Haskell source files that compose the actual functionality of our project. We won't dwell on them here.

The ChangeLog.md, README.md, LICENSE and .gitignore files have no effect on the build.

The files of interest here are package.yaml, helloworld.cabal, Setup.hs and stack.yaml.

`package.yaml`¶

Each package contains a file that describes the package. Stack uses the Cabal build system and that system uses a Cabal file named after the package (such as helloworld.cabal) to describe the package.

However, Stack's preferred package description format is the Hpack format.

The package.yaml file describes the package in the Hpack format.

If a package.yaml file is present, Stack will use its built-in Hpack functionality to create a Cabal file.

What is covered by a package description?

A package description includes information such as the package name and version, and the package's components. A package can have an optional main library component and optional named sub-library components. It can also have optional executable components, test suite components and benchmark components. The description identifies other packages on which those components depend.

The Cabal User Guide is the definitive reference for the Cabal package description format.

The Hpack documentation is the reference for the Hpack package description format.

`helloworld.cabal`¶

The helloworld.cabal file is updated automatically as part of the stack build process and should not be modified.

`Setup.hs`¶

The Setup.hs file is a component of the Cabal build system.

Technically, it is not needed by Stack, but it is considered good practice to include it. The file we're using is boilerplate:

import Distribution.Simple
main = defaultMain

`stack.yaml`¶

Stack requires a Stack project-level configuration file for every project. stack.yaml is that file. The contents of the file define project-specific options and non-project-specific options that apply to the project.

The contents of the file include comments beginning #. Ignoring those comments, the contents will look something like this:

resolver:
  url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/21.yaml
packages:
- .

The key resolver is a project-specific configuration option. Its value tells Stack how to build your package: which GHC version to use, which versions of package dependencies to use, and so on. Our value here says to use LTS Haskell 22.21, which implies GHC 9.6.5 (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 resolver, which we'll cover later.

The key packages is another project-specific configuration option. Its value 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 describe as this guide progresses.

1. A Hello World example¶

The stack new command¶

The stack build command¶

The stack exec command¶

The stack test command¶

Inner workings of Stack¶

package.yaml¶

helloworld.cabal¶

Setup.hs¶

stack.yaml¶