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hello_world_example.md

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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.

??? question "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.

??? question "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.

??? question "Are other project templates available?"

Yes. For further information about project templates, command:
~~~text
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.

??? question "How can I configure project template paramaters?"

For further information, see the
[`templates`](../configure/yaml/non-project.md#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.

??? question "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.

??? question "Where is the Stack-supplied GHC located?"

You can use the [`stack path`](../commands/path_command.md) command for path
information. To identify where GHC is installed, command:

=== "Unix-like"

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

=== "Windows"

    ~~~text
    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.

??? question "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`](../commands/exec_command.md) command,
[`stack ghc`](../commands/ghc_command.md) command, and
[`stack runghc`](../commands/ghc_command.md) 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-like"

~~~text
...
helloworld> configure (lib + exe)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe) with ghc-9.8.4
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 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-9.8.4/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-9.8.4/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..
~~~

=== "Windows"

~~~text
...
helloworld> configure (lib + exe)
Configuring helloworld-0.1.0.0...
helloworld> build (lib + exe) with ghc-9.8.4
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 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-9.8.4\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-9.8.4
...
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

??? question "Why is the output just someFunc?"

The code in the `new-template` project template is very simple. The package
has a Haskell module `Lib`:
~~~haskell
module Lib
    ( someFunc
    ) where

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

and a Haskell module `Main`:
~~~haskell
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.

??? question "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-like"

~~~text
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-9.8.4
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-9.8.4/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-9.8.4/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..
~~~

=== "Windows"

~~~text
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-9.8.4
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-9.8.4\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.