Haskell Stack is a tool used to build Haskell projects and to handle its dependencies, including GHC, Cabal, a version of the Hackage repository, and a version of the Stackage package collection tool. In this chapter, you will learn the main uses of Haskell Stack.

In Windows, go to https://get.haskellstack.org/stable/windows-x86_64-installer.exe , where you will be prompted to download the installer. Then follow the steps in the installer.

Let’s see what every command does.

• stack new: Creates a new directory and all the necessary files to begin a project. The structure will look like Figure 23-1.

• stack setup: Downloads the compiler if needed, putting it in a separate location. This means it won’t make changes outside its directory.

• stack build: Builds a minimal project, designing reproducible builds. In this process, curated package sets are used, called snapshots. The main directory contains a file called stack.yaml representing a blueprint. It contains a reference called resolver that points to the snapshot used in the build process.

• stack exec my-project-exe: Executes a command.

Another useful command is stack install <package_name> , which installs a desired package. And, of course, don’t forget about stack --help, which provides all the commands.

Now let’s see a concrete example, called hello-world, inspired from [1] (note that, in this example, we will work on the Windows operating system).

This will create a new project called hello-world using the new-template template, applying an initial setup. Don’t worry if you get a lot of messages. Now, you will see file in the hello-world directory, as shown in Figure 23-2.

In this step, Stack will check for GHC and will download and install it into the global Stack root directory. This will take a while, and you will get intermediary messages about the download progress. After the build process, a library called hello-world and an executable called hello-world-exe are created in the autocreated directory .stack-work in the hello-world directory.

You will get a someFunc message.

If you build the project, you will get an error because the acme-missiles package is not found as a dependency (Figure 23-3).

Now build again the project, and it will finally succeed (Figure 23-4).

You have created a simple project with the Haskell tool Stack.

The Haskell tool Stack is great for versioning control, focusing on reproducible build plans and multipackage projects. Of course, you can do a lot more things with it than what was presented in this chapter. For example, you can put your project into a Git repository, or you can include other projects from Git. You can find a comprehensive tutorial in [1].