1. Getting Started with Haskell and Cabal

1.1. Installing the Haskell toolchain

To install the Haskell toolchain follow the ghcup instructions.

1.2. Creating a new application

Let’s start by creating a simple Haskell application from scratch where we’ll learn about a Haskell package’s directory structure, how to run the executable, and how to add external dependencies.

1.2.1. Initializing the application

Start by creating a myfirstapp directory to hold the project files, these instructions work in unix shells and PowerShell (if you’re on Windows).

$ mkdir myfirstapp
$ cd myfirstapp

Once you have an empty directory we can initialize our package:

$ cabal init -n

Note

-n stands for --non-interactive, which means that cabal will try to guess how to set up the project for you and use the default settings, which will serve us well for the purpose of this tutorial. When setting up your projects in the future, you will likely want to ommit -n and do just cabal init, so that cabal will interactively ask you for the details on how the project should be set up (while still offering reasonable defaults on every step). Also, you can run cabal init --help to get more info on how cabal init can be used.

This will generate the following files:

$ tree
.
├── app
│   └── Main.hs
├── CHANGELOG.md
└── myfirstapp.cabal

app/Main.hs is where your package’s code lives.

myfirstapp.cabal is Cabal’s metadata file which describes your package, how it is built and its dependencies. We’ll be updating this file in a little bit when we add an external dependency to our package.

1.2.2. Running the application

When we ran cabal init -n above, it generated a package with a single executable named same as the package (in this case myfirstapp) that prints "Hello, Haskell!" to the terminal. To run the executable enter the following command:

cabal run myfirstapp

You should see the following output in the terminal:

$ cabal run myfirstapp
...
Hello, Haskell!

Notice that we didn’t need to run a build command before we ran cabal run. This is because cabal run automatically determines if the code needs to be (re)built before running the executable. If you just want to build a target without running it, you can do so with cabal build:

cabal build myfirstapp

1.2.3. Adding dependencies

Next we’ll add an external dependency to our application. Hackage is the Haskell community’s central package archive of open source software.

In our application, we’ll use a package called haskell-say to print text to the terminal with some embellishment.

Tip

If you installed cabal a while ago but haven’t used it recently you may need to update the package index, you can do this by running cabal update.

In our myfirstapp.cabal file we’ll update the build-depends attribute of the executable myfirstapp section to include haskell-say:

executable myfirstapp
    import: warnings
    main-is: Main.hs
    build-depends:
        base ^>=4.14.3.0,
        haskell-say ^>=1.0.0.0
    hs-source-dirs: app
    default-language: Haskell2010

Note

^>=1.0.0.0 means use version 1.0.0.0 of the library or any more recent minor release with the same major version. To put it simply, this means use the latest version of the library that starts with 1.0.

Next we’ll update app/Main.hs to use the HaskellSay library:

module Main where

import HaskellSay (haskellSay)

main :: IO ()
main =
  haskellSay "Hello, Haskell! You're using a function from another package!"

import HaskellSay (haskellSay) brings the haskellSay function from the module named HaskellSay into scope. The HaskellSay module is defined in the haskell-say package that we added as a dependency above.

Now you can build and re-run your code to see the new output:

$ cabal run
    ________________________________________________________
   /                                                        \
  | Hello, Haskell! You're using a function from another     |
  | package!                                                 |
   \____       _____________________________________________/
        \    /
         \  /
          \/
    _____   _____
    \    \  \    \
     \    \  \    \
      \    \  \    \
       \    \  \    \  \-----------|
        \    \  \    \  \          |
         \    \  \    \  \---------|
         /    /  /     \
        /    /  /       \  \-------|
       /    /  /    ^    \  \      |
      /    /  /    / \    \  \ ----|
     /    /  /    /   \    \
    /____/  /____/     \____\

1.3. What Next?

Now that you know how to set up a simple Haskell package using Cabal, check out some of the resources on the Haskell website’s documentation page or read more about packages and Cabal on the introduction page.