1. Getting Started with Haskell and Cabal¶
1.1. Installing the Haskell toolchain¶
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 --cabal-version=2.4 --license=NONE -p myfirstapp
This will generate the following files:
$ ls CHANGELOG.md Main.hs myfirstapp.cabal Setup.hs
Main.hs is where your package’s code lives. By default
creates an executable with the same name as the package
myfirstapp in this
case, you can instruct
cabal init to generate just a library (with
--lib) or both a library and executable with (
--libandexe); for the full
set of options see
cabal init --help.
myfirstapp.cabal is Cabal’s metadata file which describes your package 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¶
As mentioned above,
cabal init with no arguments generates a package with a
single executable 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!
: prefix in
:myfirstapp signifies that the
myfirstapp target is part of the current package.
Notice that we didn’t need to run a build command before
cabal run, this
cabal run first determines if the code needs to be re-built
before running the executable. If you just want to build a target you can do so
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.
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
myfirstapp.cabal file we’ll update the
build-depends attribute of
executable myfirstapp section to include
executable myfirstapp main-is: Main.hs build-depends: base >=4.11 && <4.12, haskell-say ^>=126.96.36.199
^>=188.8.131.52 means use version 184.108.40.206 of the library or any more recent
minor release with the same major version.
Next we’ll update
Main.hs to use the
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
HaskellSay into scope. The
HaskellSay module is defined in
haskell-say packages that we added a dependency on 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! | \____ _____________________________________________/ \ / \ / \/ _____ _____ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \-----------| \ \ \ \ \ | \ \ \ \ \---------| / / / \ / / / \ \-------| / / / ^ \ \ | / / / / \ \ \ ----| / / / / \ \ /____/ /____/ \____\