Getting Set Up

Haskell

The simplest way to get started with Haskell is the Stack build system. Stack handles sandboxing (allowing different projects to use different versions of the GHCThe Glasgow Haskell Compiler is by far the most commonly-used Haskell compiler, and provides a variety of extensions to the standard Haskell language.

compiler and packages), dependency resolution, and building Haskell applications.

Installation

On Mac, you can install Stack using the Homebrew package manager.

brew install stack

For other operating systems or if you do not want to use Homebrew, see the installation instructions.

Starting a New Project

The following command will create a new project called first-project using the simple template

stack new first-project simple
cd first-project

with the following directory structure.

.
├── LICENSE
├── README.md
├── Setup.hs
├── first-project.cabal
├── src
│   └── Main.hs
└── stack.yaml

Now, the project can be built into an executable and run.

stack build
stack exec first-project

For details on Stack, see the user manual.

The REPL

An important tool in Haskell development is ghci, a REPLRead-Eval-Print-Loop

provided by GHC. We can start ghci and load our project’s code as follows.

stack ghci

The only function currently defined in our project (in src/Main.hs) is main, which we can run in the repl.

*Main> main
hello world

We can run Haskell code in ghci. It is important to note that ghci does not execute code in the same way it is executed in a Haskell program. Instead, it is executed in a computational context known as a monad, which allows symbols to be redefined to take on new values, among other things. This will become clearer in the next set of notes.

ghci also supports a variety of specialty commands. The most useful will be :r, which reloads the project code, :t x, which gives the type of x, and :q, which quits the repl.

*Main> :r
Ok, one module loaded.
*Main> :t "Hello"
"Hello" :: [Char]

Tooling

A variety of tooling is available for Haskell. We recommend, at least at first, setting up syntax highlighting in the text editor of your choice and relying only on it and ghci.