Haskell/Getting set up

Installing Haskell

Haskell is a programming language, i.e. a language in which humans can express how computers should behave. It's like writing a cooking recipe: you write the recipe and the computer executes it.

To use Haskell programs, you need a special program called a Haskell compiler. A compiler takes code written in Haskell and translates it into machine code, a more elementary language that the computer understands. Using the cooking analogy, you write a recipe (your Haskell program) and a cook (a compiler program) does the work of putting together actual ingredients into an edible dish (an executable file). Of course, you can't easily get the recipe from a final dish (and you can't get the Haskell code from the executable after it's compiled).

To get started, visit haskell.org/downloads for the latest instructions. The current recommended way is to use GHCup. Use it to install the latest recommended version of GHC, cabal (not stack), and the Haskell language server. You should use cabal to install any Haskell library you need. We will cover using cabal in depth in Haskell/Packaging

Note

Linux users:

It is heavily, heavily discouraged to install anything pertaining to Haskell using your system's package repositories and package manager (unless you're using Fedora or NixOS). Specifically distributions like Arch Linux and Debian manage Haskell packages very badly, and will lead to a bad experience with the tooling. Use GHCup and Cabal as noted above.

To just test some Haskell basics without downloading and installing, there is a playground which includes a few packages by default. Most instructions here on the Wikibook will also work on the playground, though it doesn't accept user input.

First code

After installation, we will do our first Haskell coding with the program called GHCi (the 'i' stands for 'interactive'). Depending on your operating system, perform the following steps:

• On Windows: Click Start, then Run, then type 'cmd' and hit Enter, then type ghci and hit Enter once more.
• On MacOS: Open the application "Terminal" found in the "Applications/Utilities" folder, type the letters ghci into the window that appears, and hit the Enter key.
• On Linux: Open a terminal and run ghci.

You should get output that looks something like the following:

GHCi, version 8.10.7: http://www.haskell.org/ghc/  :? for help
Prelude> 

The first bit is GHCi's version, and tells your how to get help in GHCi. The Prelude> bit is known as the prompt. This is where you enter commands, and GHCi will respond with their results. The prompt also tells you that the current loaded module is Prelude, which gives you access to most of the built-in functions.

Now let's try some basic arithmetic:

Prelude> 2 + 2
4
Prelude> 5 + 4 * 3
17
Prelude> 2 ^ 5
32

These operators match most other programming languages: + is addition, * is multiplication, and ^ is exponentiation (raising to the power of, or $a^{b}$). As shown in the second example, Haskell follows standard order of math operations (e.g. multiplication before addition).

Now you know how to use Haskell as a calculator. Actually, Haskell is always a calculator — just a really powerful one, able to deal not only with numbers but also with other objects like characters, lists, functions, trees, and even other programs (if you aren't familiar with these terms yet, don't worry).

To leave GHCi once you are done, use :quit (or just :q):

Prelude> :quit
Leaving GHCi.

GHCi is a powerful development environment. As we progress, we will learn how to load files with source code into GHCi and evaluate different parts of them.

Assuming you're clear on everything so far (if not, use the talk page and help us improve this Wikibook!), then you are ready for next chapter where we will introduce some of the basic concepts of Haskell and make our first Haskell functions.