# Haskell

Haskell is a general purpose, purely functional, programming language.

## Installation

Haskell generates machine code that can be run natively on Linux. There is nothing special required to run a binary (already compiled) software, like the ones provided in the official repositories or by the ArchHaskell group. On the other side, AUR packages or source codes requires a compiler to build the software.

Installing the compiler alone permits to build Haskell source code. For development there is a Haskell-Platform bundle which offers the complete set of tools to get started with Haskell.

### Compiler

To build a Haskell source–code into native–code, a compiler must be installed. There are several implementations available, but the most used one (which became de facto the reference) is the GHC (Glasgow Haskell Compiler). It is available in the official repositories as ghc.

You can try it with the following file:

    Main.hs

    main = putStrLn "Hello, World"

and by running:

    $ ghc Main.hs
    $ ./Main

    Hello, World

### Haskell platform

To start developing in Haskell easily, there is a haskell-platform bundle which is described as:

  The easiest way to get started with programming Haskell. It comes with all you need to get up and running. Think of it as "Haskell: batteries included".

An AUR package exists (haskell-platform), but can advantageously be replaced by installing the following packages from the official repositories:

- ghc (ghc) — The compiler
- cabal-install (cabal-install) — A command line interface for Cabal and Hackage
- haddock (haddock) — Tools for generating documentation
- happy (happy) — Parser generator
- alex (alex) — Lexical analyzer generator

## Managing Haskell packages

There are a lot of Haskell libraries and executables grouped in packages. They are all available on Hackage. To install and manage these packages, several methods are available and unusual ones are explained in the rest of this section.

The recommended workflow is the following:

- Official repositories or ArchHaskell repository as principal source of Haskell packages (the or is exclusive here)
- cabal-install (possibly with sandboxes) for Haskell development
- Arch User Repository for packages that are not available elsewhere

A wrapper around cblrepo exists, called cabal2pkgbuild (cabal2pkgbuild-git), that you can use to create PKGBUILD files for Hackage packages.

### Pros/Cons of the different methods

- Official repositories

  Pros: Provided by ArchLinux developers, consistent versions of packages, already compiled
  Cons: Only a few packages available

- ArchHaskell repository

  Pros: Provided by ArchHaskell group, consistent versions of packages, already compiled
  Cons: Need manual intervention to get started with

- cabal-install

  Pros: All packages available
  Cons: Installed in home folder, cabal-install is not a package manager, incompatible versions of packages possible (aka. cabal hell)

- Arch User Repository

  Pros: Simple to get started
  Cons: Risk of unmaintained or orphaned packages, incompatible versions of packages possible

### ArchHaskell repository

#### Preparation

Add the haskell-core unofficial user repository to your configuration. Be sure to put [haskell-core] above [extra] to ensure that the packages from [haskell-core] take precedence, in case of duplicate packages in the two repositories.

Note: A note for xmonad users: if you switch to this option, you need to install the haskell-xmonad package instead of xmonad (which is in the official community repo and has different dependencies).

You also need need to do the following to add Magnus Therning's key (the maintainer of the repository):

    # pacman-key -r 4209170B
    # pacman-key --lsign-key 4209170B

and then force the refresh of the package list:

    # pacman -Syy

#### Installing and removing packages

Use pacman for installing or removing packages.

### cabal-install

Warning: Discouraged method, keep in mind that cabal-install is not a package manager.
Note: The only exception is for Haskell development, where cabal-install is the recommended tool. Since version 1.18, cabal provides a sandbox system that permits to isolate different versions of libraries for different projects. There is an introduction to cabal sandbox here.

#### Preparation

Install cabal-install from the official repositories.

To run installed executables without specifying the path, the cabal binary folder `~/.cabal/bin` must be added to the `$PATH` variable. That can be done by putting the following line in your shell configuration file, for instance `~/.bashrc` for bash or `~/.zshrc` for zsh:

    PATH=$PATH:~/.cabal/bin

#### Installing packages

    $ cabal update
    $ cabal install <pkg>

You can add `-j` for parallel compilation. It is also possible to install a package system–wide with the `--global` flag, but this is strongly discouraged. With the user–wide install, all files are kept in `~/.cabal` and libraries are registered to `~/.ghc`, offering the possibility to do a clean-up easily by simply removing these folders. With system–wide install, the files will be dispersed in the file system and difficult to manage.

#### Removing packages

There is no easy way to do it.

## Troubleshooting

### Switching to ArchHaskell repository

There can be some problems switching to ArchHaskell repository when some Haskell packages are already installed from official repositories. The surest way is to remove all Haskell related packages, synchronize the pacman packages database, and reinstall all the needed packages. Also for Xmonad users, be sure to install `haskell-xmonad` package instead of xmonad.

Category:

- Programming language