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Haskell is a purely-functional programming language.

Getting started

First, put the following entries into /etc/portage/package.accept_keywords:

FILE /etc/portage/package.accept_keywords

# Haskell has no stable keywords in Gentoo
dev-haskell/*

# Only needed if using ::haskell, but harmless if not.
# May as well put it in, just in case for future use.
*/*::haskell

If interested in doing Haskell development, or if the needed packages required are not in the main Gentoo ebuild repository, please enable and configure the haskell repository. The Haskell repository (::haskell) has its own instructions too. The most important part is that this repository requires a specific unmasking procedure to prevent blockers.

Install git to fetch the overlay and eselect-repository to setup the overlay:

root #emerge --ask app-eselect/eselect-repository dev-vcs/git

Add the overlay:

root #eselect repository enable haskell

Update system:

root #emerge --sync

root #emerge -uvDU @world

Compiler and interpreter

The probably most important and up-to-date Haskell-implementation is the Glasgow Haskell Compiler (GHC). Install it with:

root #emerge --ask dev-lang/ghc

The package also includes an interpreter called GHCI (except on the ARM-architecture).

Furthermore, there's Hugs, an (meanwhile (almost) out-dated) interpreter for Haskell98. Install it with:

root #emerge --ask dev-lang/hugs98

cabal tool

With cabal tool, it is possible to package and build libraries and programs. Install it with:

root #emerge --ask dev-haskell/cabal-install

Warning
The Gentoo Haskell team does not recommend the use of dev-haskell/cabal-install to install Haskell packages. Instead, emerge should be used, or the package should be installed locally in a sandbox. If a package is needed that isn't in the main Gentoo tree, it may be present in the Haskell ebuild repository. If not, please file a bug/issue, or ask us in #gentoo-haskell on IRC!

Updating Haskell packages

Sometimes:

root #emerge -auvDN --keep-going @world

has trouble figuring out how to update Haskell packages. Providing emerge with the full list of dev-haskell packages that have upgrades available can sometimes help:

root #eix-update

root #emerge -av --oneshot --keep-going `eix --only-names --upgrade -C dev-haskell`

root #haskell-updater

Sometimes, if there are sub-slot blockers (when updating ghc or some specific package there are a list of blockers), this issue could be solved via running:

root #haskell-updater --all -- =dev-lang/ghc-<latest.version>

Hoogle with local installation

The Hoogle ebuild is currently only available in the official gentoo-haskell ebuild repository. So add that first.

Important
Remember to unmask the each package as described in the repository README

root #eselect repository enable haskell

root #emerge --sync haskell

In order to get the an offline installation of all hoogle data, enable the doc, hscolour, and hoogle USE flag values.

root #echo "dev-haskell/* doc hoogle hscolour" >> /etc/portage/package.use

Enable the relevant USE flags for hoogle to store all data local, and emerge hoogle:

root #echo "dev-haskell/hoogle fetchdb fetchdb-ghc localdb" >> /etc/portage/package.use

root #emerge --ask dev-haskell/hoogle

After emerging haskell packages, the hoogle database of the locally installed packages is updated by running:

root #hoogle-build-localdb

(It's normal for hoogle-build-localdb to output a bunch of junk. Redirect stdout to /dev/null if it has been added to e.g. /etc/cron.daily.)

At this point, Hoogle should work from the command-line. For example, the following will search for the splitOn function:

user $$ hoogle splitOn

Data.Text.Lazy splitOn :: Text -> Text -> [Text]
Data.Text splitOn :: Text -> Text -> [Text]
Data.List.Extra splitOn :: Eq a => [a] -> [a] -> [[a]]
Extra splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split.Internals splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split.Internals splitOneOf :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split splitOneOf :: Eq a => [a] -> [a] -> [[a]]

Integration with GHCi

To use Hoogle within GHCi, make a little modification to local ~/.ghci file. Add the following:

FILE ~/.ghci

-- Surround a string in single quotes.
let single_quote s = concat ["'", s, "'"]

-- Escape a single quote in the shell. (This mess actually works.)
let escape_single_quote c = if c == '\'' then "'\"'\"'" else [c]

-- Simple heuristic to escape shell command arguments.
let simple_shell_escape = single_quote . (concatMap escape_single_quote)

:def hoogle \x -> return $ ":!hoogle --color " ++ (simple_shell_escape x)
:def doc \x -> return $ ":!hoogle --info --color " ++ (simple_shell_escape x)

Now, within GHCi, there should be access to two new commands, :hoogle and :doc. The first will perform a normal Hoogle search and print the output:

user $ghci

ghci> :hoogle splitOn
Searching for: splitOn
Data.Text.Lazy splitOn :: Text -> Text -> [Text]
Data.Text splitOn :: Text -> Text -> [Text]
Data.List.Extra splitOn :: Eq a => [a] -> [a] -> [[a]]
Extra splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split.Internals splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split splitOn :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split.Internals splitOneOf :: Eq a => [a] -> [a] -> [[a]]
Data.List.Split splitOneOf :: Eq a => [a] -> [a] -> [[a]]

The second will display the Haddock documentation for the method:

user $ghci

ghci> :doc splitOn
Searching for: splitOn
Data.Text.Lazy splitOn :: Text -> Text -> [Text]

O(m+n) Break a Text into pieces separated by the first Text argument (which cannot be an empty string), consuming the delimiter. An empty delimiter is invalid, and will cause an error to be raised.

Examples:

> splitOn "\r\n" "a\r\nb\r\nd\r\ne" == ["a","b","d","e"]
> splitOn "aaa"  "aaaXaaaXaaaXaaa"  == ["","X","X","X",""]
> splitOn "x"    "x"                == ["",""]

and

> intercalate s . splitOn s         == id
> splitOn (singleton c)             == split (==c)

(Note: the string s to split on above cannot be empty.)

This function is strict in its first argument, and lazy in its second.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m). 

From package text
splitOn :: Text -> Text -> [Text]

HLint

HLint checks and simplifies the Haskell source code! Install it with:

root #eselect repository enable haskell

root #emerge --sync haskell

root #emerge --ask dev-haskell/hlint

Editor plugins

Emacs

Haskell mode

The Haskell-Mode for (X)Emacs makes programming easier; for GNU Emacs it can be installed with:

root #emerge --ask app-emacs/haskell-mode

Or, for XEmacs:

root #emerge --ask app-xemacs/haskell-mode

Now it can be configured with M-x customize-group RET haskell RET.

ghc-mod

The official Gentoo Haskell ebuild repository provides an ebuild for ghc-mod, an extension of Haskell mode. Add the repository with:

root #eselect repository enable haskell

root #emerge --sync haskell

Then, install the ebuild:

root #emerge --ask app-emacs/ghc-mod

Haskell-Mode for Vim

There's also a Haskell-Mode for Vim.

Troubleshooting

Haskell ebuilds failing with out of memory error

When MAKEOPTS is set to allow parallel jobs, ghc may fail in Haskell ebuilds with ghc: failed to create OS thread: Cannot allocate memory. To fix this, lower the amount of jobs set in MAKEOPTS, or do not allow parallel jobs at all. MAKEOPTS can be overridden for failing ebuilds as described in Overriding environment variables per package.

External resources

• The #haskell (webchat) and #gentoo-haskell (webchat) channels on code.freenode.net.