Nixpkgs distributes build instructions for all Haskell packages registered on Hackage, but strangely enough normal Nix package lookups don't seem to discover any of them: $ nix-env -qa cabal-install error: selector ‘cabal-install’ matches no derivations $ nix-env -i ghc error: selector ‘ghc’ matches no derivations The Haskell package set is not registered in the top-level namespace because it is huge. If all Haskell packages were visible to these commands, then name-based search/install operations would be much slower than they are now. We avoided that by keeping all Haskell-related packages in a separate attribute set called haskellPackages, which the following command will list: $ nix-env -f "<nixpkgs>" -qaP -A haskellPackages haskellPackages.a50 a50-0.5 haskellPackages.abacate haskell-abacate-0.0.0.0 haskellPackages.abcBridge haskell-abcBridge-0.12 haskellPackages.afv afv-0.1.1 haskellPackages.alex alex-3.1.4 haskellPackages.Allure Allure-0.4.101.1 haskellPackages.alms alms-0.6.7 [... some 8000 entries omitted ...] To install any of those packages into your profile, refer to them by their attribute path (first column): $ nix-env -f "<nixpkgs>" -iA haskellPackages.Allure ... The attribute path of any Haskell packages corresponds to the name of that particular package on Hackage: the package cabal-install has the attribute haskellPackages.cabal-install, and so on. (Actually, this convention causes trouble with packages like 3dmodels and 4Blocks, because these names are invalid identifiers in the Nix language. The issue of how to deal with these rare corner cases is currently unresolved.) Haskell packages who's Nix name (second column) begins with a haskell- prefix are packages that provide a library whereas packages without that prefix provide just executables. Libraries may provide executables too, though: the package haskell-pandoc, for example, installs both a library and an application. You can install and use Haskell executables just like any other program in Nixpkgs, but using Haskell libraries for development is a bit trickier and we'll address that subject in great detail in section How to create a development environment. Attribute paths are deterministic inside of Nixpkgs, but the path necessary to reach Nixpkgs varies from system to system. We dodged that problem by giving nix-env an explicit -f "<nixpkgs>" parameter, but if you call nix-env without that flag, then chances are the invocation fails: $ nix-env -iA haskellPackages.cabal-install error: attribute ‘haskellPackages’ in selection path ‘haskellPackages.cabal-install’ not found On NixOS, for example, Nixpkgs does not exist in the top-level namespace by default. To figure out the proper attribute path, it's easiest to query for the path of a well-known Nixpkgs package, i.e.: $ nix-env -qaP coreutils nixos.coreutils coreutils-8.23 If your system responds like that (most NixOS installations will), then the attribute path to haskellPackages is nixos.haskellPackages. Thus, if you want to use nix-env without giving an explicit -f flag, then that's the way to do it: $ nix-env -qaP -A nixos.haskellPackages $ nix-env -iA nixos.haskellPackages.cabal-install Our current default compiler is GHC 7.10.x and the haskellPackages set contains packages built with that particular version. Nixpkgs contains the latest major release of every GHC since 6.10.4, however, and there is a whole family of package sets available that defines Hackage packages built with each of those compilers, too: $ nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc6123 $ nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc763 The name haskellPackages is really just a synonym for haskell.packages.ghc7101, because we prefer that package set internally and recommend it to our users as their default choice, but ultimately you are free to compile your Haskell packages with any GHC version you please. The following command displays the complete list of available compilers: $ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler haskell.compiler.ghc6104 ghc-6.10.4 haskell.compiler.ghc6123 ghc-6.12.3 haskell.compiler.ghc704 ghc-7.0.4 haskell.compiler.ghc722 ghc-7.2.2 haskell.compiler.ghc742 ghc-7.4.2 haskell.compiler.ghc763 ghc-7.6.3 haskell.compiler.ghc784 ghc-7.8.4 haskell.compiler.ghc7101 ghc-7.10.1 haskell.compiler.ghcHEAD ghc-7.11.20150402 haskell.compiler.ghcNokinds ghc-nokinds-7.11.20150704 haskell.compiler.ghcjs ghcjs-0.1.0 haskell.compiler.jhc jhc-0.8.2 haskell.compiler.uhc uhc-1.1.9.0 We have no package sets for jhc or uhc yet, unfortunately, but for every version of GHC listed above, there exists a package set based on that compiler. Also, the attributes haskell.compiler.ghcXYC and haskell.packages.ghcXYC.ghc are synonymous for the sake of convenience.

A simple development environment consists of a Haskell compiler and the tool cabal-install, and we saw in section How to install Haskell packages how you can install those programs into your user profile: $ nix-env -f "<nixpkgs>" -iA haskellPackages.ghc haskellPackages.cabal-install Instead of the default package set haskellPackages, you can also use the more precise name haskell.compiler.ghc7101, which has the advantage that it refers to the same GHC version regardless of what Nixpkgs considers "default" at any given time. Once you've made those tools available in $PATH, it's possible to build Hackage packages the same way people without access to Nix do it all the time: $ cabal get lens-4.11 && cd lens-4.11 $ cabal install -j --dependencies-only $ cabal configure $ cabal build If you enjoy working with Cabal sandboxes, then that's entirely possible too: just execute the command $ cabal sandbox init before installing the required dependencies. The nix-shell utility makes it easy to switch to a different compiler version; just enter the Nix shell environment with the command $ nix-shell -p haskell.compiler.ghc784 to bring GHC 7.8.4 into $PATH. Re-running cabal configure switches your build to use that compiler instead. If you're working on a project that doesn't depend on any additional system libraries outside of GHC, then it's sufficient even to run the cabal configure command inside of the shell: $ nix-shell -p haskell.compiler.ghc784 --command "cabal configure" Afterwards, all other commands like cabal build work just fine in any shell environment, because the configure phase recorded the absolute paths to all required tools like GHC in its build configuration inside of the dist/ directory. Please note, however, that nix-collect-garbage can break such an environment because the Nix store paths created by nix-shell aren't "alive" anymore once nix-shell has terminated. If you find that your Haskell builds no longer work after garbage collection, then you'll have to re-run cabal configure inside of a new nix-shell environment.

GHC expects to find all installed libraries inside of its own lib directory. This approach works fine on traditional Unix systems, but it doesn't work for Nix, because GHC's store path is immutable once it's built. We cannot install additional libraries into that location. As a consequence, our copies of GHC don't know any packages except their own core libraries, like base, containers, Cabal, etc. We can register additional libraries to GHC, however, using a special build function called ghcWithPackages. That function expects one argument: a function that maps from an attribute set of Haskell packages to a list of packages, which determines the libraries known to that particular version of GHC. For example, the Nix expression ghcWithPackages (pkgs: [pkgs.mtl]) generates a copy of GHC that has the mtl library registered in addition to its normal core packages: $ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])" [nix-shell:~]$ ghc-pkg list mtl /nix/store/zy79...-ghc-7.10.1/lib/ghc-7.10.1/package.conf.d: mtl-2.2.1 This function allows users to define their own development environment by means of an override. After adding the following snippet to ~/.nixpkgs/config.nix, { packageOverrides = super: let self = super.pkgs; in { myHaskellEnv = self.haskell.packages.ghc7101.ghcWithPackages (haskellPackages: with haskellPackages; [ # libraries arrows async cgi criterion # tools cabal-install haskintex ]); }; } it's possible to install that compiler with nix-env -f "<nixpkgs>" -iA myHaskellEnv. If you'd like to switch that development environment to a different version of GHC, just replace the ghc7101 bit in the previous definition with the appropriate name. Of course, it's also possible to define any number of these development environments! (You can't install two of them into the same profile at the same time, though, because that would result in file conflicts.) The generated ghc program is a wrapper script that re-directs the real GHC executable to use a new lib directory --- one that we specifically constructed to contain all those packages the user requested: $ cat $(type -p ghc) #! /nix/store/xlxj...-bash-4.3-p33/bin/bash -e export NIX_GHC=/nix/store/19sm...-ghc-7.10.1/bin/ghc export NIX_GHCPKG=/nix/store/19sm...-ghc-7.10.1/bin/ghc-pkg export NIX_GHC_DOCDIR=/nix/store/19sm...-ghc-7.10.1/share/doc/ghc/html export NIX_GHC_LIBDIR=/nix/store/19sm...-ghc-7.10.1/lib/ghc-7.10.1 exec /nix/store/j50p...-ghc-7.10.1/bin/ghc "-B$NIX_GHC_LIBDIR" "$@" The variables $NIX_GHC, $NIX_GHCPKG, etc. point to the new store path ghcWithPackages constructed specifically for this environment. The last line of the wrapper script then executes the real ghc, but passes the path to the new lib directory using GHC's -B flag. The purpose of those environment variables is to work around an impurity in the popular ghc-paths library. That library promises to give its users access to GHC's installation paths. Only, the library can't possible know that path when it's compiled, because the path GHC considers its own is determined only much later, when the user configures it through ghcWithPackages. So we patched ghc-paths to return the paths found in those environment variables at run-time rather than trying to guess them at compile-time. To make sure that mechanism works properly all the time, we recommend that you set those variables to meaningful values in your shell environment, too, i.e. by adding the following code to your ~/.bashrc: if type >/dev/null 2>&1 -p ghc; then eval "$(egrep ^export "$(type -p ghc)")" fi If you are certain that you'll use only one GHC environment which is located in your user profile, then you can use the following code, too, which has the advantage that it doesn't contain any paths from the Nix store, i.e. those settings always remain valid even if a nix-env -u operation updates the GHC environment in your profile: if [ -e ~/.nix-profile/bin/ghc ]; then export NIX_GHC="$HOME/.nix-profile/bin/ghc" export NIX_GHCPKG="$HOME/.nix-profile/bin/ghc-pkg" export NIX_GHC_DOCDIR="$HOME/.nix-profile/share/doc/ghc/html" export NIX_GHC_LIBDIR="$HOME/.nix-profile/lib/ghc-$($NIX_GHC --numeric-version)" fi

The easiest way to create an ad hoc development environment is to run nix-shell with the appropriate GHC environment given on the command-line: nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [mtl pandoc])" For more sophisticated use-cases, however, it's more convenient to save the desired configuration in a file called shell.nix that looks like this: { nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7101" }: let inherit (nixpkgs) pkgs; ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [ monad-par mtl ]); in pkgs.stdenv.mkDerivation { name = "my-haskell-env-0"; buildInputs = [ ghc ]; shellHook = "eval $(egrep ^export ${ghc}/bin/ghc)"; } Now run nix-shell --- or even nix-shell --pure --- to enter a shell environment that has the appropriate compiler in $PATH. If you use --pure, then add all other packages that your development environment needs into the buildInputs attribute. If you'd like to switch to a different compiler version, then pass an appropriate compiler argument to the expression, i.e. nix-shell --argstr compiler ghc784. If you need such an environment because you'd like to compile a Hackage package outside of Nix --- i.e. because you're hacking on the latest version from Git ---, then the package set provides suitable nix-shell environments for you already! Every Haskell package has an env attribute that provides a shell environment suitable for compiling that particular package. If you'd like to hack the lens library, for example, then you just have to check out the source code and enter the appropriate environment: $ cabal get lens-4.11 && cd lens-4.11 Downloading lens-4.11... Unpacking to lens-4.11/ $ nix-shell "<nixpkgs>" -A haskellPackages.lens.env [nix-shell:/tmp/lens-4.11]$ At point, you can run cabal configure, cabal build, and all the other development commands. Note that you need cabal-install installed in your $PATH already to use it here --- the nix-shell environment does not provide it.

If your own Haskell packages have build instructions for Cabal, then you can convert those automatically into build instructions for Nix using the cabal2nix utility, which you can install into your profile by running nix-env -i cabal2nix.

For example, let's assume that you're working on a private project called foo. To generate a Nix build expression for it, change into the project's top-level directory and run the command: $ cabal2nix . >foo.nix Then write the following snippet into a file called default.nix: { nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7101" }: nixpkgs.pkgs.haskell.packages.${compiler}.callPackage ./foo.nix { } Finally, store the following code in a file called shell.nix: { nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7101" }: (import ./default.nix { inherit nixpkgs compiler; }).env At this point, you can run nix-build to have Nix compile your project and install it into a Nix store path. The local directory will contain a symlink called result after nix-build returns that points into that location. Of course, passing the flag --argstr compiler ghc763 allows switching the build to any version of GHC currently supported. Furthermore, you can call nix-shell to enter an interactive development environment in which you can use cabal configure and cabal build to develop your code. That environment will automatically contain a proper GHC derivation with all the required libraries registered as well as all the system-level libraries your package might need. If your package does not depend on any system-level libraries, then it's sufficient to run $ nix-shell --command "cabal configure" once to set up your build. cabal-install determines the absolute paths to all resources required for the build and writes them into a config file in the dist/ directory. Once that's done, you can run cabal build and any other command for that project even outside of the nix-shell environment. This feature is particularly nice for those of us who like to edit their code with an IDE, like Emacs' haskell-mode, because it's not necessary to start Emacs inside of nix-shell just to make it find out the necessary settings for building the project; cabal-install has already done that for us. If you want to do some quick-and-dirty hacking and don't want to bother setting up a default.nix and shell.nix file manually, then you can use the --shell flag offered by cabal2nix to have it generate a stand-alone nix-shell environment for you. With that feature, running $ cabal2nix --shell . >shell.nix $ nix-shell --command "cabal configure" is usually enough to set up a build environment for any given Haskell package. You can even use that generated file to run nix-build, too: $ nix-build shell.nix

If you have multiple private Haskell packages that depend on each other, then you'll have to register those packages in the Nixpkgs set to make them visible for the dependency resolution performed by callPackage. First of all, change into each of your projects top-level directories and generate a default.nix file with cabal2nix: $ cd ~/src/foo && cabal2nix . >default.nix $ cd ~/src/bar && cabal2nix . >default.nix Then edit your ~/.nixpkgs/config.nix file to register those builds in the default Haskell package set: { packageOverrides = super: let self = super.pkgs; in { haskellPackages = super.haskellPackages.override { overrides = self: super: { foo = self.callPackage ../src/foo {}; bar = self.callPackage ../src/bar {}; }; }; }; } Once that's accomplished, nix-env -f "<nixpkgs>" -qA haskellPackages will show your packages like any other package from Hackage, and you can build them $ nix-build "<nixpkgs>" -A haskellPackages.foo or enter an interactive shell environment suitable for building them: $ nix-shell "<nixpkgs>" -A haskellPackages.bar.env

Every Haskell package set takes a function called overrides that you can use to manipulate the package as much as you please. One useful application of this feature is to replace the default mkDerivation function with one that enables library profiling for all packages. To accomplish that, add configure the following snippet in your ~/.nixpkgs/config.nix file: { packageOverrides = super: let self = super.pkgs; in { profiledHaskellPackages = self.haskellPackages.override { overrides = self: super: { mkDerivation = args: super.mkDerivation (args // { enableLibraryProfiling = true; }); }; }; }; } Then, replace instances of haskellPackages in the cabal2nix-generated default.nix or shell.nix files with profiledHaskellPackages.

Nixpkgs provides the latest version of ghc-events, which is 0.4.4.0 at the time of this writing. This is fine for users of GHC 7.10.x, but GHC 7.8.4 cannot compile that binary. Now, one way to solve that problem is to register an older version of ghc-events in the 7.8.x-specific package set. The first step is to generate Nix build instructions with cabal2nix: $ cabal2nix cabal://ghc-events-0.4.3.0 >~/.nixpkgs/ghc-events-0.4.3.0.nix Then add the override in ~/.nixpkgs/config.nix: { packageOverrides = super: let self = super.pkgs; in { haskell = super.haskell // { packages = super.haskell.packages // { ghc784 = super.haskell.packages.ghc784.override { overrides = self: super: { ghc-events = self.callPackage ./ghc-events-0.4.3.0.nix {}; }; }; }; }; }; } This code is a little crazy, no doubt, but it's necessary because the intuitive version haskell.packages.ghc784 = super.haskell.packages.ghc784.override { overrides = self: super: { ghc-events = self.callPackage ./ghc-events-0.4.3.0.nix {}; }; }; doesn't do what we want it to: that code replaces the haskell package set in Nixpkgs with one that contains only one entry,packages, which contains only one entry ghc784. This override loses the haskell.compiler set, and it loses the haskell.packages.ghcXYZ sets for all compilers but GHC 7.8.4. To avoid that problem, we have to perform the convoluted little dance from above, iterating over each step in hierarchy. Once it's accomplished, however, we can install a variant of ghc-events that's compiled with GHC 7.8.4: nix-env -f "<nixpkgs>" -iA haskell.packages.ghc784.ghc-events Unfortunately, it turns out that this build fails again while executing the test suite! Apparently, the release archive on Hackage is missing some data files that the test suite requires, so we cannot run it. We accomplish that by re-generating the Nix expression with the --no-check flag: $ cabal2nix --no-check cabal://ghc-events-0.4.3.0 >~/.nixpkgs/ghc-events-0.4.3.0.nix Now the builds succeeds. Of course, in the concrete example of ghc-events this whole exercise is not an ideal solution, because ghc-events can analyze the output emitted by any version of GHC later than 6.12 regardless of the compiler version that was used to build the `ghc-events' executable, so strictly speaking there's no reason to prefer one built with GHC 7.8.x in the first place. However, for users who cannot use GHC 7.10.x at all for some reason, the approach of downgrading to an older version might be useful.

GHC and distributed build farms don't get along well: https://ghc.haskell.org/trac/ghc/ticket/4012 When you see an error like this one package foo-0.7.1.0 is broken due to missing package text-1.2.0.4-98506efb1b9ada233bb5c2b2db516d91 then you have to download and re-install foo and all its dependents from scratch: # nix-store -q --referrers /nix/store/*-haskell-text-1.2.0.4 \ | xargs -L 1 nix-store --repair-path --option binary-caches http://hydra.nixos.org If you're using additional Hydra servers other than hydra.nixos.org, then it might be necessary to purge the local caches that store data from those machines to disable these binary channels for the duration of the previous command, i.e. by running: rm /nix/var/nix/binary-cache-v3.sqlite rm /nix/var/nix/manifests/* rm /nix/var/nix/channel-cache/*

Users of GHC on Darwin have occasionally reported that builds fail, because the compiler complains about a missing include file: fatal error: 'math.h' file not found The issue has been discussed at length in ticket 6390, and so far no good solution has been proposed. As a work-around, users who run into this problem can configure the environment variables export NIX_CFLAGS_COMPILE="-idirafter /usr/include" export NIX_CFLAGS_LINK="-L/usr/lib" in their ~/.bashrc file to avoid the compiler error.

The Youtube video Nix Loves Haskell provides an introduction into Haskell NG aimed at beginners. The slides are available at http://cryp.to/nixos-meetup-3-slides.pdf and also -- in a form ready for cut & paste -- at https://github.com/NixOS/cabal2nix/blob/master/doc/nixos-meetup-3-slides.md. Another Youtube video is Escaping Cabal Hell with Nix, which discusses the subject of Haskell development with Nix but also provides a basic introduction to Nix as well, i.e. it's suitable for viewers with almost no prior Nix experience. Oliver Charles wrote a very nice Tutorial how to develop Haskell packages with Nix. The Journey into the Haskell NG infrastructure series of postings describe the new Haskell infrastructure in great detail: Part 1 explains the differences between the old and the new code and gives instructions how to migrate to the new setup. Part 2 looks in-depth at how to tweak and configure your setup by means of overrides. Part 3 describes the infrastructure that keeps the Haskell package set in Nixpkgs up-to-date.