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title	author	date
User's Guide for Haskell in Nixpkgs	Peter Simons	2015-06-01

Haskell

How to install Haskell packages

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, except for the default version of ghc, cabal-install, and stack:

$ nix-env -i alex
error: selector ‘alex’ matches no derivations
$ nix-env -qa ghc
ghc-7.10.2

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.AAI                                             AAI-0.2.0.1
haskellPackages.abacate                                         abacate-0.0.0.0
haskellPackages.abc-puzzle                                      abc-puzzle-0.2.1
haskellPackages.abcBridge                                       abcBridge-0.15
haskellPackages.abcnotation                                     abcnotation-1.9.0
haskellPackages.abeson                                          abeson-0.1.0.1
[... some 14000 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 whose 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 8.8.x and the haskellPackages set contains packages built with that particular version. Nixpkgs contains the last three major releases of GHC 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.ghc865
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc8101

The name haskellPackages is really just a synonym for haskell.packages.ghc882, 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.ghc8101                 ghc-8.10.1
haskell.compiler.integer-simple.ghc8101  ghc-8.10.1
haskell.compiler.ghcHEAD                 ghc-8.11.20200505
haskell.compiler.integer-simple.ghcHEAD  ghc-8.11.20200505
haskell.compiler.ghc822Binary            ghc-8.2.2-binary
haskell.compiler.ghc844                  ghc-8.4.4
haskell.compiler.ghc863Binary            ghc-8.6.3-binary
haskell.compiler.ghc865                  ghc-8.6.5
haskell.compiler.integer-simple.ghc865   ghc-8.6.5
haskell.compiler.ghc882                  ghc-8.8.2
haskell.compiler.integer-simple.ghc882   ghc-8.8.2
haskell.compiler.ghc883                  ghc-8.8.3
haskell.compiler.integer-simple.ghc883   ghc-8.8.3
haskell.compiler.ghcjs                   ghcjs-8.6.0.1

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.

How to create a development environment

How to install a compiler

A simple development environment consists of a Haskell compiler and one or both of the tools cabal-install and stack. 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.ghc7102, 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. Alternatively, you can use Stack instead of nix-shell directly to select compiler versions and other build tools per-project. It uses nix-shell under the hood when Nix support is turned on. See How to build a Haskell project using Stack.

If you're using cabal-install, re-running cabal configure inside the spawned shell 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 even sufficient to just 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.

How to install a compiler with libraries

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-7.10.2/lib/ghc-7.10.2/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 ~/.config/nixpkgs/config.nix,

{
  packageOverrides = super: let self = super.pkgs; in
  {
    myHaskellEnv = self.haskell.packages.ghc7102.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 ghc7102 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 $(type -p ghc)
#! /nix/store/xlxj...-bash-4.3-p33/bin/bash -e
export NIX_GHC=/nix/store/19sm...-ghc-7.10.2/bin/ghc
export NIX_GHCPKG=/nix/store/19sm...-ghc-7.10.2/bin/ghc-pkg
export NIX_GHC_DOCDIR=/nix/store/19sm...-ghc-7.10.2/share/doc/ghc/html
export NIX_GHC_LIBDIR=/nix/store/19sm...-ghc-7.10.2/lib/ghc-7.10.2
exec /nix/store/j50p...-ghc-7.10.2/bin/ghc "-B$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

How to install a compiler with libraries, hoogle and documentation indexes

If you plan to use your environment for interactive programming, not just compiling random Haskell code, you might want to replace ghcWithPackages in all the listings above with ghcWithHoogle.

This environment generator not only produces an environment with GHC and all the specified libraries, but also generates a hoogle and haddock indexes for all the packages, and provides a wrapper script around hoogle binary that uses all those things. A precise name for this thing would be "ghcWithPackagesAndHoogleAndDocumentationIndexes", which is, regrettably, too long and scary.

For example, installing the following environment

{
  packageOverrides = super: let self = super.pkgs; in
  {
    myHaskellEnv = self.haskellPackages.ghcWithHoogle
                     (haskellPackages: with haskellPackages; [
                       # libraries
                       arrows async cgi criterion
                       # tools
                       cabal-install haskintex
                     ]);
  };
}

allows one to browse module documentation index not too dissimilar to this for all the specified packages and their dependencies by directing a browser of choice to ~/.nix-profile/share/doc/hoogle/index.html (or /run/current-system/sw/share/doc/hoogle/index.html in case you put it in environment.systemPackages in NixOS).

After you've marveled enough at that try adding the following to your ~/.ghc/ghci.conf

:def hoogle \s -> return $ ":! hoogle search -cl --count=15 \"" ++ s ++ "\""
:def doc \s -> return $ ":! hoogle search -cl --info \"" ++ s ++ "\""

and test it by typing into ghci:

:hoogle a -> a
:doc a -> a

Be sure to note the links to haddock files in the output. With any modern and properly configured terminal emulator you can just click those links to navigate there.

Finally, you can run

hoogle server --local -p 8080

and navigate to http://localhost:8080/ for your own local Hoogle. The --local flag makes the hoogle server serve files from your nix store over http, without the flag it will use file:// URIs. Note, however, that Firefox and possibly other browsers disallow navigation from http:// to file:// URIs for security reasons, which might be quite an inconvenience. Versions before v5 did not have this flag. See this page for workarounds.

For NixOS users there's a service which runs this exact command for you. Specify the packages you want documentation for and the haskellPackages set you want them to come from. Add the following to configuration.nix.

services.hoogle = {
  enable = true;
  packages = (hpkgs: with hpkgs; [text cryptonite]);
  haskellPackages = pkgs.haskellPackages;
};

How to install haskell-language-server

In short: Install pkgs.haskell-language-server and use the haskell-language-server-wrapper command to run it. See the hls README on how to configure your text editor to use hls and how to test your setup.

Hls needs to be compiled with the ghc version of the project you use it on.

pkgs.haskell-language-server provides haskell-language-server-wrapper, haskell-language-server, haskell-language-server-x.x and haskell-language-server-x.x.x binaries, where x.x.x is the ghc version for which it is compiled. By default it includes binaries for all ghc versions that are provided in the binary caches. You can override that list with e.g.

pkgs.haskell-language-server.override { supportedGhcVersions = [ "884" "901" ]; }

When you run haskell-language-server-wrapper it will detect the ghc version used by the project you are working on (by asking e.g. cabal or stack) and pick the appropriate above mentioned binary from your path.

Be careful when installing hls globally and using a pinned nixpkgs for a Haskell project in a nix-shell. If the nixpkgs versions deviate to much (e.g. use different glibc versions) hls might fail. It is recommended to then install hls in the nix-shell from the nixpkgs version pinned in there.

If you know, that you only use one ghc version, e.g. in a project specific nix-shell You can either use an override as given above or simply install pkgs.haskellPackages.haskell-language-server instead of the top-level attribute pkgs.haskell-language-server.

How to build a Haskell project using Stack

Stack is a popular build tool for Haskell projects. It has first-class support for Nix. Stack can optionally use Nix to automatically select the right version of GHC and other build tools to build, test and execute apps in an existing project downloaded from somewhere on the Internet. Pass the --nix flag to any stack command to do so, e.g.

git clone --recurse-submodules https://github.com/yesodweb/wai.git
cd wai
stack --nix build

If you want stack to use Nix by default, you can add a nix section to the stack.yaml file, as explained in the Stack documentation. For example:

nix:
  enable: true
  packages: [pkgconfig zeromq zlib]

The example configuration snippet above tells Stack to create an ad hoc environment for nix-shell as in the below section, in which the pkgconfig, zeromq and zlib packages from Nixpkgs are available. All stack commands will implicitly be executed inside this ad hoc environment.

Some projects have more sophisticated needs. For examples, some ad hoc environments might need to expose Nixpkgs packages compiled in a certain way, or with extra environment variables. In these cases, you'll need a shell field instead of packages:

nix:
  enable: true
  shell-file: shell.nix

For more on how to write a shell.nix file see the below section. You'll need to express a derivation. Note that Nixpkgs ships with a convenience wrapper function around mkDerivation called haskell.lib.buildStackProject to help you create this derivation in exactly the way Stack expects. However for this to work you need to disable the sandbox, which you can do by using --option sandbox relaxed or --option sandbox false to the Nix command. All of the same inputs as mkDerivation can be provided. For example, to build a Stack project that including packages that link against a version of the R library compiled with special options turned on:

with (import <nixpkgs> { });

let R = pkgs.R.override { enableStrictBarrier = true; };
in
haskell.lib.buildStackProject {
  name = "HaskellR";
  buildInputs = [ R zeromq zlib ];
}

You can select a particular GHC version to compile with by setting the ghc attribute as an argument to buildStackProject. Better yet, let Stack choose what GHC version it wants based on the snapshot specified in stack.yaml (only works with Stack >= 1.1.3):

{nixpkgs ? import <nixpkgs> { }, ghc ? nixpkgs.ghc}:

with nixpkgs;

let R = pkgs.R.override { enableStrictBarrier = true; };
in
haskell.lib.buildStackProject {
  name = "HaskellR";
  buildInputs = [ R zeromq zlib ];
  inherit ghc;
}

How to create ad hoc environments for `nix-shell`

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 ? "ghc7102" }:
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.

How to create Nix builds for your own private Haskell packages

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.

How to build a stand-alone project

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 ? "ghc7102" }:
nixpkgs.pkgs.haskell.packages.${compiler}.callPackage ./foo.nix { }

Finally, store the following code in a file called shell.nix:

{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
(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

How to build projects that depend on each other

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 ~/.config/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

Miscellaneous Topics

How to build with profiling enabled

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 the following snippet to your ~/.config/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.

How to override package versions in a compiler-specific package set

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 ~/.config/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.

How to override packages in all compiler-specific package sets

In the previous section we learned how to override a package in a single compiler-specific package set. You may have some overrides defined that you want to use across multiple package sets. To accomplish this you could use the technique that we learned in the previous section by repeating the overrides for all the compiler-specific package sets. For example:

{
  packageOverrides = super: let self = super.pkgs; in
  {
    haskell = super.haskell // {
      packages = super.haskell.packages // {
        ghc784 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            my-package = ...;
            my-other-package = ...;
          };
        };
        ghc822 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            my-package = ...;
            my-other-package = ...;
          };
        };
        ...
      };
    };
  };
}

However there's a more convenient way to override all compiler-specific package sets at once:

{
  packageOverrides = super: let self = super.pkgs; in
  {
    haskell = super.haskell // {
      packageOverrides = self: super: {
        my-package = ...;
        my-other-package = ...;
      };
    };
  };
}

How to specify source overrides for your Haskell package

When starting a Haskell project you can use developPackage to define a derivation for your package at the root path as well as source override versions for Hackage packages, like so:

# default.nix
{ compilerVersion ? "ghc842" }:
let
  # pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
  pkgs = import (fetchGit (import ./version.nix)) { };
  compiler = pkgs.haskell.packages."${compilerVersion}";
  pkg = compiler.developPackage {
    root = ./.;
    source-overrides = {
      # Let's say the GHC 8.4.2 haskellPackages uses 1.6.0.0 and your test suite is incompatible with >= 1.6.0.0
      HUnit = "1.5.0.0";
    };
  };
in pkg

This could be used in place of a simplified stack.yaml defining a Nix derivation for your Haskell package.

As you can see this allows you to specify only the source version found on Hackage and nixpkgs will take care of the rest.

You can also specify buildInputs for your Haskell derivation for packages that directly depend on external libraries like so:

# default.nix
{ compilerVersion ? "ghc842" }:
let
  # pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
  pkgs = import (fetchGit (import ./version.nix)) { };
  compiler = pkgs.haskell.packages."${compilerVersion}";
  pkg = compiler.developPackage {
    root = ./.;
    source-overrides = {
      HUnit = "1.5.0.0"; # Let's say the GHC 8.4.2 haskellPackages uses 1.6.0.0 and your test suite is incompatible with >= 1.6.0.0
    };
  };
  # in case your package source depends on any libraries directly, not just transitively.
  buildInputs = [ zlib ];
in pkg.overrideAttrs(attrs: {
  buildInputs = attrs.buildInputs ++ buildInputs;
})

Notice that you will need to override (via overrideAttrs or similar) the derivation returned by the developPackage Nix lambda as there is no buildInputs named argument you can pass directly into the developPackage lambda.

How to recover from GHC's infamous non-deterministic library ID bug

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

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 ~/.cache/nix/binary-cache*.sqlite

Builds on Darwin fail with `math.h` not found

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.

Builds using Stack complain about missing system libraries

--  While building package zlib-0.5.4.2 using:
  runhaskell -package=Cabal-1.22.4.0 -clear-package-db [... lots of flags ...]
Process exited with code: ExitFailure 1
Logs have been written to: /home/foo/src/stack-ide/.stack-work/logs/zlib-0.5.4.2.log

Configuring zlib-0.5.4.2...
Setup.hs: Missing dependency on a foreign library:
* Missing (or bad) header file: zlib.h
This problem can usually be solved by installing the system package that
provides this library (you may need the "-dev" version). If the library is
already installed but in a non-standard location then you can use the flags
--extra-include-dirs= and --extra-lib-dirs= to specify where it is.
If the header file does exist, it may contain errors that are caught by the C
compiler at the preprocessing stage. In this case you can re-run configure
with the verbosity flag -v3 to see the error messages.

When you run the build inside of the nix-shell environment, the system is configured to find libz.so without any special flags -- the compiler and linker "just know" how to find it. Consequently, Cabal won't record any search paths for libz.so in the package description, which means that the package works fine inside of nix-shell, but once you leave the shell the shared object can no longer be found. That issue is by no means specific to Stack: you'll have that problem with any other Haskell package that's built inside of nix-shell but run outside of that environment.

You can remedy this issue in several ways. The easiest is to add a nix section to the stack.yaml like the following:

nix:
  enable: true
  packages: [ zlib ]

Stack's Nix support knows to add ${zlib.out}/lib and ${zlib.dev}/include as an --extra-lib-dirs and extra-include-dirs, respectively. Alternatively, you can achieve the same effect by hand. First of all, run

$ nix-build --no-out-link "<nixpkgs>" -A zlib
/nix/store/alsvwzkiw4b7ip38l4nlfjijdvg3fvzn-zlib-1.2.8

to find out the store path of the system's zlib library. Now, you can

add that path (plus a "/lib" suffix) to your $LD_LIBRARY_PATH environment variable to make sure your system linker finds libz.so automatically. It's no pretty solution, but it will work.
As a variant of (1), you can also install any number of system libraries into your user's profile (or some other profile) and point $LD_LIBRARY_PATH to that profile instead, so that you don't have to list dozens of those store paths all over the place.
The solution I prefer is to call stack with an appropriate --extra-lib-dirs flag like so: shell stack --extra-lib-dirs=/nix/store/alsvwzkiw4b7ip38l4nlfjijdvg3fvzn-zlib-1.2.8/lib build

Typically, you'll need --extra-include-dirs as well. It's possible to add those flag to the project's stack.yaml or your user's global ~/.stack/global/stack.yaml file so that you don't have to specify them manually every time. But again, you're likely better off using Stack's Nix support instead.

The same thing applies to cabal configure, of course, if you're building with cabal-install instead of Stack.

Creating statically linked binaries

There are two levels of static linking. The first option is to configure the build with the Cabal flag --disable-executable-dynamic. In Nix expressions, this can be achieved by setting the attribute:

enableSharedExecutables = false;

That gives you a binary with statically linked Haskell libraries and dynamically linked system libraries.

To link both Haskell libraries and system libraries statically, the additional flags --ghc-option=-optl=-static --ghc-option=-optl=-pthread need to be used. In Nix, this is accomplished with:

configureFlags = [ "--ghc-option=-optl=-static" "--ghc-option=-optl=-pthread" ];

It's important to realize, however, that most system libraries in Nix are built as shared libraries only, i.e. there is just no static library available that Cabal could link!

Building GHC with integer-simple

By default GHC implements the Integer type using the GNU Multiple Precision Arithmetic (GMP) library. The implementation can be found in the integer-gmp package.

A potential problem with this is that GMP is licensed under the GNU Lesser General Public License (LGPL), a kind of "copyleft" license. According to the terms of the LGPL, paragraph 5, you may distribute a program that is designed to be compiled and dynamically linked with the library under the terms of your choice (i.e., commercially) but if your program incorporates portions of the library, if it is linked statically, then your program is a "derivative"--a "work based on the library"--and according to paragraph 2, section c, you "must cause the whole of the work to be licensed" under the terms of the LGPL (including for free).

The LGPL licensing for GMP is a problem for the overall licensing of binary programs compiled with GHC because most distributions (and builds) of GHC use static libraries. (Dynamic libraries are currently distributed only for macOS.) The LGPL licensing situation may be worse: even though The Glasgow Haskell Compiler License is essentially a "free software" license (BSD3), according to paragraph 2 of the LGPL, GHC must be distributed under the terms of the LGPL!

To work around these problems GHC can be build with a slower but LGPL-free alternative implementation for Integer called integer-simple.

To get a GHC compiler build with integer-simple instead of integer-gmp use the attribute: haskell.compiler.integer-simple."${ghcVersion}". For example:

$ nix-build -E '(import <nixpkgs> {}).haskell.compiler.integer-simple.ghc802'
...
$ result/bin/ghc-pkg list | grep integer
    integer-simple-0.1.1.1

The following command displays the complete list of GHC compilers build with integer-simple:

$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler.integer-simple
haskell.compiler.integer-simple.ghc7102  ghc-7.10.2
haskell.compiler.integer-simple.ghc7103  ghc-7.10.3
haskell.compiler.integer-simple.ghc722   ghc-7.2.2
haskell.compiler.integer-simple.ghc742   ghc-7.4.2
haskell.compiler.integer-simple.ghc783   ghc-7.8.3
haskell.compiler.integer-simple.ghc784   ghc-7.8.4
haskell.compiler.integer-simple.ghc801   ghc-8.0.1
haskell.compiler.integer-simple.ghc802   ghc-8.0.2
haskell.compiler.integer-simple.ghcHEAD  ghc-8.1.20170106

To get a package set supporting integer-simple use the attribute: haskell.packages.integer-simple."${ghcVersion}". For example use the following to get the scientific package build with integer-simple:

nix-build -A haskell.packages.integer-simple.ghc802.scientific

Quality assurance

The haskell.lib library includes a number of functions for checking for various imperfections in Haskell packages. It's useful to apply these functions to your own Haskell packages and integrate that in a Continuous Integration server like hydra to assure your packages maintain a minimum level of quality. This section discusses some of these functions.

failOnAllWarnings

Applying haskell.lib.failOnAllWarnings to a Haskell package enables the -Wall and -Werror GHC options to turn all warnings into build failures.

buildStrictly

Applying haskell.lib.buildStrictly to a Haskell package calls failOnAllWarnings on the given package to turn all warnings into build failures. Additionally the source of your package is gotten from first invoking cabal sdist to ensure all needed files are listed in the Cabal file.

checkUnusedPackages

Applying haskell.lib.checkUnusedPackages to a Haskell package invokes the packunused tool on the package. packunused complains when it finds packages listed as build-depends in the Cabal file which are redundant. For example:

$ nix-build -E 'let pkgs = import <nixpkgs> {}; in pkgs.haskell.lib.checkUnusedPackages {} pkgs.haskellPackages.scientific'
these derivations will be built:
  /nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv
...
detected package components
~~~~~~~~~~~~~~~~~~~~~~~~~~~

 - library
 - testsuite(s): test-scientific
 - benchmark(s): bench-scientific*

(component names suffixed with '*' are not configured to be built)

library
~~~~~~~

The following package dependencies seem redundant:

 - ghc-prim-0.5.0.0

testsuite(test-scientific)
~~~~~~~~~~~~~~~~~~~~~~~~~~

no redundant packages dependencies found

builder for ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv’ failed with exit code 1
error: build of ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv’ failed

As you can see, packunused finds out that although the testsuite component has no redundant dependencies the library component of scientific-0.3.5.1 depends on ghc-prim which is unused in the library.

Using hackage2nix with nixpkgs

Hackage package derivations are found in the hackage-packages.nix file within nixpkgs and are used as the initial package set for haskellPackages. The hackage-packages.nix file is not meant to be edited by hand, but rather autogenerated by hackage2nix, which by default uses the configuration-hackage2nix.yaml file to generate all the derivations.

To modify the contents configuration-hackage2nix.yaml, follow the instructions on hackage2nix.

Other resources

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.

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