Frequently Asked Questions

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 <http://hackage.haskell.org/package/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:

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

  1. 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.
  2. 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.
  3. 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

How to get an environment with GHC and LLVM

By default LLVM is not included with GHC unless it is required for native code generation:

$ nix-shell -p 'haskellPackages.ghcWithPackages (ps: with ps; [mtl])'

[nix-shell:~]$ ghc -fllvm Main.hs
<no location info>: error:
    Warning: Couldn't figure out LLVM version!
             Make sure you have installed LLVM 9
ghc: could not execute: opt

[nix-shell:~]$ llvm-ar
llvm-ar: command not found

To include LLVM in GHC you have to override the expression:

$ nix-shell -p '(haskellPackages.ghcWithPackages (ps: with ps; [mtl])).override { withLLVM = true; }'

[nix-shell:~]$ ghc -fllvm Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...

However LLVM will still not be available in the environment:

[nix-shell:~]$ llvm-ar
llvm-ar: command not found

The solution is to add it as another package to the environment:

$ nix-shell -p '(haskellPackages.ghcWithPackages (ps: with ps; [mtl])).override { withLLVM = true; }' 'haskellPackages.llvmPackages.llvm'

[nix-shell:~]$ llvm-ar --help
OVERVIEW: LLVM Archiver
...

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.