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doc: add user's guide for the Haskell infrastructure

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Resolves https://github.com/NixOS/nixpkgs/issues/4941.
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Peter Simons 2015-06-05 00:14:58 +02:00
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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="users-guide-to-the-haskell-infrastructure">
<title>User's Guide to the Haskell Infrastructure</title>
<section xml:id="how-to-install-haskell-packages">
<title>How to install Haskell packages</title>
<para>
Nixpkgs distributes build instructions for all Haskell packages
registered on
<link xlink:href="http://hackage.haskell.org/">Hackage</link>, but
strangely enough normal Nix package lookups don't seem to discover
any of them:
</para>
<programlisting>
$ nix-env -qa cabal-install
error: selector ‘cabal-install’ matches no derivations
$ nix-env -i ghc
error: selector ‘ghc’ matches no derivations
</programlisting>
<para>
The Haskell package set is not registered in the top-level namespace
because it is <emphasis>huge</emphasis>. 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 <literal>haskellPackages</literal>, which the following
command will list:
</para>
<programlisting>
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -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 ...]
</programlisting>
<para>
To install any of those packages into your profile, refer to them by
their attribute path (first column):
</para>
<programlisting>
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA haskellPackages.Allure ...
</programlisting>
<para>
The attribute path of any Haskell packages corresponds to the name
of that particular package on Hackage: the package
<literal>cabal-install</literal> has the attribute
<literal>haskellPackages.cabal-install</literal>, and so on.
(Actually, this convention causes trouble with packages like
<literal>3dmodels</literal> and <literal>4Blocks</literal>, because
these names are invalid identifiers in the Nix language. The issue
of how to deal with these rare corner cases is currently
unresolved.)
</para>
<para>
Haskell packages who's Nix name (second column) begins with a
<literal>haskell-</literal> prefix are packages that provide a
library whereas packages without that prefix provide just
executables. Libraries may provide executables too, though: the
package <literal>haskell-pandoc</literal>, 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
<link linkend="how-to-create-a-development-environment">How to
create a development environment</link>.
</para>
<para>
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 <literal>nix-env</literal> an explicit
<literal>-f &quot;&lt;nixpkgs&gt;&quot;</literal> parameter, but if
you call <literal>nix-env</literal> without that flag, then chances
are the invocation fails:
</para>
<programlisting>
$ nix-env -iA haskellPackages.cabal-install
error: attribute ‘haskellPackages’ in selection path
‘haskellPackages.cabal-install’ not found
</programlisting>
<para>
On NixOS, for example, Nixpkgs does <emphasis>not</emphasis> 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.:
</para>
<programlisting>
$ nix-env -qaP coreutils
nixos.pkgs.coreutils coreutils-8.23
</programlisting>
<para>
If your system responds like that (most NixOS installatios will),
then the attribute path to <literal>haskellPackages</literal> is
<literal>nixos.pkgs.haskellPackages</literal>. Thus, if you want to
use <literal>nix-env</literal> without giving an explicit
<literal>-f</literal> flag, then that's the way to do it:
</para>
<programlisting>
$ nix-env -qaP -A nixos.pkgs.haskellPackages
$ nix-env -iA nixos.pkgs.haskellPackages.cabal-install
</programlisting>
<para>
Our current default compiler is GHC 7.10.x and the
<literal>haskellPackages</literal> 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:
</para>
<programlisting>
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -qaP -A haskell.packages.ghc6123
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -qaP -A haskell.packages.ghc763
</programlisting>
<para>
The name <literal>haskellPackages</literal> is really just a synonym
for <literal>haskell.packages.ghc7101</literal>, 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:
</para>
<programlisting>
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -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.ghcjs ghcjs-0.1.0
haskell.compiler.jhc jhc-0.8.2
haskell.compiler.uhc uhc-1.1.9.0
</programlisting>
<para>
We have no package sets for <literal>jhc</literal> or
<literal>uhc</literal> yet, unfortunately, but for every version of
GHC listed above, there exists a package set based on that compiler.
Also, the attributes <literal>haskell.compiler.ghcXYC</literal> and
<literal>haskell.packages.ghcXYC.ghc</literal> are synonymous for
the sake of convenience.
</para>
</section>
<section xml:id="how-to-create-a-development-environment">
<title>How to create a development environment</title>
<section xml:id="how-to-install-a-compiler">
<title>How to install a compiler</title>
<para>
A simple development environment consists of a Haskell compiler
and the tool <literal>cabal-install</literal>, and we saw in
section <link linkend="how-to-install-haskell-packages">How to
install Haskell packages</link> how you can install those programs
into your user profile:
</para>
<programlisting>
$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA haskellPackages.ghc haskellPackages.cabal-install
</programlisting>
<para>
Instead of the default package set
<literal>haskellPackages</literal>, you can also use the more
precise name <literal>haskell.compiler.ghc7101</literal>, which
has the advantage that it refers to the same GHC version
regardless of what Nixpkgs considers &quot;default&quot; at any
given time.
</para>
<para>
Once you've made those tools available in
<literal>$PATH</literal>, it's possible to build Hackage packages
the same way people without access to Nix do it all the time:
</para>
<programlisting>
$ cabal get lens-4.11 &amp;&amp; cd lens-4.11
$ cabal install -j --dependencies-only
$ cabal configure
$ cabal build
</programlisting>
<para>
If you enjoy working with Cabal sandboxes, then that's entirely
possible too: just execute the command
</para>
<programlisting>
$ cabal sandbox init
</programlisting>
<para>
before installing the required dependencies.
</para>
<para>
The <literal>nix-shell</literal> utility makes it easy to switch
to a different compiler version; just enter the Nix shell
environment with the command
</para>
<programlisting>
$ nix-shell -p haskell.compiler.ghc784
</programlisting>
<para>
to bring GHC 7.8.4 into <literal>$PATH</literal>. Re-running
<literal>cabal configure</literal> 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 <literal>cabal configure</literal>
command inside of the shell:
</para>
<programlisting>
$ nix-shell -p haskell.compiler.ghc784 --command &quot;cabal configure&quot;
</programlisting>
<para>
Afterwards, all other commands like <literal>cabal build</literal>
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 <literal>dist/</literal>
directory. Please note, however, that
<literal>nix-collect-garbage</literal> can break such an
environment because the Nix store paths created by
<literal>nix-shell</literal> aren't &quot;alive&quot; anymore once
<literal>nix-shell</literal> has terminated. If you find that your
Haskell builds no longer work after garbage collection, then
you'll have to re-run <literal>cabal configure</literal> inside of
a new <literal>nix-shell</literal> environment.
</para>
</section>
<section xml:id="how-to-install-a-compiler-with-libraries">
<title>How to install a compiler with libraries</title>
<para>
GHC expects to find all installed libraries inside of its own
<literal>lib</literal> 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 <literal>base</literal>,
<literal>containers</literal>, <literal>Cabal</literal>, etc.
</para>
<para>
We can register additional libraries to GHC, however, using a
special build function called <literal>ghcWithPackages</literal>.
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
<literal>ghcWithPackages (pkgs: [pkgs.mtl])</literal> generates a
copy of GHC that has the <literal>mtl</literal> library registered
in addition to its normal core packages:
</para>
<programlisting>
$ nix-shell -p &quot;haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])&quot;
[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
</programlisting>
<para>
This function allows users to define their own development
environment by means of an override. After adding the following
snippet to <literal>~/.nixpkgs/config.nix</literal>,
</para>
<programlisting>
{
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
]);
};
}
</programlisting>
<para>
it's possible to install that compiler with
<literal>nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA myHaskellEnv</literal>.
If you'd like to switch that development environment to a
different version of GHC, just replace the
<literal>ghc7101</literal> 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.)
</para>
<para>
The generated <literal>ghc</literal> program is a wrapper script
that re-directs the real GHC executable to use a new
<literal>lib</literal> directory --- one that we specifically
constructed to contain all those packages the user requested:
</para>
<programlisting>
$ cat $(type -p ghc)
#! /nix/store/xlxj...-bash-4.3-p33/bin/bash -e
export NIX_GHC=/nix/store/19sm...-ghc-7.10.1/bin/ghc
export NIX_GHCPKG=/nix/store/19sm...-ghc-7.10.1/bin/ghc-pkg
export NIX_GHC_DOCDIR=/nix/store/19sm...-ghc-7.10.1/share/doc/ghc/html
export NIX_GHC_LIBDIR=/nix/store/19sm...-ghc-7.10.1/lib/ghc-7.10.1
exec /nix/store/j50p...-ghc-7.10.1/bin/ghc &quot;-B$NIX_GHC_LIBDIR&quot; &quot;$@&quot;
</programlisting>
<para>
The variables <literal>$NIX_GHC</literal>,
<literal>$NIX_GHCPKG</literal>, etc. point to the
<emphasis>new</emphasis> store path
<literal>ghcWithPackages</literal> constructed specifically for
this environment. The last line of the wrapper script then
executes the real <literal>ghc</literal>, but passes the path to
the new <literal>lib</literal> directory using GHC's
<literal>-B</literal> flag.
</para>
<para>
The purpose of those environment variables is to work around an
impurity in the popular
<link xlink:href="http://hackage.haskell.org/package/ghc-paths">ghc-paths</link>
library. That library promises to give its users access to GHC's
installation paths. Only, the library can't possible know that
path when it's compiled, because the path GHC considers its own is
determined only much later, when the user configures it through
<literal>ghcWithPackages</literal>. So we
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/ghc-paths-nix.patch">patched
ghc-paths</link> to return the paths found in those environment
variables at run-time rather than trying to guess them at
compile-time.
</para>
<para>
To make sure that mechanism works properly all the time, we
recommend that you set those variables to meaningful values in
your shell environment, too, i.e. by adding the following code to
your <literal>~/.bashrc</literal>:
</para>
<programlisting>
if type &gt;/dev/null 2&gt;&amp;1 -p ghc; then
eval &quot;$(egrep ^export &quot;$(type -p ghc)&quot;)&quot;
fi
</programlisting>
<para>
If you are certain that you'll use only one GHC environment which
is located in your user profile, then you can use the following
code, too, which has the advantage that it doesn't contain any
paths from the Nix store, i.e. those settings always remain valid
even if a <literal>nix-env -u</literal> operation updates the GHC
environment in your profile:
</para>
<programlisting>
if [ -e ~/.nix-profile/bin/ghc ]; then
export NIX_GHC=&quot;$HOME/.nix-profile/bin/ghc&quot;
export NIX_GHCPKG=&quot;$HOME/.nix-profile/bin/ghc-pkg&quot;
export NIX_GHC_DOCDIR=&quot;$HOME/.nix-profile/share/doc/ghc/html&quot;
export NIX_GHC_LIBDIR=&quot;$HOME/.nix-profile/lib/ghc-$($NIX_GHC --numeric-version)&quot;
fi
</programlisting>
</section>
<section xml:id="how-to-create-ad-hoc-environments-for-nix-shell">
<title>How to create ad hoc environments for
<literal>nix-shell</literal></title>
<para>
The easiest way to create an ad hoc development environment is to
run <literal>nix-shell</literal> with the appropriate GHC
environment given on the command-line:
</para>
<programlisting>
nix-shell -p &quot;haskellPackages.ghcWithPackages (pkgs: with pkgs; [mtl pandoc])&quot;
</programlisting>
<para>
For more sophisticated use-cases, however, it's more convenient to
save the desired configuration in a file called
<literal>shell.nix</literal> that looks like this:
</para>
<programlisting>
{ nixpkgs ? import &lt;nixpkgs&gt; {}, compiler ? &quot;ghc7101&quot; }:
let
inherit (nixpkgs) pkgs;
ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [
monad-par mtl
]);
in
pkgs.stdenv.mkDerivation {
name = &quot;my-haskell-env-0&quot;;
buildInputs = [ ghc ];
shellHook = &quot;eval $(egrep ^export ${ghc}/bin/ghc)&quot;;
}
</programlisting>
<para>
Now run <literal>nix-shell</literal> --- or even
<literal>nix-shell --pure</literal> --- to enter a shell
environment that has the appropriate compiler in
<literal>$PATH</literal>. If you use <literal>--pure</literal>,
then add all other packages that your development environment
needs into the <literal>buildInputs</literal> attribute. If you'd
like to switch to a different compiler version, then pass an
appropriate <literal>compiler</literal> argument to the
expression, i.e.
<literal>nix-shell --argstr compiler ghc784</literal>.
</para>
<para>
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 <literal>env</literal> attribute that provides a
shell environment suitable for compiling that particular package.
If you'd like to hack the <literal>lens</literal> library, for
example, then you just have to check out the source code and enter
the appropriate environment:
</para>
<programlisting>
$ cabal get lens-4.11 &amp;&amp; cd lens-4.11
Downloading lens-4.11...
Unpacking to lens-4.11/
$ nix-shell &quot;&lt;nixpkgs&gt;&quot; -A haskellPackages.lens.env
[nix-shell:/tmp/lens-4.11]$
</programlisting>
<para>
At point, you can run <literal>cabal configure</literal>,
<literal>cabal build</literal>, and all the other development
commands. Note that you need <literal>cabal-install</literal>
installed in your <literal>$PATH</literal> already to use it here
--- the <literal>nix-shell</literal> environment does not provide
it.
</para>
</section>
</section>
<section xml:id="how-to-create-nix-builds-for-your-own-private-haskell-packages">
<title>How to create Nix builds for your own private Haskell
packages</title>
<para>
If your own Haskell packages have build instructions for Cabal, then
you can convert those automatically into build instructions for Nix
using the <literal>cabal2nix</literal> utility, which you can
install into your profile by running
<literal>nix-env -i cabal2nix</literal>.
</para>
<section xml:id="how-to-build-a-stand-alone-project">
<title>How to build a stand-alone project</title>
<para>
For example, let's assume that you're working on a private project
called <literal>foo</literal>. To generate a Nix build expression
for it, change into the project's top-level directory and run the
command:
</para>
<programlisting>
$ cabal2nix . &gt;foo.nix
</programlisting>
<para>
Then write the following snippet into a file called
<literal>default.nix</literal>:
</para>
<programlisting>
{ nixpkgs ? import &lt;nixpkgs&gt; {}, compiler ? &quot;ghc7101&quot; }:
nixpkgs.pkgs.haskell.packages.${compiler}.callPackage ./foo.nix { }
</programlisting>
<para>
Finally, store the following code in a file called
<literal>shell.nix</literal>:
</para>
<programlisting>
{ nixpkgs ? import &lt;nixpkgs&gt; {}, compiler ? &quot;ghc7101&quot; }:
(import ./default.nix { inherit nixpkgs compiler; }).env
</programlisting>
<para>
At this point, you can run <literal>nix-build</literal> to have
Nix compile your project and install it into a Nix store path. The
local directory will contain a symlink called
<literal>result</literal> after <literal>nix-build</literal>
returns that points into that location. Of course, passing the
flag <literal>--argstr compiler ghc763</literal> allows switching
the build to any version of GHC currently supported.
</para>
<para>
Furthermore, you can call <literal>nix-shell</literal> to enter an
interactive development environment in which you can use
<literal>cabal configure</literal> and
<literal>cabal build</literal> 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.
</para>
<para>
If your package does not depend on any system-level libraries,
then it's sufficient to run
</para>
<programlisting>
$ nix-shell --command &quot;cabal configure&quot;
</programlisting>
<para>
once to set up your build. <literal>cabal-install</literal>
determines the absolute paths to all resources required for the
build and writes them into a config file in the
<literal>dist/</literal> directory. Once that's done, you can run
<literal>cabal build</literal> and any other command for that
project even outside of the <literal>nix-shell</literal>
environment. This feature is particularly nice for those of us who
like to edit their code with an IDE, like Emacs'
<literal>haskell-mode</literal>, 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;
<literal>cabal-install</literal> has already done that for us.
</para>
<para>
If you want to do some quick-and-dirty hacking and don't want to
bother setting up a <literal>default.nix</literal> and
<literal>shell.nix</literal> file manually, then you can use the
<literal>--shell</literal> flag offered by
<literal>cabal2nix</literal> to have it generate a stand-alone
<literal>nix-shell</literal> environment for you. With that
feature, running
</para>
<programlisting>
$ cabal2nix --shell . &gt;shell.nix
$ nix-shell --command &quot;cabal configure&quot;
</programlisting>
<para>
is usually enough to set up a build environment for any given
Haskell package. You can even use that generated file to run
<literal>nix-build</literal>, too:
</para>
<programlisting>
$ nix-build shell.nix
</programlisting>
</section>
<section xml:id="how-to-build-projects-that-depend-on-each-other">
<title>How to build projects that depend on each other</title>
<para>
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 <literal>callPackage</literal>. First of all, change into each
of your projects top-level directories and generate a
<literal>default.nix</literal> file with
<literal>cabal2nix</literal>:
</para>
<programlisting>
$ cd ~/src/foo &amp;&amp; cabal2nix . &gt;default.nix
$ cd ~/src/bar &amp;&amp; cabal2nix . &gt;default.nix
</programlisting>
<para>
Then edit your <literal>~/.nixpkgs/config.nix</literal> file to
register those builds in the default Haskell package set:
</para>
<programlisting>
{
packageOverrides = super: let self = super.pkgs; in
{
haskellPackages = super.haskellPackages.override {
overrides = self: super: {
foo = self.callPackage ../src/foo {};
bar = self.callPackage ../src/bar {};
};
};
};
}
</programlisting>
<para>
Once that's accomplished,
<literal>nix-env -f &quot;&lt;nixpkgs&gt;&quot; -qA haskellPackages</literal>
will show your packages like any other package from Hackage, and
you can build them
</para>
<programlisting>
$ nix-build &quot;&lt;nixpkgs&gt;&quot; -A haskellPackages.foo
</programlisting>
<para>
or enter an interactive shell environment suitable for building
them:
</para>
<programlisting>
$ nix-shell &quot;&lt;nixpkgs&gt;&quot; -A haskellPackages.bar.env
</programlisting>
</section>
</section>
<section xml:id="miscellaneous-topics">
<title>Miscellaneous Topics</title>
<section xml:id="how-to-build-with-profiling-enabled">
<title>How to build with profiling enabled</title>
<para>
Every Haskell package set takes a function called
<literal>overrides</literal> that you can use to manipulate the
package as much as you please. One useful application of this
feature is to replace the default <literal>mkDerivation</literal>
function with one that enables library profiling for all packages.
To accomplish that, add configure the following snippet in your
<literal>~/.nixpkgs/config.nix</literal> file:
</para>
<programlisting>
{
packageOverrides = super: let self = super.pkgs; in
{
profiledHaskellPackages = self.haskellPackages.override {
overrides = self: super: {
mkDerivation = args: super.mkDerivation (args // {
enableLibraryProfiling = true;
});
};
};
};
}
</programlisting>
</section>
<section xml:id="how-to-override-package-versions-in-a-compiler-specific-package-set">
<title>How to override package versions in a compiler-specific
package set</title>
<para>
Nixpkgs provides the latest version of
<link xlink:href="http://hackage.haskell.org/package/ghc-events"><literal>ghc-events</literal></link>,
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 <literal>ghc-events</literal> in the 7.8.x-specific package
set. The first step is to generate Nix build instructions with
<literal>cabal2nix</literal>:
</para>
<programlisting>
$ cabal2nix cabal://ghc-events-0.4.3.0 &gt;~/.nixpkgs/ghc-events-0.4.3.0.nix
</programlisting>
<para>
Then add the override in <literal>~/.nixpkgs/config.nix</literal>:
</para>
<programlisting>
{
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 {};
};
};
};
};
};
}
</programlisting>
<para>
This code is a little crazy, no doubt, but it's necessary because
the intuitive version
</para>
<programlisting>
haskell.packages.ghc784 = super.haskell.packages.ghc784.override {
overrides = self: super: {
ghc-events = self.callPackage ./ghc-events-0.4.3.0.nix {};
};
};
</programlisting>
<para>
doesn't do what we want it to: that code replaces the
<literal>haskell</literal> package set in Nixpkgs with one that
contains only one entry,<literal>packages</literal>, which
contains only one entry <literal>ghc784</literal>. This override
loses the <literal>haskell.compiler</literal> set, and it loses
the <literal>haskell.packages.ghcXYZ</literal> 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.
</para>
<para>
Once it's accomplished, however, we can install a variant of
<literal>ghc-events</literal> that's compiled with GHC 7.8.4:
</para>
<programlisting>
nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA haskell.packages.ghc784.ghc-events
</programlisting>
<para>
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 <literal>--no-check</literal> flag:
</para>
<programlisting>
$ cabal2nix --no-check cabal://ghc-events-0.4.3.0 &gt;~/.nixpkgs/ghc-events-0.4.3.0.nix
</programlisting>
<para>
Now the builds succeeds.
</para>
<para>
Of course, in the concrete example of
<literal>ghc-events</literal> this whole exercise is not an ideal
solution, because <literal>ghc-events</literal> 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.
</para>
</section>
<section xml:id="how-to-recover-from-ghcs-infamous-non-deterministic-library-id-bug">
<title>How to recover from GHC's infamous non-deterministic library
ID bug</title>
<para>
GHC and distributed build farms don't get along well:
</para>
<programlisting>
https://ghc.haskell.org/trac/ghc/ticket/4012
</programlisting>
<para>
When you see an error like this one
</para>
<programlisting>
package foo-0.7.1.0 is broken due to missing package
text-1.2.0.4-98506efb1b9ada233bb5c2b2db516d91
</programlisting>
<para>
then you have to download and re-install <literal>foo</literal>
and all its dependents from scratch:
</para>
<programlisting>
# nix-store -q --referrers /nix/store/*-haskell-text-1.2.0.4 \
| nix-store --repair-path --option binary-caches http://hydra.nixos.org
</programlisting>
<para>
If you're using additional Hydra servers other than
<literal>hydra.nixos.org</literal>, 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:
</para>
<programlisting>
rm /nix/var/nix/binary-cache-v3.sqlite
rm /nix/var/nix/manifests/*
rm /nix/var/nix/channel-cache/*
</programlisting>
</section>
<section xml:id="builds-on-darwin-fail-with-math.h-not-found">
<title>Builds on Darwin fail with <literal>math.h</literal> not
found</title>
<para>
Users of GHC on Darwin have occasionally reported that builds
fail, because the compiler complains about a missing include file:
</para>
<programlisting>
fatal error: 'math.h' file not found
</programlisting>
<para>
The issue has been discussed at length in
<link xlink:href="https://github.com/NixOS/nixpkgs/issues/6390">ticket
6390</link>, and so far no good solution has been proposed. As a
work-around, users who run into this problem can configure the
environment variables
</para>
<programlisting>
export NIX_CFLAGS_COMPILE=&quot;-idirafter /usr/include&quot;
export NIX_CFLAGS_LINK=&quot;-L/usr/lib&quot;
</programlisting>
<para>
in their <literal>~/.bashrc</literal> file to avoid the compiler
error.
</para>
</section>
</section>
</chapter>

1
doc/manual.xml
View File

@ -18,5 +18,6 @@
<xi:include href="package-notes.xml" />
<xi:include href="coding-conventions.xml" />
<xi:include href="contributing.xml" />
<xi:include href="haskell-users-guide.xml" />
</book>