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nix_integration.md

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Nix integration

(since 0.1.10.0)

When using the Nix integration, Haskell dependencies are handled as usual: They are downloaded from Stackage and built locally by Stack. Nix is used by Stack to provide the non-Haskell dependencies needed by these Haskell packages.

stack can automatically create a build environment (the equivalent of a "container" in Docker parlance) using nix-shell, provided Nix is already installed on your system. To do so, please visit the Nix download page.

There are two ways to create a build environment:

  • providing a list of packages (by "attribute name") from Nixpkgs, or
  • providing a custom shell.nix file containing a Nix expression that determines a derivation, i.e. a specification of what resources are available inside the shell.

The second requires writing code in Nix's custom language. So use this option only if you already know Nix and have special requirements, such as using custom Nix packages that override the standard ones or using system libraries with special requirements.

Checking Nix installation

Follow the instructions on the Nix download page to install Nix. After doing so, when opening a terminal, the nix commands (nix-build, nix-shell, etc) should be available. If they are not, it should be because the file located at $HOME/.nix-profile/etc/profile.d/nix.sh is not sourced by your shell.

You should either run source ~/.nix-profile/etc/profile.d/nix.sh manually every time you open a terminal and need Nix or add this command to your ~/.bashrc or ~/.bash_profile.

Additions to your stack.yaml

Add a section to your stack.yaml as follows:

nix:
  enable: true
  packages: [glpk, pcre]

This will instruct stack to build inside a local build environment that will have the glpk and pcre libraries installed and available. Further, the build environment will implicitly also include a version of GHC matching the configured resolver. Enabling Nix support means packages will always be built using a GHC available inside the shell, rather than your globally installed one if any.

Note that in this mode stack can use only GHC versions that have already been mirrored into the Nix package repository. The Nixpkgs master branch usually picks up new versions quickly, but it takes two or three days before those updates arrive in the unstable channel. Release channels, like nixos-15.09, receive those updates only occasionally -- say, every two or three months --, so you should not expect them to have the latest compiler available. Fresh NixOS installs use a release version by default.

To know for sure whether a given compiler is available on your system, you can use the command

$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler.ghc801
haskell.compiler.ghc801  ghc-8.0.1

to check whether it's available. If Nix doesn't know that resolver yet, then you'll see the following error message instead:

$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler.ghc999
error: attribute ‘ghc999’ in selection path ‘haskell.compiler.ghc999’ not found

You can list all known Haskell compilers in Nix with the following:

$ nix-instantiate --eval -E "with import <nixpkgs> {}; lib.attrNames haskell.compiler"

Alternatively, install nix-repl, a convenient tool to explore nixpkgs:

$ nix-env -i nix-repl
$ nix-repl

In the REPL, load nixpkgs and get the same information through autocomplete:

nix-repl> :l <nixpkgs>
nix-repl> haskell.compiler.ghc<Tab>

You can type and evaluate any nix expression in the nix-repl, such as the one we gave to nix-instantiate earlier.

Note: currently, stack only discovers dynamic and static libraries in the lib/ folder of any nix package, and likewise header files in the include/ folder. If you're dealing with a package that doesn't follow this standard layout, you'll have to deal with that using a custom shell file (see below).

Use stack as normal

With Nix enabled, stack build and stack exec will automatically launch themselves in a local build environment (using nix-shell behind the scenes).

stack setup will start a nix-shell, so it will gather all the required packages, but given nix handles GHC installation, instead of stack, this will happen when running stack build if no setup has been performed before. Therefore it is not longer necessary to run stack setup unless you want to cache a GHC installation before running the build.

If enable: is omitted or set to false, you can still build in a nix-shell by passing the --nix flag to stack, for instance stack --nix build. Passing any --nix* option to the command line will do the same.

Known limitation on macOS: currently, stack --nix ghci fails on macOS, due to a bug in GHCi when working with external shared libraries.

The Nix shell

By default, stack will run the build in a pure Nix build environment (or shell), which means two important things:

  • basically no environment variable will be forwarded from your user session to the nix-shell (variables like HTTP_PROXY or PATH notably will not be available),
  • the build should fail if you haven't specified all the dependencies in the packages: section of the stack.yaml file, even if these dependencies are installed elsewhere on your system. This behaviour enforces a complete description of the build environment to facilitate reproducibility.

To override this behaviour, add pure: false to your stack.yaml or pass the --no-nix-pure option to the command line.

Note: On macOS shells are non-pure by default currently. This is due soon to be resolved locale issues. So on macOS you'll need to be a bit more careful to check that you really have listed all dependencies.

Package sources

By default, nix-shell will look for the nixpkgs package set located by your NIX_PATH environment variable.

You can override this by passing --nix-path="nixpkgs=/my/own/nixpkgs/clone" to ask Nix to use your own local checkout of the nixpkgs repository. You could in this way use a bleeding edge nixpkgs, cloned from the nixpkgs master branch, or edit the nix descriptions of some packages. Setting

nix:
  path: [nixpkgs=/my/own/nixpkgs/clone]

in your stack.yaml will do the same.

Command-line options

The configuration present in your stack.yaml can be overridden on the command-line. See stack --nix-help for a list of all Nix options.

Configuration

stack.yaml contains a nix: section with Nix settings. Without this section, Nix will not be used.

Here is a commented configuration file, showing the default values:

nix:

  # false by default. Must be present and set to `true` to enable Nix, except on
  # NixOS where it is enabled by default (see #3938).  You can set set it in your
  # `$HOME/.stack/config.yaml` to enable Nix for all your projects without having
  # to repeat it
  # enable: true

  # true by default. Tells Nix whether to run in a pure shell or not.
  pure: true

  # Empty by default. The list of packages you want to be
  # available in the nix-shell at build time (with `stack
  # build`) and run time (with `stack exec`).
  packages: []

  # Unset by default. You cannot set this option if `packages:`
  # is already present and not empty.
  shell-file: shell.nix

  # A list of strings, empty by default. Additional options that
  # will be passed verbatim to the `nix-shell` command.
  nix-shell-options: []

  # A list of strings, empty by default, such as
  # `[nixpkgs=/my/local/nixpkgs/clone]` that will be used to override
  # NIX_PATH.
  path: []
  
  # false by default. Whether to add your nix dependencies as nix garbage
  # collection roots. This way, calling nix-collect-garbage will not remove
  # those packages from the nix store, saving you some time when running
  # stack build again with nix support activated.
  # This creates a `nix-gc-symlinks` directory in the project `.stack-work`.
  # To revert that, just delete this `nix-gc-symlinks` directory.
  add-gc-roots: false

Using a custom shell.nix file

Nix is also a programming language, and as specified here if you know it you can provide to the shell a fully customized derivation as an environment to use. Here is the equivalent of the configuration used in this section, but with an explicit shell.nix file (make sure you're using a nixpkgs version later than 2015-03-05):

{ghc}:
with (import <nixpkgs> {});

haskell.lib.buildStackProject {
  inherit ghc;
  name = "myEnv";
  buildInputs = [ glpk pcre ];
}

Defining manually a shell.nix file gives you the possibility to override some Nix derivations ("packages"), for instance to change some build options of the libraries you use, or to set additional environment variables. See the Nix manual for more. The buildStackProject utility function is documented in the Nixpkgs manual. In such case, stack expect this file to define a function of exactly one argument that should be called ghc (as arguments within a set are non-positional), which you should give to buildStackProject. This is the ghc from the resolver you set in the stack.yaml.

And now for the stack.yaml file:

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

The stack build command will behave exactly the same as above. Note that specifying both packages: and a shell-file: results in an error. (Comment one out before adding the other.)