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Building with nix

This directory contains the simplest possible Haskell project and the simplest possible Nix derivation to build that project.

You can build this Haskell package by running:

$ nix-build release0.nix
these derivations will be built:
  /nix/store/3l7f5v3aibz9rnxxafm6afvihjc04aiq-project0-1.0.0.drv
building path(s) ‘/nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0’
...
Configuring project0-1.0.0...
Dependency base <5: using base-4.9.0.0
...
Building project0-1.0.0...
Preprocessing executable 'project0' for project0-1.0.0...
[1 of 1] Compiling Main             ( Main.hs, dist/build/project0/project0-tmp/Main.dyn_o )
Linking dist/build/project0/project0 ...
...
Installing executable(s) in
/nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0/bin
...
/nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0

I've highlighted the parts that matter the most for our simple example. Don't expect the hashed paths in the above example output to necessarily match the ones you get (See below for more details about why they might differ).

The project0.cabal file only specifies a single dependency of base < 5 and Nix picks base-4.9.0.0 to satisfy that dependency. Nix then builds the project, stores the build output in /nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0 and creates a symlink in the current directory named result pointing to that directory in the /nix/store:

$ readlink result
/nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0

Right now the contents of that directory are just an executable file and an empty library directory:

$ tree result
result
├── bin
│   └── project0
└── lib
    └── links

3 directories, 1 file

As the project grows more complex we'll see additional build outputs in this directory.

We can run the executable stored in the result/bin subdirectory:

$ result/bin/project0 
Hello, world!

This output makes sense since our Main.hs file is just a simple "Hello, world!" program:

module Main where

main :: IO ()
main = putStrLn "Hello, world!"

What happens if we try to build the project again?

$ nix-build release0.nix
these derivations will be built:
  /nix/store/vvw0v8ys7dadck747vj48vb0jgs7isqm-project0-1.0.0.drv
building path(s) ‘/nix/store/ysrqcpdl51jaa4gqzx1xmb7m0h05rdwq-project0-1.0.0’
setupCompilerEnvironmentPhase
...
[1 of 1] Compiling Main             ( Main.hs, dist/build/project0/project0-tmp/Main.dyn_o )
Linking dist/build/project0/project0 ...
...
/nix/store/ysrqcpdl51jaa4gqzx1xmb7m0h05rdwq-project0-1.0.0

This might seem odd at first since you'd expect Nix to reuse the cached result from our first build. However, Nix does not reuse the first build because our project subtly changed: our nix-build deposited a result symlink which was not there before! Our Nix derivation depends on the current project directory, so if anything in the current directory changes then Nix performs a complete rebuild of our project.

We can verify this by removing the symlink and then performing the build again:

$ rm result
$ nix-build release0.nix
/nix/store/x28vx2rfnffl1clmxn5054bxwqyln2j0-project0-1.0.0

This time we get a cache hit and reuse the first build since our directory is now bit-for-bit identical to when we first ran nix-build.

These wasteful rebuilds are one reason that I don't recommend using nix-build to build the root Haskell project. Instead, the next section describes how to use cabal with Nix to avoid the issue of wasteful rebuilds.

Creating your own project

If you ever need to bootstrap your own project using cabal init, then run:

$ nix-shell --packages ghc --run 'cabal init'

cabal init requires ghc to be on the executable search path, but we do not plan to install GHC globally. Instead, we can use a nix-shell to transiently provide ghc just for the duration of a cabal init command. Later on we rely on the project's Nix configuration to provide the desired GHC for project development.

Building with cabal

You can open up a development environment for this project inside of a "Nix shell" by running:

$ nix-shell --attr env release0.nix

Normally nix-shell wouldn't require the --attr flag since nix-shell is designed to automatically compute the necessary development environment from the original derivation. However, Haskell derivations are different and nix-shell doesn't work out of the box on them.

Haskell derivations are records that include an env field which nix-shell can use to compute the correct development environment. In Nix a field is called an "attribute" so we pass the --attr env flag to specify that nix-shell should compute the development environment from the derivation record's env "attribute".

Once we open up the development environment we can use cabal to build and run the project0 executable:

$ cabal configure
Resolving dependencies...
Configuring project0-1.0.0...
$ cabal run project0
Preprocessing executable 'project0' for project0-1.0.0...
[1 of 1] Compiling Main             ( Main.hs, dist/build/project0/project0-tmp/Main.o )
Linking dist/build/project0/project0 ...
Running project0...
Hello, world!

Unlike Nix, cabal will be smart and won't wastefully rebuild things that haven't changed. This means we can safely re-run cabal without rebuilding the entire project from scratch:

$ cabal run project0
Up to date
Hello, world!

You can exit from the Nix shell using the exit command or typing Ctrl-D.

This Nix shell provides all necessary dependencies for your project and the Haskell toolchain except for cabal. For example, inside the Nix shell you will see that you are using a ghc provided by Nix, regardless of whether or not you have a global ghc installed:

$ which ghc  # The exact hash in the path might differ
/nix/store/dg7ak1hvlj66vgn4fwvddnnr4pfncd04-ghc-8.0.1/bin/ghc

The cabal configure step automatically picks up the ghc tool-chain and package database provisioned by Nix and uses them for all subsequent cabal commands.

The nixpkgs manual notes that if you only have Haskell dependencies you can also just run the following command once:

$ nix-shell --attr env release0.nix --run 'cabal configure'

... and then run all the other cabal commands without the Nix shell. However, if you have non-Haskell dependencies then this won't work. When in doubt, just get used to development inside of a Nix shell since that habit will translate well to non-Haskell projects managed by Nix.

NOTE: I recommend using cabal to build the root project during Haskell package development, but subsequent examples will still use nix-build to keep the examples short.

Nix derivations

The release0.nix file specifies how to build the project using Nix:

let
  pkgs = import <nixpkgs> { };

in
  pkgs.haskellPackages.callPackage ./project0.nix { }

I don't recommend reusing the above derivation for your Haskell projects. There are several ways that we can improve upon this derivation that we'll address in later examples.

This derivation begins by importing nixpkgs, which is a Nix channel. You can find all the officially supported Nix channels here:

The default Nix installation will subscribe you some channel (i.e. release) of Nixpkgs (a package repository for Nix).

You can tell which release of Nixpkgs you are using by running this command:

$ nix-instantiate --eval --expr 'builtins.readFile <nixpkgs/.version>'
"18.03\n"

... and you can also obtain the exact git revision of the Nixpkgs repository that the channel was cut from using this command:

$ nix-instantiate --eval --expr 'builtins.readFile <nixpkgs/.git-revision>'
"411cc559c052feb6e20a01fc6d5fa63cba09ce9a"

You should probably use the default channel selected for you. If you are using a Linux operating system other than NixOS, you can safely change to a stable channel if you prefer by running:

$ nix-channel --add https://nixos.org/channels/nixos-18.09-small nixpkgs
$ nix-channel --update nixpkgs

... replacing 18.09 with whatever stable release version you wish to use.

However, you should be very careful about using a stable release on OS X because the public binary cache only caches OS X build products for the unstable channel. If you try to use a stable channel on OS X you run a very high risk of compiling things from scratch (including ghc).

Pinning nixpkgs

Even "stable" channels are still not frozen. Stable channels are like major releases and they periodically receive minor releases for security patches, bug fixes, and new packages that successfully build. However, you have to specifically opt in to channel updates by running nix-channel --update nixpkgs. If you do nothing then your channel will remain frozen at whatever minor release you downloaded when you first installed Nix. When you do upgrade your channel you can only upgrade to the latest minor release.

Nix's channel mechanism works okay for personal or open source development, but does not work well in a corporate environment, since you can't easily ensure that every person or deployment is on the exact same minor release.

In a corporate environment, you can pin nixpkgs to a specific git revision as illustrated in release1.nix:

let
  bootstrap = import <nixpkgs> { };

  nixpkgs = builtins.fromJSON (builtins.readFile ./nixpkgs.json);

  src = bootstrap.fetchFromGitHub {
    owner = "NixOS";
    repo  = "nixpkgs";
    inherit (nixpkgs) rev sha256;
  };

  pkgs = import src { };

in
  pkgs.haskellPackages.callPackage ./project0.nix { }

... where nixpkgs.json was generated using the nix-prefetch-git tool:

$ nix-prefetch-git https://github.com/NixOS/nixpkgs.git 2c288548b93b657365c27a0132a43ba0080870cc > nixpkgs.json
$ cat nixpkgs.json
{
  "url": "https://github.com/NixOS/nixpkgs.git",
  "rev": "2c288548b93b657365c27a0132a43ba0080870cc",
  "date": "2017-01-02T00:10:04+01:00",
  "sha256": "1a365am90a1zy99k4qwddj8s3bdlyfisrsq4a3r00kghjcz89zld"
}

Replace 2c288548b93b657365c27a0132a43ba0080870cc with the git revision that you want to pin nixpkgs to. You can also omit the revision to pin to the current master.

However, if you choose to go this route then you will need to set up an internal Hydra server to build and cache your project.

Without an internal cache your developers will likely need to build these tools from scratch whenever your pinned nixpkgs drifts too far from the publicly cached channels. ghc in particular is very expensive to rebuild.

This guide does not (yet) cover how to set up an internal Hydra server for this purpose, but may do so in a future draft. Until then, the remaining examples will not use a pinned nixpkgs for simplicity.

cabal2nix

The second half of our release0.nix derivation contains the instructions to build our project:

let
  pkgs = import <nixpkgs> { };

in
  pkgs.haskellPackages.callPackage ./project0.nix { }

This references another file in this same project called project0.nix. This file was generated using cabal2nix by running:

$ cabal2nix . > project0.nix

... and the generated project0.nix file for this project is:

{ mkDerivation, base, stdenv }:
mkDerivation {
  pname = "project0";
  version = "1.0.0";
  src = ./.;
  isLibrary = false;
  isExecutable = true;
  executableHaskellDepends = [ base ];
  license = stdenv.lib.licenses.bsd3;
}

All that cabal2nix does is translate our project0.cabal file into a corresponding Nix expression. For comparison, here is the original project0.cabal file that project0.nix was generated from:

name: project0
version: 1.0.0
license: BSD3
license-file: LICENSE
cabal-version: >= 1.18
build-type: Simple

executable project0
    build-depends: base < 5
    main-is: Main.hs
    default-language: Haskell2010

Any time you update a Haskell project's cabal file you need to regenerate the project0.nix file using cabal2nix.

Hydra compatibility

release2.nix illustrates the next improvement we can make to our project's Nix derivation:

let
  pkgs = import <nixpkgs> { };

in
  { project0 = pkgs.haskellPackages.callPackage ./project0.nix { };
  }

The only difference is that now our file returns a "set" (the Nix term for a dictionary) of derivations. This "set" only has one "attribute" (i.e. key) named project0 whose value is the derivation to build our Haskell project.

The main motivation for this change is that Hydra (Nix's continuous integration server) requires that project build files are sets of derivations with one attribute per build product. If you try to build a naked derivation with Hydra you will get weird errors.

A second lesser reason for this change is that this makes it easy to add additional build products to your project. This comes in handy when you want to build and test dependencies or extra tools that you rely on.

You can still build the project using nix-build either by specifying to build all derivations in the set:

$ nix-build release2.nix

... or by specifying the attribute of the derivation you want to build using the same --attr flag we introduced before for nix-shell:

$ nix-build --attr project0 release2.nix

This --attr flag specifies that we only want to build the project0 field of the record, and this flag comes in handy once the record has more than one field.

You can also still open up a Nix shell, but you need to change the attribute you pass on the command line from env to project0.env:

$ nix-shell --attr project0.env release2.nix

Like before, nix-shell and nix-build take slightly different attributes: we specify the project0 attribute when using nix-build and the project0.env attribute when using nix-shell.

You can also avoid having to type this every time you initialize the project by creating the following shell.nix file:

(import ./release2.nix).project0.env

... replacing release2.nix with the name of your project's derivation file. Then you can just type:

$ nix-shell

... and that will automatically use the contents of shell.nix

Note that cabal2nix provides a --shell option to generate a shell.nix file suitable for the current project. However, this does not play nice with advanced dependency management (covered in the next section) so I do not recommend this approach in general.

Conclusion

That concludes Nix workflow basics for Haskell development. The next section covers dependency management.

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