Learning Haskell with nix and Emacs

haskell-servant 和 miso 等haskell开源项目竟然都是用nix管理依赖的。时隔多年nix竟然被赋予了这样的魔法，不仅让我大跌眼镜。在 Nix and Haskell in production 一文中指出，nix对标的其实是stack，他和stack一样有独立的haskell包缓存，用以达成reproducible的构建。nix胜过stack的唯一个地方，就是他不仅能解决haskell包依赖，还能解决其他任何依赖，甚至可以为你打造一个独立的开发环境。本文就是介绍如何在 macos catalina 上用nix打造一个haskell开发环境。

本来这一个步骤应该是非常简单的。但是在最新版的macos catalina上并不那么容易了。nix需要在根目录上创建文件夹 /nix ，在系统默认开启 SIP 的情况下，根目录是以只读度方式挂载的，所以无法创建。首先需要关闭SIP，创建目录，重新启用SIP，由于启用后根目录仍然只读，还需要创建分区，将分区挂载到 /nix 。

关闭SIP方法可参考文档： How to disable Systems Integrity Protection (SIP) in macOS

  PASSPHRASE=$(openssl rand -base64 32)
  echo "Creating encrypted APFS volume with passphrase: $PASSPHRASE" >&2
  sudo diskutil apfs addVolume disk1 'Case-sensitive APFS' Nix -mountpoint /nix -passphrase "$PASSPHRASE"
  UUID=$(diskutil info -plist /nix | plutil -extract VolumeUUID xml1 - -o - | plutil -p - | sed -e 's/"//g')
  echo $UUID
  security add-generic-password -l Nix -a "$UUID" -s "$UUID" -D "Encrypted Volume Password" -w "$PASSPHRASE" \\n -T "/System/Library/CoreServices/APFSUserAgent" -T "/System/Library/CoreServices/CSUserAgent"
  sudo diskutil enableOwnership /nix
  echo 'LABEL=Nix /nix apfs rw' | sudo tee -a /etc/fstab >/dev/null

挂载完后， df -h 命令查看当前磁盘状态如下。

  Filesystem      Size   Used  Avail Capacity iused      ifree %iused  Mounted on
  /dev/disk1s5   466Gi   10Gi  211Gi     5%  483643 4881969237    0%   /
  devfs          199Ki  199Ki    0Bi   100%     690          0  100%   /dev
  /dev/disk1s1   466Gi  232Gi  211Gi    53% 3527520 4878925360    0%   /System/Volumes/Data
  /dev/disk1s4   466Gi  2.0Gi  211Gi     1%       2 4882452878    0%   /private/var/vm
  map auto_home    0Bi    0Bi    0Bi   100%       0          0  100%   /System/Volumes/Data/home
  /dev/disk1s6   466Gi  9.8Gi  211Gi     5%  256476 4882196404    0%   /nix

之后就可以按照官方文档所述，安装 nix 了。

  curl https://nixos.org/nix/install | sh

关于nix和macos catalina和sip的问题可以看下，nix的一个issues： /nix will not be writable on macOS Catalina #2925

nix安装完之后就可以安装开发工具了。

  nix-env --install cabal2nix stack cabal-install

  stack new helloworld new-template

工程结构如下

  .
  ├── ChangeLog.md
  ├── LICENSE
  ├── README.md
  ├── Setup.hs
  ├── app
  │   └── Main.hs
  ├── helloworld.cabal
  ├── package.yaml
  ├── src
  │   └── Lib.hs
  ├── stack.yaml
  └── test
      └── Spec.hs

修改 .gitignore

  .stack-work/
  *~
  /result
  *.hi
  *.o
  dist-newstyle/

初始化git仓库

  git init

此时可以用 stack run 编译并运行。

  Building all executables for `helloworld' once. After a successful build of all of them, only specified executables will be rebuilt.
  helloworld> configure (lib + exe)
  Configuring helloworld-0.1.0.0...
  helloworld> build (lib + exe)
  Preprocessing library for helloworld-0.1.0.0..
  Building library for helloworld-0.1.0.0..
  [1 of 2] Compiling Lib
  [2 of 2] Compiling Paths_helloworld
  Preprocessing executable 'helloworld-exe' for helloworld-0.1.0.0..
  Building executable 'helloworld-exe' for helloworld-0.1.0.0..
  [1 of 2] Compiling Main
  [2 of 2] Compiling Paths_helloworld
  Linking .stack-work/dist/x86_64-osx/Cabal-2.4.0.1/build/helloworld-exe/helloworld-exe ...
  helloworld> copy/register
  Installing library in /Users/shane/src/github.com/shanexu/helloworld/.stack-work/install/x86_64-osx/12a66049aaca4a8b492b9641af325fbaf91178a6bf5ad4ae4f7714e26944d4c7/8.6.5/lib/x86_64-osx
  -ghc-8.6.5/helloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp
  Installing executable helloworld-exe in /Users/shane/src/github.com/shanexu/helloworld/.stack-work/install/x86_64-osx/12a66049aaca4a8b492b9641af325fbaf91178a6bf5ad4ae4f7714e26944d4c7/8.
  6.5/bin
  Registering library for helloworld-0.1.0.0..
  someFunc

说是编写，其实就是中 cabal2nix 生成。

  cabal2nix . > helloworld.nix

helloworld.nix 的内容如下：

 1:   { mkDerivation, base, hpack, stdenv }:
 2:   mkDerivation {
 3:     pname = "helloworld";
 4:     version = "0.1.0.0";
 5:     src = ./.;
 6:     isLibrary = true;
 7:     isExecutable = true;
 8:     libraryHaskellDepends = [ base ];
 9:     libraryToolDepends = [ hpack ];
10:     executableHaskellDepends = [ base ];
11:     testHaskellDepends = [ base ];
12:     prePatch = "hpack";
13:     homepage = "https://github.com/shanexu/helloworld#readme";
14:     license = stdenv.lib.licenses.bsd3;
15:   }

编写 default.nix

1:   let
2:     pkgs = import <nixpkgs> { };
3:   in
4:     pkgs.haskellPackages.callPackage ./helloworld.nix { }

使用nix build

  nix-build

默认 nix-build 会在当前目录下找 default.nix 文件，并使用这个文件执行build任务。

  these derivations will be built:
    /nix/store/lq4nibqgmvrb8j3yl460jqpc7ysbi417-helloworld-0.1.0.0.drv
  building '/nix/store/lq4nibqgmvrb8j3yl460jqpc7ysbi417-helloworld-0.1.0.0.drv'...
  setupCompilerEnvironmentPhase
  Build with /nix/store/kdmykixl5nafbygjp5i8a6b4iclmfm1l-ghc-8.6.5.
  unpacking sources
  unpacking source archive /nix/store/dfa15ibdmddj3pvq5zr1g01df2zr186d-helloworld
  source root is helloworld
  patching sources
  helloworld.cabal is up-to-date
  compileBuildDriverPhase
  setupCompileFlags: -package-db=/private/var/folders/8x/6h3nms2s34z7vwk5blbsz3100000gn/T/nix-build-helloworld-0.1.0.0.drv-0/setup-package.conf.d -j4 -threaded
  [1 of 1] Compiling Main             ( Setup.hs, /private/var/folders/8x/6h3nms2s34z7vwk5blbsz3100000gn/T/nix-build-helloworld-0.1.0.0.drv-0/Main.o )
  Linking Setup ...

build成功后会在当前目录下生成 result 目录，其结构如下：

  result
  ├── bin
  │   └── helloworld-exe
  ├── lib
  │   ├── ghc-8.6.5
  │   │   ├── package.conf.d
  │   │   │   └── helloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp.conf
  │   │   └── x86_64-osx-ghc-8.6.5
  │   │       ├── helloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp
  │   │       │   ├── Lib.dyn_hi
  │   │       │   ├── Lib.hi
  │   │       │   ├── Lib.p_hi
  │   │       │   ├── Paths_helloworld.dyn_hi
  │   │       │   ├── Paths_helloworld.hi
  │   │       │   ├── Paths_helloworld.p_hi
  │   │       │   ├── libHShelloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp.a
  │   │       │   └── libHShelloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp_p.a
  │   │       └── libHShelloworld-0.1.0.0-5AFk17aNCLW4CrcpWxCXgp-ghc8.6.5.dylib
  │   └── links
  └── nix-support
      └── propagated-build-inputs

  8 directories, 12 files

确认build结果：

  ❯ result/bin/helloworld-exe
  someFunc

实际上这个result是一个软链接

  ❯ readlink result
  /nix/store/zdp1n3yifw6ikpn8hjsza40iwdmk5fnz-helloworld-0.1.0.0

此时再执行一遍 nix-build 会发现nix又会重新build一遍

  ❯ nix-build
  these derivations will be built:
    /nix/store/43qdjaq019p4w81hqqds7wysbxaz0w4x-helloworld-0.1.0.0.drv
  building '/nix/store/43qdjaq019p4w81hqqds7wysbxaz0w4x-helloworld-0.1.0.0.drv'...
  setupCompilerEnvironmentPhase
  Build with /nix/store/kdmykixl5nafbygjp5i8a6b4iclmfm1l-ghc-8.6.5.
  unpacking sources
  unpacking source archive /nix/store/smxcdw5xy2kdmdn2931vgny4l7cxxq65-helloworld
  source root is helloworld
  patching sources
  helloworld.cabal is up-to-date
  compileBuildDriverPhase
  setupCompileFlags: -package-db=/private/var/folders/8x/6h3nms2s34z7vwk5blbsz3100000gn/T/nix-build-helloworld-0.1.0.0.drv-0/setup-package.conf.d -j4 -threaded
  [1 of 1] Compiling Main             ( Setup.hs, /private/var/folders/8x/6h3nms2s34z7vwk5blbsz3100000gn/T/nix-build-helloworld-0.1.0.0.drv-0/Main.o )
  Linking Setup ...

明明没有任何变更为什么会重新编译，问题出在 helloworld.nix 文件的第五行。

  src = ./.;

这里定义了当前目录所有文件为源码文件，第一次build时生成了result文件，所以文件夹内容有改变，所以就会认为文件变化，就会重新build。

这里可以使用nix内置函数，过滤掉软链接：

  src = builtins.filterSource (path: type: type != "symlink") ./.;

更通用的是可以使用gitignore来过滤非源码文件，修改helloworld.nix：

 1:   { nix-gitignore, mkDerivation, base, hpack, stdenv }:
 2:   mkDerivation {
 3:     pname = "helloworld";
 4:     version = "0.1.0.0";
 5:     src = nix-gitignore.gitignoreSourcePure [./.gitignore] ./.;
 6:     isLibrary = true;
 7:     isExecutable = true;
 8:     libraryHaskellDepends = [ base ];
 9:     libraryToolDepends = [ hpack ];
10:     executableHaskellDepends = [ base ];
11:     testHaskellDepends = [ base ];
12:     prePatch = "hpack";
13:     homepage = "https://github.com/githubuser/helloworld#readme";
14:     license = stdenv.lib.licenses.bsd3;
15:   }

nix-shell - start an interactive shell based on a Nix expression

所以使用nix-shell就可以打开一个跟build时相同的环境。

  ❯ nix-shell

  [nix-shell:~/src/github.com/shanexu/helloworld]$ ghc --version
  The Glorious Glasgow Haskell Compilation System, version 8.6.5

  [nix-shell:~/src/github.com/shanexu/helloworld]$
  /Users/shane/.nix-profile/bin/cabal

默认nix-shell会找shell.nix和default.nix文件。

如果需要定制nix-shell，则可以自行编写shell.nix文件：

 1:   let
 2:     pkgs = import <nixpkgs> {};
 3:     default = (import ./default.nix);
 4:   in
 5:     pkgs.haskellPackages.shellFor {
 6:       name = "helloworld-shell";
 7:       packages = p: [default];
 8:       buildInputs = [
 9:         pkgs.cabal-install
10:         pkgs.haskellPackages.apply-refact
11:         pkgs.haskellPackages.hlint
12:         pkgs.haskellPackages.stylish-haskell
13:         pkgs.haskellPackages.hasktags
14:         pkgs.haskellPackages.hoogle
15:         pkgs.haskellPackages.hindent
16:       ];
17:     }

这里在nix-shell里面安装了cabal，hlint等工具：

  ❯ nix-shell

  [nix-shell:~/src/github.com/shanexu/helloworld]$ which cabal
  /nix/store/h4wzzvjzpggyprf40n20y1adzbvhh9xj-cabal-install-3.0.0.0/bin/cabal

  [nix-shell:~/src/github.com/shanexu/helloworld]$ which hlint
  /nix/store/8s5q0l63cs2fn0hlwnjsps7iwh8ma5w8-hlint-2.2.3/bin/hlint

安装cachix

  nix-env -iA cachix -f https://cachix.org/api/v1/install

使用预编译缓存

  cachix use all-hies

可以在全局环境安装HIE

  nix-env -iA selection --arg selector 'p: { inherit (p) ghc865; }' -f https://github.com/infinisil/all-hies/tarball/master

也可以加入到shell.nix中

 1:   let
 2:     pkgs = import <nixpkgs> {};
 3:     default = (import ./default.nix);
 4:     all-hie = (import (fetchTarball "https://github.com/infinisil/all-hies/tarball/master") {});
 5:   in
 6:     pkgs.haskellPackages.shellFor {
 7:       name = "helloworld-shell";
 8:       packages = p: [default];
 9:       buildInputs = [
10:         pkgs.cabal-install
11:         pkgs.haskellPackages.apply-refact
12:         pkgs.haskellPackages.hlint
13:         pkgs.haskellPackages.stylish-haskell
14:         pkgs.haskellPackages.hasktags
15:         pkgs.haskellPackages.hoogle
16:         pkgs.haskellPackages.hindent
17:         (all-hie.selection { selector = p: { inherit (p) ghc865; }; })
18:       ];
19:     }

nix-shell默认使用bash，但是我平时都用 oh-my-zsh 。direnv已经支持nix，可以通过stdlib的use_nix函数，启用nix的环境配置。只要在当前目录下增加.envrc文件便可启用。

  use_nix

但是每次进入目录都会非常慢。

  ❯ direnv allow
  direnv: loading .envrc
  direnv: ([/usr/local/bin/direnv export zsh]) is taking a while to execute. Use CTRL-C to give up.

查看stdlib中的use_nix函数：

 1:   # Usage: use_nix [...]
 2:   #
 3:   # Load environment variables from `nix-shell`.
 4:   # If you have a `default.nix` or `shell.nix` these will be
 5:   # used by default, but you can also specify packages directly
 6:   # (e.g `use nix -p ocaml`).
 7:   #
 8:   use_nix() {
 9:     direnv_load nix-shell --show-trace "$@" --run "$(join_args "$direnv" dump)"
10:     if [[ $# == 0 ]]; then
11:       watch_file default.nix
12:       watch_file shell.nix
13:     fi
14:   }

可见每次进入目录的时候都需要执行nix-shell命令，这其实非常慢。

修改 ~/.direnvrc 增加 use_nix 函数，覆盖stdlib中的方法。这个方法根据目录中的shell.nix和default.nix文件的hash值，在.direnv中保存nix-shell环境的缓存。同时为了防止 nix-store --gc 回收 nix-shell 的依赖包。在 .direnv 目录里面生成nix的gcroots，也就是几个软链接。

 1:   use_nix() {
 2:       local path="$(nix-instantiate --find-file nixpkgs)"
 3: 
 4:       if [ -f "${path}/.version-suffix" ]; then
 5:           local version="$(< $path/.version-suffix)"
 6:       elif [ -f "${path}/.git" ]; then
 7:           local version="$(< $(< ${path}/.git/HEAD))"
 8:       fi
 9: 
10:       local cache=".direnv/cache-${version:-unknown}"
11: 
12:       local update_drv=0
13:       if [[ ! -e "$cache" ]] || \
14:              [[ "$HOME/.direnvrc" -nt "$cache" ]] || \
15:              [[ .envrc -nt "$cache" ]] || \
16:              [[ default.nix -nt "$cache" ]] || \
17:              [[ shell.nix -nt "$cache" ]];
18:       then
19:           [ -d .direnv ] || mkdir .direnv
20:           nix-shell --show-trace "$@" --run "\"$direnv\" dump bash" > "$cache"
21:           update_drv=1
22:       else
23:           log_status using cached derivation
24:       fi
25:       local term_backup=$TERM path_backup=$PATH
26:       if [ -n ${TMPDIR+x} ]; then
27:           local tmp_backup=$TMPDIR
28:       fi
29: 
30:       eval "$(< $cache)"
31:       export PATH=$PATH:$path_backup TERM=$term_backup TMPDIR=$tmp_backup
32:       if [ -n ${tmp_backup+x} ]; then
33:           export TMPDIR=${tmp_backup}
34:       else
35:           unset TMPDIR
36:       fi
37: 
38:       if [ "$out" ] && (( $update_drv )); then
39:           local drv_link=".direnv/shell.drv"
40:           local drv_dep_link=".direnv/shell.dep"
41:           local drv="$(nix show-derivation $out | grep -E -o -m1 '/nix/store/.*.drv')"
42:           local stripped_pwd=${PWD/\//}
43:           local escaped_pwd=${stripped_pwd//-/--}
44:           local escaped_pwd=${escaped_pwd//\//-}
45:           ln -fs "$drv" "$drv_link"
46:           ln -fs "$PWD/$drv_link" "/nix/var/nix/gcroots/per-user/$LOGNAME/$escaped_pwd"
47:           rm -f ${drv_dep_link}*
48:           nix-store --indirect --add-root $drv_dep_link --realise $(nix-store --query --references $drv_link)
49:           log_status renewed cache and derivation link
50:       fi
51: 
52:       if [[ $# = 0 ]]; then
53:           watch_file default.nix
54:           watch_file shell.nix
55:       fi
56:   }

我使用的是 spacemacs 的 develop 分支。

为了是emacs和direnv能够很好结合可以额外增加包，并在 .spacemacs 文件中 dotspacemacs/user-config 函数中添加配置。

1:   ;; direnv-mode
2:   (use-package direnv
3:     :config
4:     (direnv-mode))

Shane Xu's Home

Learning Haskell with nix and Emacs

前言

安装nix和其他工具

使用stack初始化工程

编写第一个nix表达式

添加shell.nix文件

安装HIE

nix-shell和direnv

配置emacs(spacemacs)

仓库地址

参考文档

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