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nixpkgs/doc/languages-frameworks/dhall.section.md
Valentin Gagarin 3b5b8e31a3 fixup note on dhall-to-nixpkgs example
2023-11-02 17:38:03 +01:00

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Dhall

The Nixpkgs support for Dhall assumes some familiarity with Dhall's language support for importing Dhall expressions, which is documented here:

Remote imports

Nixpkgs bypasses Dhall's support for remote imports using Dhall's semantic integrity checks. Specifically, any Dhall import can be protected by an integrity check like:

https://prelude.dhall-lang.org/v20.1.0/package.dhall
  sha256:26b0ef498663d269e4dc6a82b0ee289ec565d683ef4c00d0ebdd25333a5a3c98

… and if the import is cached then the interpreter will load the import from cache instead of fetching the URL.

Nixpkgs uses this trick to add all of a Dhall expression's dependencies into the cache so that the Dhall interpreter never needs to resolve any remote URLs. In fact, Nixpkgs uses a Dhall interpreter with remote imports disabled when packaging Dhall expressions to enforce that the interpreter never resolves a remote import. This means that Nixpkgs only supports building Dhall expressions if all of their remote imports are protected by semantic integrity checks.

Instead of remote imports, Nixpkgs uses Nix to fetch remote Dhall code. For example, the Prelude Dhall package uses pkgs.fetchFromGitHub to fetch the dhall-lang repository containing the Prelude. Relying exclusively on Nix to fetch Dhall code ensures that Dhall packages built using Nix remain pure and also behave well when built within a sandbox.

Packaging a Dhall expression from scratch

We can illustrate how Nixpkgs integrates Dhall by beginning from the following trivial Dhall expression with one dependency (the Prelude):

-- ./true.dhall

let Prelude = https://prelude.dhall-lang.org/v20.1.0/package.dhall

in  Prelude.Bool.not False

As written, this expression cannot be built using Nixpkgs because the expression does not protect the Prelude import with a semantic integrity check, so the first step is to freeze the expression using dhall freeze, like this:

$ dhall freeze --inplace ./true.dhall

… which gives us:

-- ./true.dhall

let Prelude =
      https://prelude.dhall-lang.org/v20.1.0/package.dhall
        sha256:26b0ef498663d269e4dc6a82b0ee289ec565d683ef4c00d0ebdd25333a5a3c98

in  Prelude.Bool.not False

To package that expression, we create a ./true.nix file containing the following specification for the Dhall package:

# ./true.nix

{ buildDhallPackage, Prelude }:

buildDhallPackage {
  name = "true";
  code = ./true.dhall;
  dependencies = [ Prelude ];
  source = true;
}

… and we complete the build by incorporating that Dhall package into the pkgs.dhallPackages hierarchy using an overlay, like this:

# ./example.nix

let
  nixpkgs = builtins.fetchTarball {
    url    = "https://github.com/NixOS/nixpkgs/archive/94b2848559b12a8ed1fe433084686b2a81123c99.tar.gz";
    hash = "sha256-B4Q3c6IvTLg3Q92qYa8y+i4uTaphtFdjp+Ir3QQjdN0=";
  };

  dhallOverlay = self: super: {
    true = self.callPackage ./true.nix { };
  };

  overlay = self: super: {
    dhallPackages = super.dhallPackages.override (old: {
      overrides =
        self.lib.composeExtensions (old.overrides or (_: _: {})) dhallOverlay;
    });
  };

  pkgs = import nixpkgs { config = {}; overlays = [ overlay ]; };

in
  pkgs

… which we can then build using this command:

$ nix build --file ./example.nix dhallPackages.true

Contents of a Dhall package

The above package produces the following directory tree:

$ tree -a ./result
result
├── .cache
│   └── dhall
│       └── 122027abdeddfe8503496adeb623466caa47da5f63abd2bc6fa19f6cfcb73ecfed70
├── binary.dhall
└── source.dhall

… where:

  • source.dhall contains the result of interpreting our Dhall package:

    $ cat ./result/source.dhall
True

  • The .cache subdirectory contains one binary cache product encoding the same result as source.dhall:

    $ dhall decode < ./result/.cache/dhall/122027abdeddfe8503496adeb623466caa47da5f63abd2bc6fa19f6cfcb73ecfed70
True

  • binary.dhall contains a Dhall expression which handles fetching and decoding the same cache product:

    $ cat ./result/binary.dhall
missing sha256:27abdeddfe8503496adeb623466caa47da5f63abd2bc6fa19f6cfcb73ecfed70
$ cp -r ./result/.cache .cache

$ chmod -R u+w .cache

$ XDG_CACHE_HOME=.cache dhall --file ./result/binary.dhall
True

The source.dhall file is only present for packages that specify source = true;. By default, Dhall packages omit the source.dhall in order to conserve disk space when they are used exclusively as dependencies. For example, if we build the Prelude package it will only contain the binary encoding of the expression:

$ nix build --file ./example.nix dhallPackages.Prelude

$ tree -a result
result
├── .cache
│   └── dhall
│       └── 122026b0ef498663d269e4dc6a82b0ee289ec565d683ef4c00d0ebdd25333a5a3c98
└── binary.dhall

2 directories, 2 files

Typically, you only specify source = true; for the top-level Dhall expression of interest (such as our example true.nix Dhall package). However, if you wish to specify source = true for all Dhall packages, then you can amend the Dhall overlay like this:

  dhallOverrides = self: super: {
    # Enable source for all Dhall packages
    buildDhallPackage =
      args: super.buildDhallPackage (args // { source = true; });

    true = self.callPackage ./true.nix { };
  };

… and now the Prelude will contain the fully decoded result of interpreting the Prelude:

$ nix build --file ./example.nix dhallPackages.Prelude

$ tree -a result
result
├── .cache
│   └── dhall
│       └── 122026b0ef498663d269e4dc6a82b0ee289ec565d683ef4c00d0ebdd25333a5a3c98
├── binary.dhall
└── source.dhall

$ cat ./result/source.dhall
{ Bool =
  { and =
      \(_ : List Bool) ->
        List/fold Bool _ Bool (\(_ : Bool) -> \(_ : Bool) -> _@1 && _) True
  , build = \(_ : Type -> _ -> _@1 -> _@2) -> _ Bool True False
  , even =
      \(_ : List Bool) ->
        List/fold Bool _ Bool (\(_ : Bool) -> \(_ : Bool) -> _@1 == _) True
  , fold =
      \(_ : Bool) ->
…

Packaging functions

We already saw an example of using buildDhallPackage to create a Dhall package from a single file, but most Dhall packages consist of more than one file and there are two derived utilities that you may find more useful when packaging multiple files:

  • buildDhallDirectoryPackage - build a Dhall package from a local directory

  • buildDhallGitHubPackage - build a Dhall package from a GitHub repository

The buildDhallPackage is the lowest-level function and accepts the following arguments:

  • name: The name of the derivation

  • dependencies: Dhall dependencies to build and cache ahead of time

  • code: The top-level expression to build for this package

    Note that the code field accepts an arbitrary Dhall expression. You're not limited to just a file.

  • source: Set to true to include the decoded result as source.dhall in the build product, at the expense of requiring more disk space

  • documentationRoot: Set to the root directory of the package if you want dhall-docs to generate documentation underneath the docs subdirectory of the build product

The buildDhallDirectoryPackage is a higher-level function implemented in terms of buildDhallPackage that accepts the following arguments:

  • name: Same as buildDhallPackage

  • dependencies: Same as buildDhallPackage

  • source: Same as buildDhallPackage

  • src: The directory containing Dhall code that you want to turn into a Dhall package

  • file: The top-level file (package.dhall by default) that is the entrypoint to the rest of the package

  • document: Set to true to generate documentation for the package

The buildDhallGitHubPackage is another higher-level function implemented in terms of buildDhallPackage that accepts the following arguments:

  • name: Same as buildDhallPackage

  • dependencies: Same as buildDhallPackage

  • source: Same as buildDhallPackage

  • owner: The owner of the repository

  • repo: The repository name

  • rev: The desired revision (or branch, or tag)

  • directory: The subdirectory of the Git repository to package (if a directory other than the root of the repository)

  • file: The top-level file (${directory}/package.dhall by default) that is the entrypoint to the rest of the package

  • document: Set to true to generate documentation for the package

Additionally, buildDhallGitHubPackage accepts the same arguments as fetchFromGitHub, such as hash or fetchSubmodules.

dhall-to-nixpkgs

You can use the dhall-to-nixpkgs command-line utility to automate packaging Dhall code. For example:

$ nix-shell -p haskellPackages.dhall-nixpkgs nix-prefetch-git
[nix-shell]$ dhall-to-nixpkgs github https://github.com/Gabriella439/dhall-semver.git
{ buildDhallGitHubPackage, Prelude }:
  buildDhallGitHubPackage {
    name = "dhall-semver";
    githubBase = "github.com";
    owner = "Gabriella439";
    repo = "dhall-semver";
    rev = "2d44ae605302ce5dc6c657a1216887fbb96392a4";
    fetchSubmodules = false;
    hash = "sha256-n0nQtswVapWi/x7or0O3MEYmAkt/a1uvlOtnje6GGnk=";
    directory = "";
    file = "package.dhall";
    source = false;
    document = false;
    dependencies = [ (Prelude.overridePackage { file = "package.dhall"; }) ];
    }

:::{.note} nix-prefetch-git is added to the nix-shell -p invocation above, because it has to be in $PATH for dhall-to-nixpkgs to work. :::

The utility takes care of automatically detecting remote imports and converting them to package dependencies. You can also use the utility on local Dhall directories, too:

$ dhall-to-nixpkgs directory ~/proj/dhall-semver
{ buildDhallDirectoryPackage, Prelude }:
  buildDhallDirectoryPackage {
    name = "proj";
    src = ~/proj/dhall-semver;
    file = "package.dhall";
    source = false;
    document = false;
    dependencies = [ (Prelude.overridePackage { file = "package.dhall"; }) ];
    }

Remote imports as fixed-output derivations

dhall-to-nixpkgs has the ability to fetch and build remote imports as fixed-output derivations by using their Dhall integrity check. This is sometimes easier than manually packaging all remote imports.

This can be used like the following:

$ dhall-to-nixpkgs directory --fixed-output-derivations ~/proj/dhall-semver
{ buildDhallDirectoryPackage, buildDhallUrl }:
  buildDhallDirectoryPackage {
    name = "proj";
    src = ~/proj/dhall-semver;
    file = "package.dhall";
    source = false;
    document = false;
    dependencies = [
      (buildDhallUrl {
        url = "https://prelude.dhall-lang.org/v17.0.0/package.dhall";
        hash = "sha256-ENs8kZwl6QRoM9+Jeo/+JwHcOQ+giT2VjDQwUkvlpD4=";
        dhallHash = "sha256:10db3c919c25e9046833df897a8ffe2701dc390fa0893d958c3430524be5a43e";
        })
      ];
    }

Here, dhall-semver's Prelude dependency is fetched and built with the buildDhallUrl helper function, instead of being passed in as a function argument.

Overriding dependency versions

Suppose that we change our true.dhall example expression to depend on an older version of the Prelude (19.0.0):

-- ./true.dhall

let Prelude =
      https://prelude.dhall-lang.org/v19.0.0/package.dhall
        sha256:eb693342eb769f782174157eba9b5924cf8ac6793897fc36a31ccbd6f56dafe2

in  Prelude.Bool.not False

If we try to rebuild that expression the build will fail:

$ nix build --file ./example.nix dhallPackages.true
builder for '/nix/store/0f1hla7ff1wiaqyk1r2ky4wnhnw114fi-true.drv' failed with exit code 1; last 10 log lines:

  Dhall was compiled without the 'with-http' flag.

  The requested URL was: https://prelude.dhall-lang.org/v19.0.0/package.dhall


  4│       https://prelude.dhall-lang.org/v19.0.0/package.dhall
  5│         sha256:eb693342eb769f782174157eba9b5924cf8ac6793897fc36a31ccbd6f56dafe2

  /nix/store/rsab4y99h14912h4zplqx2iizr5n4rc2-true.dhall:4:7
[1 built (1 failed), 0.0 MiB DL]
error: build of '/nix/store/0f1hla7ff1wiaqyk1r2ky4wnhnw114fi-true.drv' failed

… because the default Prelude selected by Nixpkgs revision 94b2848559b12a8ed1fe433084686b2a81123c99is is version 20.1.0, which doesn't have the same integrity check as version 19.0.0. This means that version 19.0.0 is not cached and the interpreter is not allowed to fall back to importing the URL.

However, we can override the default Prelude version by using dhall-to-nixpkgs to create a Dhall package for our desired Prelude:

$ dhall-to-nixpkgs github https://github.com/dhall-lang/dhall-lang.git \
    --name Prelude \
    --directory Prelude \
    --rev v19.0.0 \
    > Prelude.nix

… and then referencing that package in our Dhall overlay, by either overriding the Prelude globally for all packages, like this:

  dhallOverrides = self: super: {
    true = self.callPackage ./true.nix { };

    Prelude = self.callPackage ./Prelude.nix { };
  };

… or selectively overriding the Prelude dependency for just the true package, like this:

  dhallOverrides = self: super: {
    true = self.callPackage ./true.nix {
      Prelude = self.callPackage ./Prelude.nix { };
    };
  };

Overrides

You can override any of the arguments to buildDhallGitHubPackage or buildDhallDirectoryPackage using the overridePackage attribute of a package. For example, suppose we wanted to selectively enable source = true just for the Prelude. We can do that like this:

  dhallOverrides = self: super: {
    Prelude = super.Prelude.overridePackage { source = true; };

    …
  };