compose.nix

# TODO(@Ericson2314): Remove `pkgs` param, which is only used for
# `buildStackProject`, `justStaticExecutables` and `checkUnusedPackages`
{ pkgs, lib }:

rec {

  /*
    This function takes a file like `hackage-packages.nix` and constructs
    a full package set out of that.
  */
  makePackageSet = import ../make-package-set.nix;

  /*
    The function overrideCabal lets you alter the arguments to the
    mkDerivation function.

    Example:

    First, note how the aeson package is constructed in hackage-packages.nix:

        "aeson" = callPackage ({ mkDerivation, attoparsec, <snip>
                               }:
                                 mkDerivation {
                                   pname = "aeson";
                                   <snip>
                                   homepage = "https://github.com/bos/aeson";
                                 })

    The mkDerivation function of haskellPackages will take care of putting
    the homepage in the right place, in meta.

        > haskellPackages.aeson.meta.homepage
        "https://github.com/bos/aeson"

        > x = haskell.lib.compose.overrideCabal (old: { homepage = old.homepage + "#readme"; }) haskellPackages.aeson
        > x.meta.homepage
        "https://github.com/bos/aeson#readme"
  */
  overrideCabal =
    f: drv:
    (drv.override (
      args:
      args
      // {
        mkDerivation = drv: (args.mkDerivation drv).override f;
      }
    ))
    // {
      overrideScope = scope: overrideCabal f (drv.overrideScope scope);
    };

  # : Map Name (Either Path VersionNumber) -> HaskellPackageOverrideSet
  # Given a set whose values are either paths or version strings, produces
  # a package override set (i.e. (self: super: { etc. })) that sets
  # the packages named in the input set to the corresponding versions
  packageSourceOverrides =
    overrides: self: super:
    pkgs.lib.mapAttrs (
      name: src:
      let
        isPath = x: builtins.substring 0 1 (toString x) == "/";
        generateExprs = if isPath src then self.callCabal2nix else self.callHackage;
      in
      generateExprs name src { }
    ) overrides;

  /*
    doCoverage modifies a haskell package to enable the generation
    and installation of a coverage report.

    See https://wiki.haskell.org/Haskell_program_coverage
  */
  doCoverage = overrideCabal (drv: {
    doCoverage = true;
  });

  /*
    dontCoverage modifies a haskell package to disable the generation
    and installation of a coverage report.
  */
  dontCoverage = overrideCabal (drv: {
    doCoverage = false;
  });

  /*
    doHaddock modifies a haskell package to enable the generation and
    installation of API documentation from code comments using the
    haddock tool.
  */
  doHaddock = overrideCabal (drv: {
    doHaddock = true;
  });

  /*
    dontHaddock modifies a haskell package to disable the generation and
    installation of API documentation from code comments using the
    haddock tool.
  */
  dontHaddock = overrideCabal (drv: {
    doHaddock = false;
  });

  /*
    doJailbreak enables the removal of version bounds from the cabal
    file. You may want to avoid this function.

    This is useful when a package reports that it can not be built
    due to version mismatches. In some cases, removing the version
    bounds entirely is an easy way to make a package build, but at
    the risk of breaking software in non-obvious ways now or in the
    future.

    Instead of jailbreaking, you can patch the cabal file.

    Note that jailbreaking at this time, doesn't lift bounds on
    conditional branches.
    https://github.com/peti/jailbreak-cabal/issues/7 has further details.
  */
  doJailbreak = overrideCabal (drv: {
    jailbreak = true;
  });

  /*
    dontJailbreak restores the use of the version bounds the check
    the use of dependencies in the package description.
  */
  dontJailbreak = overrideCabal (drv: {
    jailbreak = false;
  });

  /*
    doCheck enables dependency checking, compilation and execution
    of test suites listed in the package description file.
  */
  doCheck = overrideCabal (drv: {
    doCheck = true;
  });
  /*
    dontCheck disables dependency checking, compilation and execution
    of test suites listed in the package description file.
  */
  dontCheck = overrideCabal (drv: {
    doCheck = false;
  });
  /*
    The dontCheckIf variant sets doCheck = false if the condition
    applies. In any other case the previously set/default value is used.
    This prevents accidentally re-enabling tests in a later override.
  */
  dontCheckIf = condition: if condition then dontCheck else lib.id;

  /*
    doBenchmark enables dependency checking and compilation
    for benchmarks listed in the package description file.
    Benchmarks are, however, not executed at the moment.
  */
  doBenchmark = overrideCabal (drv: {
    doBenchmark = true;
  });
  /*
    dontBenchmark disables dependency checking, compilation and execution
    for benchmarks listed in the package description file.
  */
  dontBenchmark = overrideCabal (drv: {
    doBenchmark = false;
  });

  /*
    doDistribute enables the distribution of binaries for the package
    via hydra.
  */
  doDistribute = overrideCabal (drv: {
    # lib.platforms.all is the default value for platforms (since GHC can cross-compile)
    hydraPlatforms = lib.subtractLists (drv.badPlatforms or [ ]) (drv.platforms or lib.platforms.all);
  });
  /*
    dontDistribute disables the distribution of binaries for the package
    via hydra.
  */
  dontDistribute = overrideCabal (drv: {
    hydraPlatforms = [ ];
  });

  /*
    appendConfigureFlag adds a single argument that will be passed to the
    cabal configure command, after the arguments that have been defined
    in the initial declaration or previous overrides.

    Example:

        > haskell.lib.compose.appendConfigureFlag "--profiling-detail=all-functions" haskellPackages.servant
  */
  appendConfigureFlag = x: appendConfigureFlags [ x ];
  appendConfigureFlags =
    xs:
    overrideCabal (drv: {
      configureFlags = (drv.configureFlags or [ ]) ++ xs;
    });

  appendBuildFlag =
    x:
    overrideCabal (drv: {
      buildFlags = (drv.buildFlags or [ ]) ++ [ x ];
    });
  appendBuildFlags =
    xs:
    overrideCabal (drv: {
      buildFlags = (drv.buildFlags or [ ]) ++ xs;
    });

  /*
    removeConfigureFlag drv x is a Haskell package like drv, but with
    all cabal configure arguments that are equal to x removed.

        > haskell.lib.compose.removeConfigureFlag "--verbose" haskellPackages.servant
  */
  removeConfigureFlag =
    x:
    overrideCabal (drv: {
      configureFlags = lib.remove x (drv.configureFlags or [ ]);
    });

  addBuildTool = x: addBuildTools [ x ];
  addBuildTools =
    xs:
    overrideCabal (drv: {
      buildTools = (drv.buildTools or [ ]) ++ xs;
    });

  addExtraLibrary = x: addExtraLibraries [ x ];
  addExtraLibraries =
    xs:
    overrideCabal (drv: {
      extraLibraries = (drv.extraLibraries or [ ]) ++ xs;
    });

  addBuildDepend = x: addBuildDepends [ x ];
  addBuildDepends =
    xs:
    overrideCabal (drv: {
      buildDepends = (drv.buildDepends or [ ]) ++ xs;
    });

  addTestToolDepend = x: addTestToolDepends [ x ];
  addTestToolDepends =
    xs:
    overrideCabal (drv: {
      testToolDepends = (drv.testToolDepends or [ ]) ++ xs;
    });

  addPkgconfigDepend = x: addPkgconfigDepends [ x ];
  addPkgconfigDepends =
    xs:
    overrideCabal (drv: {
      pkg-configDepends = (drv.pkg-configDepends or [ ]) ++ xs;
    });

  addSetupDepend = x: addSetupDepends [ x ];
  addSetupDepends =
    xs:
    overrideCabal (drv: {
      setupHaskellDepends = (drv.setupHaskellDepends or [ ]) ++ xs;
    });

  enableCabalFlag = x: drv: appendConfigureFlag "-f${x}" (removeConfigureFlag "-f-${x}" drv);
  disableCabalFlag = x: drv: appendConfigureFlag "-f-${x}" (removeConfigureFlag "-f${x}" drv);

  markBroken = overrideCabal (drv: {
    broken = true;
    hydraPlatforms = [ ];
  });
  unmarkBroken = overrideCabal (drv: {
    broken = false;
  });
  markBrokenVersion =
    version: drv:
    assert drv.version == version;
    markBroken drv;
  markUnbroken = overrideCabal (drv: {
    broken = false;
  });

  /*
    disableParallelBuilding drops the -j<n> option from the GHC
    command line for the given package. This can be useful in rare
    situations where parallel building of a package causes GHC to
    fail for some reason.
  */
  disableParallelBuilding = overrideCabal (drv: {
    enableParallelBuilding = false;
  });

  enableLibraryProfiling = overrideCabal (drv: {
    enableLibraryProfiling = true;
  });
  disableLibraryProfiling = overrideCabal (drv: {
    enableLibraryProfiling = false;
  });

  enableExecutableProfiling = overrideCabal (drv: {
    enableExecutableProfiling = true;
  });
  disableExecutableProfiling = overrideCabal (drv: {
    enableExecutableProfiling = false;
  });

  enableSharedExecutables = overrideCabal (drv: {
    enableSharedExecutables = true;
  });
  disableSharedExecutables = overrideCabal (drv: {
    enableSharedExecutables = false;
  });

  enableSharedLibraries = overrideCabal (drv: {
    enableSharedLibraries = true;
  });
  disableSharedLibraries = overrideCabal (drv: {
    enableSharedLibraries = false;
  });

  enableDeadCodeElimination = overrideCabal (drv: {
    enableDeadCodeElimination = true;
  });
  disableDeadCodeElimination = overrideCabal (drv: {
    enableDeadCodeElimination = false;
  });

  enableStaticLibraries = overrideCabal (drv: {
    enableStaticLibraries = true;
  });
  disableStaticLibraries = overrideCabal (drv: {
    enableStaticLibraries = false;
  });

  enableSeparateBinOutput = overrideCabal (drv: {
    enableSeparateBinOutput = true;
  });

  appendPatch = x: appendPatches [ x ];
  appendPatches =
    xs:
    overrideCabal (drv: {
      patches = (drv.patches or [ ]) ++ xs;
    });

  /*
    Set a specific build target instead of compiling all targets in the package.
    For example, imagine we have a .cabal file with a library, and 2 executables "dev" and "server".
    We can build only "server" and not wait on the compilation of "dev" by using setBuildTarget as follows:

      > setBuildTarget "server" (callCabal2nix "thePackageName" thePackageSrc {})
  */
  setBuildTargets =
    xs:
    overrideCabal (drv: {
      buildTarget = lib.concatStringsSep " " xs;
    });
  setBuildTarget = x: setBuildTargets [ x ];

  doHyperlinkSource = overrideCabal (drv: {
    hyperlinkSource = true;
  });
  dontHyperlinkSource = overrideCabal (drv: {
    hyperlinkSource = false;
  });

  disableHardening =
    flags:
    overrideCabal (drv: {
      hardeningDisable = flags;
    });

  /*
    Let Nix strip the binary files.
    This removes debugging symbols.
  */
  doStrip = overrideCabal (drv: {
    dontStrip = false;
  });

  /*
    Stop Nix from stripping the binary files.
    This keeps debugging symbols.
  */
  dontStrip = overrideCabal (drv: {
    dontStrip = true;
  });

  /*
    Useful for debugging segfaults with gdb.
    This includes dontStrip.
  */
  enableDWARFDebugging =
    drv:
    # -g: enables debugging symbols
    # --disable-*-stripping: tell GHC not to strip resulting binaries
    # dontStrip: see above
    appendConfigureFlag "--ghc-options=-g --disable-executable-stripping --disable-library-stripping" (
      dontStrip drv
    );

  /*
    Create a source distribution tarball like those found on hackage,
    instead of building the package.
  */
  sdistTarball =
    pkg:
    lib.overrideDerivation pkg (drv: {
      name = "${drv.pname}-source-${drv.version}";
      # Since we disable the haddock phase, we also need to override the
      # outputs since the separate doc output will not be produced.
      outputs = [ "out" ];
      buildPhase = "./Setup sdist";
      haddockPhase = ":";
      checkPhase = ":";
      installPhase = "install -D dist/${drv.pname}-*.tar.gz $out/${drv.pname}-${drv.version}.tar.gz";
      fixupPhase = ":";
    });

  /*
    Create a documentation tarball suitable for uploading to Hackage instead
    of building the package.
  */
  documentationTarball =
    pkg:
    pkgs.lib.overrideDerivation pkg (drv: {
      name = "${drv.name}-docs";
      # Like sdistTarball, disable the "doc" output here.
      outputs = [ "out" ];
      buildPhase = ''
        runHook preHaddock
        ./Setup haddock --for-hackage
        runHook postHaddock
      '';
      haddockPhase = ":";
      checkPhase = ":";
      installPhase = ''
        runHook preInstall
        mkdir -p "$out"
        tar --format=ustar \
          -czf "$out/${drv.name}-docs.tar.gz" \
          -C dist/doc/html "${drv.name}-docs"
        runHook postInstall
      '';
    });

  /*
    Use the gold linker. It is a linker for ELF that is designed
    "to run as fast as possible on modern systems"
  */
  linkWithGold = appendConfigureFlag "--ghc-option=-optl-fuse-ld=gold --ld-option=-fuse-ld=gold --with-ld=ld.gold";

  /*
    link executables statically against haskell libs to reduce
    closure size
  */
  justStaticExecutables = overrideCabal (drv: {
    enableSharedExecutables = false;
    enableLibraryProfiling = drv.enableExecutableProfiling or false;
    isLibrary = false;
    doHaddock = false;
    postFixup =
      drv.postFixup or ""
      + ''

        # Remove every directory which could have links to other store paths.
        rm -rf $out/lib $out/nix-support $out/share/doc
      '';
    disallowGhcReference = true;
  });

  /*
    Build a source distribution tarball instead of using the source files
    directly. The effect is that the package is built as if it were published
    on hackage. This can be used as a test for the source distribution,
    assuming the build fails when packaging mistakes are in the cabal file.

    A faster implementation using `cabal-install` is available as
    `buildFromCabalSdist` in your Haskell package set.
  */
  buildFromSdist =
    pkg:
    overrideCabal (drv: {
      src = "${sdistTarball pkg}/${pkg.pname}-${pkg.version}.tar.gz";

      # Revising and jailbreaking the cabal file has been handled in sdistTarball
      revision = null;
      editedCabalFile = null;
      jailbreak = false;
    }) pkg;

  /*
    Build the package in a strict way to uncover potential problems.
    This includes buildFromSdist and failOnAllWarnings.
  */
  buildStrictly = pkg: buildFromSdist (failOnAllWarnings pkg);

  # Disable core optimizations, significantly speeds up build time
  disableOptimization = appendConfigureFlag "--disable-optimization";

  /*
    Turn on most of the compiler warnings and fail the build if any
    of them occur.
  */
  failOnAllWarnings = appendConfigureFlag "--ghc-option=-Wall --ghc-option=-Werror";

  /*
    Add a post-build check to verify that dependencies declared in
    the cabal file are actually used.

    The first attrset argument can be used to configure the strictness
    of this check and a list of ignored package names that would otherwise
    cause false alarms.
  */
  checkUnusedPackages =
    {
      ignoreEmptyImports ? false,
      ignoreMainModule ? false,
      ignorePackages ? [ ],
    }:
    drv:
    overrideCabal (_drv: {
      postBuild =
        let
          args = lib.concatStringsSep " " (
            lib.optional ignoreEmptyImports "--ignore-empty-imports"
            ++ lib.optional ignoreMainModule "--ignore-main-module"
            ++ map (pkg: "--ignore-package ${pkg}") ignorePackages
          );
        in
        "${pkgs.haskellPackages.packunused}/bin/packunused" + lib.optionalString (args != "") " ${args}";
    }) (appendConfigureFlag "--ghc-option=-ddump-minimal-imports" drv);

  buildStackProject = pkgs.callPackage ../generic-stack-builder.nix { };

  /*
    Add a dummy command to trigger a build despite an equivalent
    earlier build that is present in the store or cache.
  */
  triggerRebuild =
    i:
    overrideCabal (drv: {
      postUnpack =
        drv.postUnpack or ""
        + ''

          # trigger rebuild ${toString i}
        '';
    });

  /*
    Override the sources for the package and optionally the version.
    This also takes of removing editedCabalFile.
  */
  overrideSrc =
    {
      src,
      version ? null,
    }:
    drv:
    overrideCabal (_: {
      inherit src;
      version = if version == null then drv.version else version;
      editedCabalFile = null;
    }) drv;

  # Get all of the build inputs of a haskell package, divided by category.
  getBuildInputs = p: p.getBuildInputs;

  # Extract the haskell build inputs of a haskell package.
  # This is useful to build environments for developing on that
  # package.
  getHaskellBuildInputs = p: (getBuildInputs p).haskellBuildInputs;

  # Under normal evaluation, simply return the original package. Under
  # nix-shell evaluation, return a nix-shell optimized environment.
  shellAware = p: if lib.inNixShell then p.env else p;

  ghcInfo = ghc: rec {
    isCross = (ghc.cross or null) != null;
    isGhcjs = ghc.isGhcjs or false;
    nativeGhc = if isCross || isGhcjs then ghc.bootPkgs.ghc else ghc;
  };

  ### mkDerivation helpers
  # These allow external users of a haskell package to extract
  # information about how it is built in the same way that the
  # generic haskell builder does, by reusing the same functions.
  # Each function here has the same interface as mkDerivation and thus
  # can be called for a given package simply by overriding the
  # mkDerivation argument it used. See getHaskellBuildInputs above for
  # an example of this.

  # Some information about which phases should be run.
  controlPhases =
    ghc:
    let
      inherit (ghcInfo ghc) isCross;
    in
    {
      doCheck ? !isCross,
      doBenchmark ? false,
      ...
    }:
    {
      inherit doCheck doBenchmark;
    };

  # Utility to convert a directory full of `cabal2nix`-generated files into a
  # package override set
  #
  # packagesFromDirectory : { directory : Directory, ... } -> HaskellPackageOverrideSet
  packagesFromDirectory =
    { directory, ... }:

    self: super:
    let
      haskellPaths = lib.filter (lib.hasSuffix ".nix") (builtins.attrNames (builtins.readDir directory));

      toKeyVal = file: {
        name = builtins.replaceStrings [ ".nix" ] [ "" ] file;

        value = self.callPackage (directory + "/${file}") { };
      };

    in
    builtins.listToAttrs (map toKeyVal haskellPaths);

  /*
    INTERNAL function retained for backwards compatibility, use
    haskell.packages.*.generateOptparseApplicativeCompletions instead!
  */
  __generateOptparseApplicativeCompletion =
    exeName:
    overrideCabal (drv: {
      postInstall =
        (drv.postInstall or "")
        + ''
          bashCompDir="''${!outputBin}/share/bash-completion/completions"
          zshCompDir="''${!outputBin}/share/zsh/vendor-completions"
          fishCompDir="''${!outputBin}/share/fish/vendor_completions.d"
          mkdir -p "$bashCompDir" "$zshCompDir" "$fishCompDir"
          "''${!outputBin}/bin/${exeName}" --bash-completion-script "''${!outputBin}/bin/${exeName}" >"$bashCompDir/${exeName}"
          "''${!outputBin}/bin/${exeName}" --zsh-completion-script "''${!outputBin}/bin/${exeName}" >"$zshCompDir/_${exeName}"
          "''${!outputBin}/bin/${exeName}" --fish-completion-script "''${!outputBin}/bin/${exeName}" >"$fishCompDir/${exeName}.fish"

          # Sanity check
          grep -F ${exeName} <$bashCompDir/${exeName} >/dev/null || {
            echo 'Could not find ${exeName} in completion script.'
            exit 1
          }
        '';
    });

  /*
    Retained for backwards compatibility.
    Use haskell.packages.*.generateOptparseApplicativeCompletions
    which is cross aware instead.
  */
  generateOptparseApplicativeCompletions =
    commands: pkg:
    lib.warnIf (lib.oldestSupportedReleaseIsAtLeast 2211)
      "haskellLib.generateOptparseApplicativeCompletions is deprecated in favor of haskellPackages.generateOptparseApplicativeCompletions. Please change ${pkg.name} to use the latter and make sure it uses its matching haskell.packages set!"
      (pkgs.lib.foldr __generateOptparseApplicativeCompletion pkg commands);

  /*
    Retained for backwards compatibility.
    Use haskell.packages.*.generateOptparseApplicativeCompletions
    which is cross aware instead.
  */
  generateOptparseApplicativeCompletion =
    command: pkg:
    lib.warnIf (lib.oldestSupportedReleaseIsAtLeast 2211)
      "haskellLib.generateOptparseApplicativeCompletion is deprecated in favor of haskellPackages.generateOptparseApplicativeCompletions (plural!). Please change ${pkg.name} to use the latter and make sure it uses its matching haskell.packages set!"
      (__generateOptparseApplicativeCompletion command pkg);

  # Don't fail at configure time if there are multiple versions of the
  # same package in the (recursive) dependencies of the package being
  # built. Will delay failures, if any, to compile time.
  allowInconsistentDependencies = overrideCabal (drv: {
    allowInconsistentDependencies = true;
  });

  # Work around a Cabal bug requiring pkg-config --static --libs to work even
  # when linking dynamically, affecting Cabal 3.8 and 3.9.
  # https://github.com/haskell/cabal/issues/8455
  #
  # For this, we treat the runtime system/pkg-config dependencies of a Haskell
  # derivation as if they were propagated from their dependencies which allows
  # pkg-config --static to work in most cases.
  #
  # Warning: This function may change or be removed at any time, e.g. if we find
  # a different workaround, upstream fixes the bug or we patch Cabal.
  __CabalEagerPkgConfigWorkaround =
    let
      # Take list of derivations and return list of the transitive dependency
      # closure, only taking into account buildInputs. Loosely based on
      # closePropagationFast.
      propagatedPlainBuildInputs =
        drvs:
        builtins.map (i: i.val) (
          builtins.genericClosure {
            startSet = builtins.map (drv: {
              key = drv.outPath;
              val = drv;
            }) drvs;
            operator =
              { val, ... }:
              if !lib.isDerivation val then
                [ ]
              else
                builtins.concatMap (
                  drv:
                  if !lib.isDerivation drv then
                    [ ]
                  else
                    [
                      {
                        key = drv.outPath;
                        val = drv;
                      }
                    ]
                ) (val.buildInputs or [ ] ++ val.propagatedBuildInputs or [ ]);
          }
        );
    in
    overrideCabal (old: {
      benchmarkPkgconfigDepends = propagatedPlainBuildInputs old.benchmarkPkgconfigDepends or [ ];
      executablePkgconfigDepends = propagatedPlainBuildInputs old.executablePkgconfigDepends or [ ];
      libraryPkgconfigDepends = propagatedPlainBuildInputs old.libraryPkgconfigDepends or [ ];
      testPkgconfigDepends = propagatedPlainBuildInputs old.testPkgconfigDepends or [ ];
    });

  /*
    Get the combined build inputs for a list of Haskell packages.

    This combines all of the build input attributes of the packages
    (`buildDepends`, `libraryHaskellDepends`, etc.) while being careful to
    filter out packages from the arguments from the result.

    This returns an attribute set where each key corresponds to a
    `haskellPackages.mkDerivation` argument accepting a list of build inputs.

    This logic is used by `haskellPackages.shellFor` to determine the packages
    which are needed to build a particular set of packages.

    ```
    > combineInputs { packages = [haskellPackages.lens haskellPackages.aeson]; }
    {
      buildTools = [ ];
      # ...
      libraryHaskellDepends = [
        «derivation /nix/store/pha8fwh6lc05nm2vvg1sivhyb5nmnkic-assoc-1.1.1.drv»
        «derivation /nix/store/9j5w6q1hmf50r9jydj9n7vcplknw4fvv-base-orphans-0.9.3.drv»
        # ...
        «derivation /nix/store/r28dlymqid6q31dh5a46k36bag60ca52-witherable-0.5.drv»
      ];
      # ...
      testHaskellDepends = [
        «derivation /nix/store/rl0kfrl05z94in9f2fk8hl44r9pfyh44-HUnit-1.6.2.0.drv»
        «derivation /nix/store/0dbqnww1fb3ml9f4s21phkw901yszd48-QuickCheck-2.14.3.drv»
        # ...
        «derivation /nix/store/0dml0xsw6s1v1d2qqhyk3qjyl4lw2fi4-vector-0.13.2.0.drv»
      ];
    }
    ```
  */
  combineInputs =
    {
      # A list of packages to get the transitive inputs for.
      #
      # Note that while the `packages` parameter to `haskellPackages.shellFor`
      # is a function from the Haskell package set to a list of packages, this
      # function takes the list of packages directly.
      packages,

      # Extra dependencies, in the form of `haskellPackages.mkDerivation` build
      # attributes.
      #
      # An example use case is when you have Haskell scripts that use
      # libraries that don't occur in the dependencies of `packages`.
      #
      # Example:
      #
      #   extraDependencies = p: {
      #     libraryHaskellDepends = [ p.releaser ];
      #   };
      extraDependencies ? { },

      ...
    }:
    let
      # This is a list of attribute sets, where each attribute set
      # corresponds to the build inputs of one of the packages input to shellFor.
      #
      # Each attribute has keys like buildDepends, executableHaskellDepends,
      # testPkgconfigDepends, etc.  The values for the keys of the attribute
      # set are lists of dependencies.
      #
      # Example:
      #   cabalDepsForSelected
      #   => [
      #        # This may be the attribute set corresponding to the `backend`
      #        # package in the example above.
      #        { buildDepends = [ gcc ... ];
      #          libraryHaskellDepends = [ lens conduit ... ];
      #          ...
      #        }
      #        # This may be the attribute set corresponding to the `common`
      #        # package in the example above.
      #        { testHaskellDepends = [ tasty hspec ... ];
      #          libraryHaskellDepends = [ lens aeson ];
      #          benchmarkHaskellDepends = [ criterion ... ];
      #          ...
      #        }
      #        ...
      #      ]
      cabalDepsForSelected = builtins.map (p: p.getCabalDeps) (builtins.filter (p: p != null) packages);

      # A predicate that takes a derivation as input, and tests whether it is
      # the same as any of the `packages`.
      #
      # Returns true if the input derivation is not in the list of `packages`.
      #
      # isNotSelected :: Derivation -> Bool
      #
      # Example:
      #
      #   isNotSelected common [ frontend backend common ]
      #   => false
      #
      #   isNotSelected lens [ frontend backend common ]
      #   => true
      isNotSelected = input: pkgs.lib.all (p: input.outPath or null != p.outPath) packages;

      # A function that takes a list of list of derivations, filters out all
      # the `packages` from each list, and concats the results.
      #
      #   zipperCombinedPkgs :: [[Derivation]] -> [Derivation]
      #
      # Example:
      #   zipperCombinedPkgs [ [ lens conduit ] [ aeson frontend ] ]
      #   => [ lens conduit aeson ]
      #
      # Note: The reason this isn't just the function `pkgs.lib.concat` is
      # that we need to be careful to remove dependencies that are in the
      # `packages` list.
      #
      # For instance, in the above example, if `common` is a dependency of
      # `backend`, then zipperCombinedPkgs needs to be careful to filter out
      # `common`, because cabal will end up ignoring that built version,
      # assuming new-style commands.
      zipperCombinedPkgs = vals: pkgs.lib.concatMap (drvList: pkgs.lib.filter isNotSelected drvList) vals;

      # Zip `cabalDepsForSelected` into a single attribute list, combining
      # the derivations in all the individual attributes.
      #
      # Example:
      #   packageInputs
      #   => # Assuming the value of cabalDepsForSelected is the same as
      #      # the example in cabalDepsForSelected:
      #      { buildDepends = [ gcc ... ];
      #        libraryHaskellDepends = [ lens conduit aeson ... ];
      #        testHaskellDepends = [ tasty hspec ... ];
      #        benchmarkHaskellDepends = [ criterion ... ];
      #        ...
      #      }
      #
      # See the Note in `zipperCombinedPkgs` for what gets filtered out from
      # each of these dependency lists.
      packageInputs = pkgs.lib.zipAttrsWith (_name: zipperCombinedPkgs) (
        cabalDepsForSelected ++ [ extraDependencies ]
      );
    in
    packageInputs;

}