path: root/pkgs/development/haskell-modules/make-package-set.nix

# This expression takes a file like `hackage-packages.nix` and constructs
# a full package set out of that.

{ # package-set used for build tools (all of nixpkgs)
  buildPackages

, # A haskell package set for Setup.hs, compiler plugins, and similar
  # build-time uses.
  buildHaskellPackages

, # package-set used for non-haskell dependencies (all of nixpkgs)
  pkgs

, # stdenv provides our build and host platforms
  stdenv

, # this module provides the list of known licenses and maintainers
  lib

  # needed for overrideCabal & packageSourceOverrides
, haskellLib

, # hashes for downloading Hackage packages
  # This is either a directory or a .tar.gz containing the cabal files and
  # hashes of Hackage as exemplified by this repository:
  # https://github.com/commercialhaskell/all-cabal-hashes/tree/hackage
  all-cabal-hashes

, # compiler to use
  ghc

, # A function that takes `{ pkgs, lib, callPackage }` as the first arg and
  # `self` as second, and returns a set of haskell packages
  package-set

, # The final, fully overridden package set usable with the nixpkgs fixpoint
  # overriding functionality
  extensible-self
}:

# return value: a function from self to the package set
self:

let
  inherit (stdenv) buildPlatform hostPlatform;

  inherit (lib) fix' extends makeOverridable;
  inherit (haskellLib) overrideCabal;

  mkDerivationImpl = pkgs.callPackage ./generic-builder.nix {
    inherit stdenv;
    nodejs = buildPackages.nodejs-slim;
    inherit (self) buildHaskellPackages ghc ghcWithHoogle ghcWithPackages;
    inherit (self.buildHaskellPackages) jailbreak-cabal;
    hscolour = overrideCabal (drv: {
      isLibrary = false;
      doHaddock = false;
      hyperlinkSource = false;      # Avoid depending on hscolour for this build.
      postFixup = "rm -rf $out/lib $out/share $out/nix-support";
    }) self.buildHaskellPackages.hscolour;
    cpphs = overrideCabal (drv: {
        isLibrary = false;
        postFixup = "rm -rf $out/lib $out/share $out/nix-support";
    }) (self.cpphs.overrideScope (self: super: {
      mkDerivation = drv: super.mkDerivation (drv // {
        enableSharedExecutables = false;
        enableSharedLibraries = false;
        doHaddock = false;
        useCpphs = false;
      });
    }));
  };

  mkDerivation = makeOverridable mkDerivationImpl;

  # manualArgs are the arguments that were explicitly passed to `callPackage`, like:
  #
  # callPackage foo { bar = null; };
  #
  # here `bar` is a manual argument.
  callPackageWithScope = scope: fn: manualArgs:
    let
      # this code is copied from callPackage in lib/customisation.nix
      #
      # we cannot use `callPackage` here because we want to call `makeOverridable`
      # on `drvScope` (we cannot add `overrideScope` after calling `callPackage` because then it is
      # lost on `.override`) but determine the auto-args based on `drv` (the problem here
      # is that nix has no way to "passthrough" args while preserving the reflection
      # info that callPackage uses to determine the arguments).
      drv = if lib.isFunction fn then fn else import fn;
      auto = builtins.intersectAttrs (lib.functionArgs drv) scope;

      # Converts a returned function to a functor attribute set if necessary
      ensureAttrs = v: if builtins.isFunction v then { __functor = _: v; } else v;

      # this wraps the `drv` function to add `scope` and `overrideScope` to the result.
      drvScope = allArgs: ensureAttrs (drv allArgs) // {
        inherit scope;
        overrideScope = f:
          let newScope = mkScope (fix' (extends f scope.__unfix__));
          # note that we have to be careful here: `allArgs` includes the auto-arguments that
          # weren't manually specified. If we would just pass `allArgs` to the recursive call here,
          # then we wouldn't look up any packages in the scope in the next interation, because it
          # appears as if all arguments were already manually passed, so the scope change would do
          # nothing.
          in callPackageWithScope newScope drv manualArgs;
      };
    in lib.makeOverridable drvScope (auto // manualArgs);

  mkScope = scope: let
      ps = pkgs.__splicedPackages;
      scopeSpliced = pkgs.splicePackages {
        pkgsBuildBuild = scope.buildHaskellPackages.buildHaskellPackages;
        pkgsBuildHost = scope.buildHaskellPackages;
        pkgsBuildTarget = {};
        pkgsHostHost = {};
        pkgsHostTarget = scope;
        pkgsTargetTarget = {};
      } // {
        # Don't splice these
        inherit (scope) ghc buildHaskellPackages;
      };
    in ps // ps.xorg // ps.gnome2 // { inherit stdenv; } // scopeSpliced;
  defaultScope = mkScope self;
  callPackage = drv: args: callPackageWithScope defaultScope drv args;

  # Use cabal2nix to create a default.nix for the package sources found at 'src'.
  haskellSrc2nix = { name, src, sha256 ? null, extraCabal2nixOptions ? "" }:
    let
      sha256Arg = if sha256 == null then "--sha256=" else ''--sha256="${sha256}"'';
    in buildPackages.runCommand "cabal2nix-${name}" {
      nativeBuildInputs = [ buildPackages.cabal2nix-unwrapped ];
      preferLocalBuild = true;
      allowSubstitutes = false;
      LANG = "en_US.UTF-8";
      LOCALE_ARCHIVE = pkgs.lib.optionalString (buildPlatform.libc == "glibc") "${buildPackages.glibcLocales}/lib/locale/locale-archive";
    } ''
      export HOME="$TMP"
      mkdir -p "$out"
      cabal2nix --compiler=${self.ghc.haskellCompilerName} --system=${hostPlatform.config} ${sha256Arg} "${src}" ${extraCabal2nixOptions} > "$out/default.nix"
    '';

  # Given a package name and version, e.g. name = "async", version = "2.2.4",
  # gives its cabal file and hashes (JSON file) as discovered from the
  # all-cabal-hashes value. If that's a directory, it will copy the relevant
  # files to $out; if it's a tarball, it will extract and move them to $out.
  all-cabal-hashes-component = name: version: buildPackages.runCommand "all-cabal-hashes-component-${name}-${version}" {} ''
    mkdir -p $out
    if [ -d ${all-cabal-hashes} ]
    then
      cp ${all-cabal-hashes}/${name}/${version}/${name}.json $out
      cp ${all-cabal-hashes}/${name}/${version}/${name}.cabal $out
    else
      tar --wildcards -xzvf ${all-cabal-hashes} \*/${name}/${version}/${name}.{json,cabal}
      mv */${name}/${version}/${name}.{json,cabal} $out
    fi
  '';

  hackage2nix = name: version: let component = all-cabal-hashes-component name version; in self.haskellSrc2nix {
    name   = "${name}-${version}";
    sha256 = ''$(sed -e 's/.*"SHA256":"//' -e 's/".*$//' "${component}/${name}.json")'';
    src    = "${component}/${name}.cabal";
  };

  # Adds a nix file derived from cabal2nix in the passthru of the derivation it
  # produces. This is useful to debug callHackage / callCabal2nix by looking at
  # the content of the nix file pointed by `cabal2nixDeriver`.
  # However, it does not keep a reference to that file, which may be garbage
  # collected, which may be an annoyance.
  callPackageKeepDeriver = src: args:
    overrideCabal (orig: {
      passthru = orig.passthru or {} // {
        # When using callCabal2nix or callHackage, it is often useful
        # to debug a failure by inspecting the Nix expression
        # generated by cabal2nix. This can be accessed via this
        # cabal2nixDeriver field.
        cabal2nixDeriver = src;
      };
    }) (self.callPackage src args);

in package-set { inherit pkgs lib callPackage; } self // {

    inherit mkDerivation callPackage haskellSrc2nix hackage2nix buildHaskellPackages;

    inherit (haskellLib) packageSourceOverrides;

    # callHackage :: Text -> Text -> AttrSet -> HaskellPackage
    #
    # e.g., while overriding a package set:
    #    '... foo = self.callHackage "foo" "1.5.3" {}; ...'
    callHackage = name: version: callPackageKeepDeriver (self.hackage2nix name version);

    # callHackageDirect
    #   :: { pkg :: Text, ver :: Text, sha256 :: Text }
    #   -> AttrSet
    #   -> HaskellPackage
    #
    # This function does not depend on all-cabal-hashes and therefore will work
    # for any version that has been released on hackage as opposed to only
    # versions released before whatever version of all-cabal-hashes you happen
    # to be currently using.
    callHackageDirect = {pkg, ver, sha256, rev ? { revision = null; sha256 = null; }}: args:
      let pkgver = "${pkg}-${ver}";
          firstRevision = self.callCabal2nix pkg (pkgs.fetchzip {
            url = "mirror://hackage/${pkgver}/${pkgver}.tar.gz";
            inherit sha256;
          }) args;
      in overrideCabal (orig: {
        revision = rev.revision;
        editedCabalFile = rev.sha256;
      }) firstRevision;

    # Creates a Haskell package from a source package by calling cabal2nix on the source.
    callCabal2nixWithOptions = name: src: opts: args:
      let
        extraCabal2nixOptions = if builtins.isString opts
                                then opts
                                else opts.extraCabal2nixOptions or "";
        srcModifier = opts.srcModifier or null;
        defaultFilter = path: type:
                   pkgs.lib.hasSuffix ".cabal" path ||
                   baseNameOf path == "package.yaml";
        expr = self.haskellSrc2nix {
          inherit name extraCabal2nixOptions;
          src = if srcModifier != null
                then srcModifier src
                else if pkgs.lib.canCleanSource src
                then pkgs.lib.cleanSourceWith { inherit src; filter = defaultFilter; }
                else src;
        };
      in overrideCabal (orig: {
           inherit src;
         }) (callPackageKeepDeriver expr args);

    callCabal2nix = name: src: args: self.callCabal2nixWithOptions name src "" args;

    # : { root : Path
    #   , name : Defaulted String
    #   , source-overrides : Defaulted (Either Path VersionNumber)
    #   , overrides : Defaulted (HaskellPackageOverrideSet)
    #   , modifier : Defaulted
    #   , returnShellEnv : Defaulted
    #   , withHoogle : Defaulted
    #   , cabal2nixOptions : Defaulted
    #   } -> NixShellAwareDerivation
    #
    # Given a path to a haskell package directory, an optional package name
    # which defaults to the base name of the path, an optional set of source
    # overrides as appropriate for the 'packageSourceOverrides' function, an
    # optional set of arbitrary overrides, and an optional haskell package
    # modifier, return a derivation appropriate for nix-build or nix-shell to
    # build that package.
    #
    # If 'returnShellEnv' is true this returns a derivation which will give you
    # an environment suitable for developing the listed packages with an
    # incremental tool like cabal-install.
    #
    # If 'withHoogle' is true (the default if a shell environment is requested)
    # then 'ghcWithHoogle' is used to generate the derivation (instead of
    # 'ghcWithPackages'), see the documentation there for more information.
    #
    # 'cabal2nixOptions' can contain extra command line arguments to pass to
    # 'cabal2nix' when generating the package derivation, for example setting
    # a cabal flag with '--flag=myflag'.
    developPackage =
      { root
      , name ? lib.optionalString (builtins.typeOf root == "path") (builtins.baseNameOf root)
      , source-overrides ? {}
      , overrides ? self: super: {}
      , modifier ? drv: drv
      , returnShellEnv ? pkgs.lib.inNixShell
      , withHoogle ? returnShellEnv
      , cabal2nixOptions ? "" }:
      let drv =
        (extensible-self.extend
           (pkgs.lib.composeExtensions
              (self.packageSourceOverrides source-overrides)
              overrides))
        .callCabal2nixWithOptions name root cabal2nixOptions {};
      in if returnShellEnv
           then (modifier drv).envFunc {inherit withHoogle;}
           else modifier drv;

    # This can be used to easily create a derivation containing GHC and the specified set of Haskell packages.
    #
    # Example:
    #   $ nix-shell -p 'haskellPackages.ghcWithPackages (hpkgs: [ hpkgs.mtl hpkgs.lens ])'
    #   $ ghci    # in the nix-shell
    #   Prelude > import Control.Lens
    #
    # GHC is setup with a package database with all the specified Haskell packages.
    #
    # ghcWithPackages :: (HaskellPkgSet -> [ HaskellPkg ]) -> Derivation
    ghcWithPackages = buildHaskellPackages.callPackage ./with-packages-wrapper.nix {
      haskellPackages = self;
      inherit (self) hoogleWithPackages;
    };


    # Put 'hoogle' into the derivation's PATH with a database containing all
    # the package's dependencies; run 'hoogle server --local' in a shell to
    # host a search engine for the dependencies.
    #
    # Example usage:
    #  $ nix-shell -p 'haskellPackages.hoogleWithPackages (p: [ p.mtl p.lens ])'
    #  [nix-shell] $ hoogle server
    #
    # hoogleWithPackages :: (HaskellPkgSet -> [ HaskellPkg ]) -> Derivation
    #
    # To reload the Hoogle server automatically on .cabal file changes try
    # this:
    # echo *.cabal | entr -r -- nix-shell --run 'hoogle server --local'
    hoogleWithPackages = self.callPackage ./hoogle.nix {
      haskellPackages = self;
    };
    hoogleLocal =
      { packages ? [] }:
      lib.warn "hoogleLocal is deprecated, use hoogleWithPackages instead" (
        self.hoogleWithPackages (_: packages)
      );
    # This is like a combination of ghcWithPackages and hoogleWithPackages.
    # It provides a derivation containing both GHC and Hoogle with an index of
    # the given Haskell package database.
    #
    # Example:
    #   $ nix-shell -p 'haskellPackages.ghcWithHoogle (hpkgs: [ hpkgs.conduit hpkgs.lens ])'
    #
    # ghcWithHoogle :: (HaskellPkgSet -> [ HaskellPkg ]) -> Derivation
    ghcWithHoogle = self.ghcWithPackages.override {
      withHoogle = true;
    };

    # Returns a derivation whose environment contains a GHC with only
    # the dependencies of packages listed in `packages`, not the
    # packages themselves. Using nix-shell on this derivation will
    # give you an environment suitable for developing the listed
    # packages with an incremental tool like cabal-install.
    #
    # In addition to the "packages" arg and "withHoogle" arg, anything that
    # can be passed into stdenv.mkDerivation can be included in the input attrset
    #
    #     # default.nix
    #     with import <nixpkgs> {};
    #     haskellPackages.extend (haskell.lib.compose.packageSourceOverrides {
    #       frontend = ./frontend;
    #       backend = ./backend;
    #       common = ./common;
    #     })
    #
    #     # shell.nix
    #     let pkgs = import <nixpkgs> {} in
    #     (import ./.).shellFor {
    #       packages = p: [p.frontend p.backend p.common];
    #       withHoogle = true;
    #       buildInputs = [ pkgs.python pkgs.cabal-install ];
    #     }
    #
    #     -- cabal.project
    #     packages:
    #       frontend/
    #       backend/
    #       common/
    #
    #     bash$ nix-shell --run "cabal new-build all"
    #     bash$ nix-shell --run "python"
    shellFor =
      { # Packages to create this development shell for.  These are usually
        # your local packages.
        packages
      , # Whether or not to generate a Hoogle database for all the
        # dependencies.
        withHoogle ? false
      , # Whether or not to include benchmark dependencies of your local
        # packages.  You should set this to true if you have benchmarks defined
        # in your local packages that you want to be able to run with cabal benchmark
        doBenchmark ? false
        # An optional function that can modify the generic builder arguments
        # for the fake package that shellFor uses to construct its environment.
        #
        # Example:
        #   let
        #     # elided...
        #     haskellPkgs = pkgs.haskell.packages.ghc884.override (hpArgs: {
        #       overrides = pkgs.lib.composeExtensions (hpArgs.overrides or (_: _: { })) (
        #         _hfinal: hprev: {
        #           mkDerivation = args: hprev.mkDerivation ({
        #             doCheck = false;
        #             doBenchmark = false;
        #             doHoogle = true;
        #             doHaddock = true;
        #             enableLibraryProfiling = false;
        #             enableExecutableProfiling = false;
        #           } // args);
        #         }
        #       );
        #     });
        #   in
        #   haskellPkgs.shellFor {
        #     packages = p: [ p.foo ];
        #     genericBuilderArgsModifier = args: args // { doCheck = true; doBenchmark = true };
        #   }
        #
        # This will disable tests and benchmarks for everything in "haskellPkgs"
        # (which will invalidate the binary cache), and then re-enable them
        # for the "shellFor" environment (ensuring that any test/benchmark
        # dependencies for "foo" will be available within the nix-shell).
      , genericBuilderArgsModifier ? (args: args)

        # Extra dependencies, in the form of cabal2nix build attributes.
        #
        # An example use case is when you have Haskell scripts that use
        # libraries that don't occur in your packages' dependencies.
        #
        # Example:
        #
        #   extraDependencies = p: {
        #     libraryHaskellDepends = [ p.releaser ];
        #   };
      , extraDependencies ? p: {}
      , ...
      } @ args:
      let
        # A list of the packages we want to build a development shell for.
        # This is a list of Haskell package derivations.
        selected = packages self;

        # 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 = map (p: p.getCabalDeps) selected;

        # A predicate that takes a derivation as input, and tests whether it is
        # the same as any of the `selected` packages.
        #
        # Returns true if the input derivation is not in the list of `selected`
        # 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) selected;

        # A function that takes a list of list of derivations, filters out all
        # the `selected` 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
        # `selected` packages.
        #
        # 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 self) ]);

        # A attribute set to pass to `haskellPackages.mkDerivation`.
        #
        # The important thing to note here is that all the fields from
        # packageInputs are set correctly.
        genericBuilderArgs = {
          pname =
            if pkgs.lib.length selected == 1
            then (pkgs.lib.head selected).name
            else "packages";
          version = "0";
          license = null;
        }
        // packageInputs
        // pkgs.lib.optionalAttrs doBenchmark {
          # `doBenchmark` needs to explicitly be set here because haskellPackages.mkDerivation defaults it to `false`.  If the user wants benchmark dependencies included in their development shell, it has to be explicitly enabled here.
          doBenchmark = true;
        };

        # This is a pseudo Haskell package derivation that contains all the
        # dependencies for the packages in `selected`.
        #
        # This is a derivation created with `haskellPackages.mkDerivation`.
        #
        # pkgWithCombinedDeps :: HaskellDerivation
        pkgWithCombinedDeps = self.mkDerivation (genericBuilderArgsModifier genericBuilderArgs);

        # The derivation returned from `envFunc` for `pkgWithCombinedDeps`.
        #
        # This is a derivation that can be run with `nix-shell`.  It provides a
        # GHC with a package database with all the dependencies of our
        # `selected` packages.
        #
        # This is a derivation created with `stdenv.mkDerivation` (not
        # `haskellPackages.mkDerivation`).
        #
        # pkgWithCombinedDepsDevDrv :: Derivation
        pkgWithCombinedDepsDevDrv = pkgWithCombinedDeps.envFunc { inherit withHoogle; };

        mkDerivationArgs = builtins.removeAttrs args [ "genericBuilderArgsModifier" "packages" "withHoogle" "doBenchmark" "extraDependencies" ];

      in pkgWithCombinedDepsDevDrv.overrideAttrs (old: mkDerivationArgs // {
        nativeBuildInputs = old.nativeBuildInputs ++ mkDerivationArgs.nativeBuildInputs or [];
        buildInputs = old.buildInputs ++ mkDerivationArgs.buildInputs or [];
      });

    ghc = ghc // {
      withPackages = self.ghcWithPackages;
      withHoogle = self.ghcWithHoogle;
    };

    /*
      Run `cabal sdist` on a source.

      Unlike `haskell.lib.sdistTarball`, this does not require any dependencies
      to be present, as it uses `cabal-install` instead of building `Setup.hs`.
      This makes `cabalSdist` faster than `sdistTarball`.
    */
    cabalSdist = {
      src,
      name ? if src?name then "${src.name}-sdist.tar.gz" else "source.tar.gz"
    }:
      pkgs.runCommandLocal name
        {
          inherit src;
          nativeBuildInputs = [
            buildHaskellPackages.cabal-install

            # TODO after https://github.com/haskell/cabal/issues/8352
            #      remove ghc
            self.ghc
          ];
          dontUnpack = false;
        } ''
        unpackPhase
        cd "''${sourceRoot:-.}"
        patchPhase
        mkdir out
        HOME=$PWD cabal sdist --output-directory out
        mv out/*.tar.gz $out
      '';

    /*
      Like `haskell.lib.buildFromSdist`, but using `cabal sdist` instead of
      building `./Setup`.

      Unlike `haskell.lib.buildFromSdist`, this does not require any dependencies
      to be present. This makes `buildFromCabalSdist` faster than `haskell.lib.buildFromSdist`.
    */
    buildFromCabalSdist = pkg:
      haskellLib.overrideSrc
        {
          src = self.cabalSdist { inherit (pkg) src; };
          version = pkg.version;
        }
        pkg;

    /*
      Modify a Haskell package to add shell completion scripts for the
      given executables produced by it. These completion scripts will be
      picked up automatically if the resulting derivation is installed,
      e.g. by `nix-env -i`.

      This depends on the `--*-completion` flag `optparse-applicative` provides
      automatically. Since we need to invoke installed executables, completions
      are not generated if we are cross-compiling.

       commands: names of the executables built by the derivation
            pkg: Haskell package that builds the executables

      Example:
        generateOptparseApplicativeCompletions [ "exec1" "exec2" ] pkg

       Type: [str] -> drv -> drv
    */
    generateOptparseApplicativeCompletions =
      (self.callPackage (
        { stdenv }:

        commands:
        pkg:

        if stdenv.buildPlatform.canExecute stdenv.hostPlatform
        then lib.foldr haskellLib.__generateOptparseApplicativeCompletion pkg commands
        else pkg
      ) { }) // { __attrsFailEvaluation = true; };

    /*
      Modify given Haskell package to force GHC to employ the LLVM
      codegen backend when compiling. Useful when working around bugs
      in a native codegen backend GHC defaults to.

      Example:
        forceLlvmCodegenBackend tls

      Type: drv -> drv
    */
    forceLlvmCodegenBackend = overrideCabal (drv: {
      configureFlags = drv.configureFlags or [ ] ++ [ "--ghc-option=-fllvm" ];
      buildTools = drv.buildTools or [ ] ++ [ self.llvmPackages.llvm ];
    });
  }