diff --git a/default.nix b/default.nix
index 20c57fd093..b612e796cb 100644
--- a/default.nix
+++ b/default.nix
@@ -38,6 +38,11 @@ let
   # overridden with NIX_PATH.
   fetchExternal = import ./lib/fetch-external.nix;
 
+  mkHackageIndex = indexState: import ./lib/hackageIndex.nix {
+    inherit (pkgs) runCommand cabal-install;
+    inherit indexState;
+  };
+
   # All packages from Hackage as Nix expressions
   hackage = import (fetchExternal {
     name     = "hackage-exprs-source";
@@ -123,6 +128,20 @@ let
       update-stackage = self.callPackage ./scripts/update-stackage.nix {};
       update-pins = self.callPackage ./scripts/update-pins.nix {};
     };
+
+    # Make this handy overridable fetch function available.
+    inherit fetchExternal;
+
+    # Takes a haskell src directory runs cabal new-configure and plan-to-nix.
+    # Resulting nix files are added to nix-plan subdirectory.
+    callCabalProjectToNix = import ./lib/cabalProjectToNix.nix {
+      inherit mkHackageIndex;
+      inherit pkgs;
+      inherit (pkgs) runCommand cabal-install ghc;
+      inherit (pkgs.haskellPackages) hpack;
+      inherit (self) nix-tools;
+      inherit (pkgs) symlinkJoin;
+    };
   });
 
 in
diff --git a/lib/cabalProjectToNix.nix b/lib/cabalProjectToNix.nix
new file mode 100644
index 0000000000..26fd5da044
--- /dev/null
+++ b/lib/cabalProjectToNix.nix
@@ -0,0 +1,62 @@
+{ mkHackageIndex, pkgs, runCommand, nix-tools, cabal-install, ghc, hpack, symlinkJoin }:
+let defaultGhc = ghc;
+    defaultCabalInstall = cabal-install;
+in { hackageIndexState, src, ghc ? defaultGhc, cabal-install ? defaultCabalInstall }:
+let
+  cabalFiles =
+    pkgs.lib.cleanSourceWith {
+      inherit src;
+      filter = path: type:
+        type == "directory" ||
+        pkgs.lib.any (i: (pkgs.lib.hasSuffix i path)) [ ".project" ".cabal" "package.yaml" ];
+    };
+  plan = if (builtins.compareVersions cabal-install.version "2.4.0.0") < 0
+         # cabal-install versions before 2.4 will generate insufficient plan information.
+         then throw "cabal-install (current version: ${cabal-install.version}) needs to be at least 2.4 for plan-to-nix to work without cabal-to-nix"
+         else runCommand "plan" {
+    nativeBuildInputs = [ nix-tools ghc hpack cabal-install pkgs.rsync ];
+  } ''
+    tmp=$(mktemp -d)
+    cd $tmp
+    cp -r ${cabalFiles}/* .
+    chmod +w -R .
+    # warning: this may not generate the proper cabal file.
+    # hpack allows globbing, and turns that into module lists
+    # without the source available (we cleaneSourceWith'd it),
+    # this may not produce the right result.
+    find . -name package.yaml -exec hpack "{}" \;
+    HOME=${mkHackageIndex hackageIndexState} cabal new-configure
+
+    export LANG=C.utf8 # Needed or stack-to-nix will die on unicode inputs
+    mkdir -p $out
+
+    # ensure we have all our .cabal files (also those generated from package.yaml) files.
+    # otherwise we'd need to be careful about putting the `cabal-generator = hpack` into
+    # the nix expression.  As we already called `hpack` on all `package.yaml` files we can
+    # skip that step and just package the .cabal files up as well.
+    #
+    # This is also important as `plan-to-nix` will look for the .cabal files when generating
+    # the relevant `pkgs.nix` file with the local .cabal expressions.
+    rsync -a --prune-empty-dirs \
+          --include '*/' --include '*.cabal' --include 'package.yaml' \
+          --exclude '*' \
+          $tmp/ $out/
+
+    # make sure the path's in the plan.json are relative to $out instead of $tmp
+    # this is necessary so that plan-to-nix relative path logic can work.
+    substituteInPlace $tmp/dist-newstyle/cache/plan.json --replace "$tmp" "$out"
+
+    # run `plan-to-nix` in $out.  This should produce files right there with the
+    # proper relative paths.
+    (cd $out && plan-to-nix --plan-json $tmp/dist-newstyle/cache/plan.json -o .)
+
+    # move pkgs.nix to default.nix ensure we can just nix `import` the result.
+    mv $out/pkgs.nix $out/default.nix
+  '';
+in
+  runCommand "plan-and-src" { nativeBuildInputs = [ pkgs.xorg.lndir pkgs.rsync ]; } ''
+    mkdir $out
+    # todo: should we clean `src` to drop any .git, .nix, ... other irelevant files?
+    lndir -silent "${src}" "$out"
+    rsync -a ${plan}/ $out/
+  ''
diff --git a/lib/hackageIndex.nix b/lib/hackageIndex.nix
new file mode 100644
index 0000000000..14d6d4b460
--- /dev/null
+++ b/lib/hackageIndex.nix
@@ -0,0 +1,16 @@
+{ runCommand, cabal-install
+, indexState ? "2019-04-24T21:34:04Z"
+} :
+let
+  # To avoid downloading more data than necessary this will provide a base.
+  cachedState = runCommand "hackage-${builtins.substring 0 4 indexState}" {} ''
+    mkdir -p $out
+    HOME=$out ${cabal-install}/bin/cabal update --index-state='${builtins.substring 0 4 indexState}-01-01T00:00:00Z'
+  '';
+in runCommand "hackage-${builtins.replaceStrings [":"] [""] indexState}" {} ''
+    mkdir -p $out
+    cp -r ${cachedState}/.cabal $out
+    chmod +w -R $out/.cabal
+    sed -i.back -e "s|${cachedState}|$out|g" $out/.cabal/config
+    HOME=$out ${cabal-install}/bin/cabal update --index-state='${indexState}'
+  ''
diff --git a/test/call-cabal-project-to-nix/default.nix b/test/call-cabal-project-to-nix/default.nix
new file mode 100644
index 0000000000..c18b6a3363
--- /dev/null
+++ b/test/call-cabal-project-to-nix/default.nix
@@ -0,0 +1,33 @@
+{ stdenv, mkCabalProjectPkgSet, callCabalProjectToNix }:
+
+with stdenv.lib;
+
+let
+  pkgSet = mkCabalProjectPkgSet {
+    plan-pkgs = import (callCabalProjectToNix {
+      hackageIndexState = "2019-04-24T21:34:04Z";
+      # reuse the cabal-simple test project
+      src = ../cabal-simple;
+    });
+  };
+  packages = pkgSet.config.hsPkgs;
+in
+  stdenv.mkDerivation {
+    name = "callStackToNix-test";
+
+    buildCommand = ''
+      exe="${packages.cabal-simple.components.exes.cabal-simple}/bin/cabal-simple"
+
+      printf "checking whether executable runs... " >& 2
+      $exe
+
+      touch $out
+    '';
+
+    meta.platforms = platforms.all;
+
+    passthru = {
+      # Attributes used for debugging with nix repl
+      inherit pkgSet packages;
+    };
+  }
diff --git a/test/default.nix b/test/default.nix
index 0789f84ab3..601339fe4d 100644
--- a/test/default.nix
+++ b/test/default.nix
@@ -16,6 +16,7 @@ in {
   builder-haddock = haskell.callPackage ./builder-haddock {};
   stack-simple = haskell.callPackage ./stack-simple {};
   callStackToNix = haskell.callPackage ./callStackToNix {};
+  callCabalProjectToNix = haskell.callPackage ./call-cabal-project-to-nix {};
 
   # Run unit tests with: nix-instantiate --eval --strict -A unit
   # An empty list means success.