dhallDirectoryToNix: add this utility function

pkgs/build-support/dhall/directory-to-nix.nix

@@ -0,0 +1,25 @@
+{ dhallPackages, dhallPackageToNix}:
+
+# `dhallDirectoryToNix is a utility function to take a directory of Dhall files
+# and read them in as a Nix expression.
+#
+# This function is similar to `dhallToNix`, but takes a Nixpkgs Dhall package
+# as input instead of raw Dhall code.
+#
+# Note that this uses "import from derivation" (IFD), meaning that Nix will
+# perform a build during the evaluation phase if you use this
+# `dhallDirectoryToNix` utility.  It is not possible to use
+# `dhallDirectoryToNix` in Nixpkgs, since the Nixpkgs Hydra doesn't allow IFD.
+
+{ src
+, # The file to import, relative to the src root directory
+  file ? "package.dhall"
+}@args:
+
+let
+  generatedPkg = dhallPackages.generateDhallDirectoryPackage args;
+
+  builtPkg = dhallPackages.callPackage generatedPkg { };
+
+in
+  dhallPackageToNix builtPkg

pkgs/top-level/all-packages.nix

@@ -301,6 +301,8 @@ with pkgs;
 
   crow-translate = libsForQt5.callPackage ../applications/misc/crow-translate { };
 
+  dhallDirectoryToNix = callPackage ../build-support/dhall/directory-to-nix.nix { };
+
   dhallPackageToNix = callPackage ../build-support/dhall/package-to-nix.nix { };
 
   dhallToNix = callPackage ../build-support/dhall/to-nix.nix { };