Add a pkgs.dhallToNix utility

This adds a `dhallToNix` utility which compiles expression from the Dhall
configuration language to Nix using Nix's support for "import from derivation".
The main motivation of this compiler is to allow users to carve out small typed
subsets of Nix projects.  Everything in the Dhall language (except `Double`s)
can be translated to Nix in this way, including functions.

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

@@ -0,0 +1,38 @@
+/* `dhallToNix` is a utility function to convert expressions in the Dhall
+    configuration language to their corresponding Nix expressions.
+
+    Example:
+      dhallToNix "{ foo = 1, bar = True }"
+      => { foo = 1; bar = true; }
+      dhallToNix "λ(x : Bool) → x == False"
+      => x : x == false
+      dhallToNix "λ(x : Bool) → x == False" false
+      => true
+
+    See https://hackage.haskell.org/package/dhall-nix/docs/Dhall-Nix.html for
+    a longer tutorial
+
+    Note that this uses "import from derivation", meaning that Nix will perform
+    a build during the evaluation phase if you use this `dhallToNix` utility
+*/
+{ stdenv, dhall-nix }:
+
+let
+  dhallToNix = code :
+    let
+      file = builtins.toFile "dhall-expression" code;
+
+      drv = stdenv.mkDerivation {
+        name = "dhall-compiled.nix";
+
+        buildCommand = ''
+          dhall-to-nix <<< "${file}" > $out
+        '';
+
+        buildInputs = [ dhall-nix ];
+      };
+
+    in
+      import "${drv}";
+in
+  dhallToNix

pkgs/top-level/all-packages.nix

@@ -91,6 +91,10 @@ with pkgs;
 
   cmark = callPackage ../development/libraries/cmark { };
 
+  dhallToNix = callPackage ../build-support/dhall-to-nix.nix {
+    inherit (haskellPackages) dhall-nix;
+  };
+
   dockerTools = callPackage ../build-support/docker { };
 
   dotnetenv = callPackage ../build-support/dotnetenv {