Merge pull request #22193 from Gabriel439/gabriel/dhallToNix

Add a `pkgs.dhallToNix` utility

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 {