fix?

nix/dhall.nix

@@ -1,5 +1,16 @@
 let
   sources = import ./sources.nix;
   pkgs = import sources.nixpkgs { };
-  dhall = import sources.easy-dhall-nix { inherit pkgs; };
-in dhall.dhall-yaml-simple
+in pkgs.stdenv.mkDerivation rec {
+  name = "dhall-yaml-simple";
+
+  src = sources.dhall-yaml;
+
+  installPhase = ''
+    mkdir -p $out/bin
+    DHALL_TO_YAML=$out/bin/dhall-to-yaml-ng
+    install -D -m555 -T dhall-to-yaml-ng $DHALL_TO_YAML
+    mkdir -p $out/etc/bash_completion.d/
+    $DHALL_TO_YAML --bash-completion-script $DHALL_TO_YAML > $out/etc/bash_completion.d/dhall-to-yaml-completion.bash
+  '';
+}

nix/sources.json

@@ -1,4 +1,10 @@
 {
+    "dhall-yaml": {
+        "sha256": "0mnqn6by839v1n2ccb33qxdh7jga0x5xq3kkwdsnic6q7fgcsds6",
+        "type": "file",
+        "url": "https://github.com/dhall-lang/dhall-haskell/releases/download/1.33.1/dhall-yaml-1.2.0-x86_64-linux.tar.bz2",
+        "url_template": "https://github.com/dhall-lang/dhall-haskell/releases/download/1.33.1/dhall-yaml-1.2.0-x86_64-linux.tar.bz2"
+    },
     "easy-dhall-nix": {
         "branch": "master",
         "description": "Derivations for easily downloading Dhall binaries and putting them to use.",