diff --git a/flake.nix b/flake.nix
index 95d693f5..f4c5c711 100644
--- a/flake.nix
+++ b/flake.nix
@@ -73,33 +73,11 @@
     }
     // computedOutputs
     // flake-utils.lib.eachDefaultSystem (
-      system: let
-        lib = pkgs.callPackage ./lib/lib.nix {
+      system: {
+        lib = nixpkgs.legacyPackages.${system}.callPackage ./lib/lib.nix {
           organistSrc = self;
           nickel = inputs.nickel.packages."${system}".nickel-lang-cli;
         };
-        pkgs = nixpkgs.legacyPackages.${system};
-      in {
-        inherit lib;
-
-        apps =
-          computedOutputs.apps.${system}
-          // {
-            run-test = let
-              testScript = pkgs.writeShellApplication {
-                name = "test-templates";
-                runtimeInputs = [
-                  inputs.nickel.packages."${system}".nickel-lang-cli
-                  pkgs.parallel
-                  pkgs.gnused
-                ];
-                text = builtins.readFile ./run-test.sh;
-              };
-            in {
-              type = "app";
-              program = pkgs.lib.getExe testScript;
-            };
-          };
       }
     );
 }
diff --git a/lib/builders.ncl b/lib/builders.ncl
index ed2f231f..1a40a864 100644
--- a/lib/builders.ncl
+++ b/lib/builders.ncl
@@ -8,6 +8,15 @@ let concat_strings_sep = fun sep values =>
   else
     std.array.reduce_left (fun acc value => nix-s%"%{acc}%{sep}%{value}"%) values
 in
+
+let MutExclusiveWith = fun other name other_name label value =>
+  if value == null && other == null then
+    std.fail_with "You must specify either %{name} or %{other_name} field"
+  else if value != null && other != null then
+    std.fail_with "You can set only one of %{name} or %{other_name} fields"
+  else
+    value
+in
 {
   NickelPkg
   # we should only need two '%%', but a current Nickel bug (#XXX) bug makes the example being
@@ -84,4 +93,51 @@ in
         structured_env.buildInputs = packages,
       }
         | (NickelPkg & { packages | { _ : Derivation } }),
+
+  ShellApplication
+    | doc m%"
+    "%
+    =
+      NixpkgsPkg
+      & {
+        name,
+        version | default = "0.0.0",
+        content.text | NixString | optional,
+        content.file | NixString | default = lib.to_file name content.text,
+        runtime_inputs | { _ : Derivation } = {},
+        shell_binary | NixString | default = lib.import_nix "nixpkgs#runtimeShell",
+
+        env.buildInputs.coreutils = lib.import_nix "nixpkgs#coreutils",
+        env.buildCommand = nix-s%"
+          mkdir -p "${outputs[out]}/bin"
+          target="${outputs[out]}/bin/${name}"
+          (cat <<<"$scriptHeader"; cat "%{content.file}") > "$target"
+          chmod +x "$target"
+          "%{lib.import_nix "nixpkgs#stdenv.shellDryRun"}" "$target"
+          "%{lib.import_nix "nixpkgs#shellcheck"}/bin/shellcheck" "$target"
+        "%,
+        nix_drv =
+          let path_line =
+            if std.record.is_empty runtime_inputs then
+              ""
+            else
+              let paths =
+                runtime_inputs
+                |> std.record.values
+                |> std.array.map (fun s => nix-s%"%{s}/bin"%)
+                |> concat_strings_sep ":"
+              in
+              nix-s%"export PATH="%{paths}:$PATH""%
+          in
+          {
+            script_header = nix-s%"
+            #!%{shell_binary}
+            set -o errexit
+            set -o nounset
+            set -o pipefail
+            %{path_line}
+          "%
+          },
+      }
+        | NickelPkg,
 }
diff --git a/project.ncl b/project.ncl
index 1dd4d62d..389dfa6f 100644
--- a/project.ncl
+++ b/project.ncl
@@ -16,6 +16,22 @@ let import_nix = organist.lib.import_nix in
       },
     },
 
+  flake.apps.run-test =
+    let testScript | organist.builders.ShellApplication = {
+        name = "run-test.sh",
+        file = "./run-test.sh",
+        runtime_inputs = {
+          nickel = organist.lib.import_nix "nickel#nickel-lang-cli",
+          parallel = import_nix "nixpkgs#parallel",
+          gnused = import_nix "nixpkgs#gnused",
+        },
+      }
+      in
+    {
+      type = "app",
+      program | organist.contracts.NixString = nix-s%"%{testScript}/bin/run-test.sh"%
+    },
+
   flake.checks
     | {
       _ | {