init

CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.9)
+project(nix-scope-plugin)
+
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(NIX_EXPR REQUIRED IMPORTED_TARGET nix-expr)
+
+add_library(nix-scope-plugin MODULE scope.cc)
+target_link_libraries(nix-scope-plugin PkgConfig::NIX_EXPR)
+set_target_properties(nix-scope-plugin PROPERTIES PREFIX "lib")
+
+install(TARGETS nix-scope-plugin DESTINATION lib/nix/plugins)

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1771923393,
+        "narHash": "sha256-Fy0+UXELv9hOE8WjYhJt8fMDLYTU2Dqn3cX4BwoGBos=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "ea7f1f06811ce7fcc81d6c6fd4213150c23edcf2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,38 @@
+{
+  description = "Nix plugin providing a `scope` builtin for constructing nested attrsets from dotted paths";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+    in
+    {
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          nix = pkgs.nix;
+        in
+        {
+          default = pkgs.stdenv.mkDerivation {
+            pname = "nix-scope-plugin";
+            version = "0.1.0";
+            src = self;
+            nativeBuildInputs = [ pkgs.cmake pkgs.pkg-config ];
+            buildInputs = [ nix nix.dev pkgs.boost pkgs.nlohmann_json ];
+          };
+        });
+
+      devShells = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          nix = pkgs.nix;
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [ pkgs.cmake pkgs.pkg-config nix nix.dev pkgs.boost pkgs.nlohmann_json ];
+          };
+        });
+    };
+}

scope.cc

@@ -0,0 +1,52 @@
+#include <nix/expr/primops.hh>
+#include <nix/expr/eval.hh>
+#include <nix/expr/eval-inline.hh>
+#include <nix/expr/nixexpr.hh>
+#include <nix/expr/attr-set.hh>
+
+using namespace nix;
+
+static void prim_scope(EvalState & state, const PosIdx pos, Value ** args, Value & v)
+{
+    std::vector<Symbol> path;
+
+    state.forceStringNoCtx(*args[0], pos, "'scope' path argument");
+    auto s = std::string(args[0]->string_view());
+    if (s.empty())
+        state.error<TypeError>("'scope' requires a non-empty path")
+            .atPos(pos)
+            .debugThrow();
+    size_t start = 0;
+    while (start < s.size()) {
+        auto dot = s.find('.', start);
+        if (dot == std::string::npos) {
+            path.push_back(state.symbols.create(s.substr(start)));
+            break;
+        }
+        path.push_back(state.symbols.create(s.substr(start, dot - start)));
+        start = dot + 1;
+    }
+
+    // Build nested attrset from inside out
+    // Given path [a, b, c] and value v: { a = { b = { c = v; }; }; }
+    Value * inner = args[1];
+    for (auto it = path.rbegin(); it != path.rend(); ++it) {
+        auto attrs = state.buildBindings(1);
+        attrs.insert(*it, inner);
+        Value * wrapper = state.allocValue();
+        wrapper->mkAttrs(attrs.finish());
+        inner = wrapper;
+    }
+    v = *inner;
+}
+
+static RegisterPrimOp rp({
+    .name = "scope",
+    .args = {"path", "value"},
+    .doc = R"(
+      Construct a nested attribute set from a dotted string path and a value.
+
+      Example: `scope "a.b.c" 42` evaluates to `{ a = { b = { c = 42; }; }; }`.
+    )",
+    .fun = prim_scope,
+});