fix: make update operator lazy

nix-js/runtime-ts/src/helpers.ts

@@ -50,7 +50,7 @@ export const select = (obj: NixValue, attrpath: NixValue[]): NixValue => {
   let attrs = forceAttrs(obj);
 
   for (const attr of attrpath.slice(0, -1)) {
-    const key = forceString(attr)
+    const key = forceString(attr);
     if (!(key in attrs)) {
       throw new Error(`Attribute '${key}' not found`);
     }
@@ -63,7 +63,7 @@ export const select = (obj: NixValue, attrpath: NixValue[]): NixValue => {
     attrs = cur;
   }
 
-  const last = forceString(attrpath[attrpath.length - 1])
+  const last = forceString(attrpath[attrpath.length - 1]);
   if (!(last in attrs)) {
     throw new Error(`Attribute '${last}' not found`);
   }
@@ -74,9 +74,9 @@ export const selectWithDefault = (obj: NixValue, attrpath: NixValue[], default_v
   let attrs = forceAttrs(obj);
 
   for (const attr of attrpath.slice(0, -1)) {
-    const key = forceString(attr)
+    const key = forceString(attr);
     if (!(key in attrs)) {
-      return default_val
+      return default_val;
     }
     const cur = force(attrs[key]);
     if (!isAttrs(cur)) {
@@ -87,7 +87,7 @@ export const selectWithDefault = (obj: NixValue, attrpath: NixValue[], default_v
 
   const last = forceString(attrpath[attrpath.length - 1]);
   if (last in attrs) {
-    return attrs[last]
+    return attrs[last];
   }
   return default_val;
 };

nix-js/runtime-ts/src/operators.ts

@@ -175,6 +175,26 @@ export const op = {
   },
 
   update: (a: NixValue, b: NixValue): NixAttrs => {
-    return { ...forceAttrs(a), ...forceAttrs(b) };
+    const forcedA = forceAttrs(a);
+    const forcedB = forceAttrs(b);
+    const newAttrs: NixAttrs = {};
+
+    for (const key in forcedA) {
+      Object.defineProperty(newAttrs, key, {
+        get: () => force(forcedA[key]),
+        enumerable: true,
+        configurable: true,
+      });
+    }
+
+    for (const key in forcedB) {
+      Object.defineProperty(newAttrs, key, {
+        get: () => force(forcedB[key]),
+        enumerable: true,
+        configurable: true,
+      });
+    }
+
+    return newAttrs;
   },
 };