add pushScope/popScope for Power Users

src/vm.ts

@@ -60,6 +60,14 @@ export class VM {
     this.scope.set(name, { type: 'native', fn, value: '<function>' })
   }
 
+  pushScope() {
+    this.scope = new Scope(this.scope)
+  }
+
+  popScope() {
+    this.scope = this.scope.parent!
+  }
+
   async run(): Promise<Value> {
     this.pc = 0
     this.stopped = false

tests/vm.test.ts

@@ -0,0 +1,42 @@
+import { test, expect, describe } from "bun:test"
+import { VM } from "#vm"
+import { toBytecode } from "#bytecode"
+import { toValue } from "#value"
+
+describe("VM scope methods", () => {
+  test("pushScope creates isolated child scope", async () => {
+    const bytecode = toBytecode([["HALT"]])
+    const vm = new VM(bytecode)
+
+    vm.set("x", 42)
+
+    vm.pushScope()
+
+    const xValue = vm.scope.get("x")
+    expect(xValue).toEqual({ type: "number", value: 42 })
+
+    vm.set("y", 100)
+
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toEqual({ type: "number", value: 100 })
+  })
+
+  test("popScope returns to parent scope and child variables are not accessible", async () => {
+    const bytecode = toBytecode([["HALT"]])
+    const vm = new VM(bytecode)
+
+    vm.set("x", 42)
+
+    vm.pushScope()
+    vm.set("y", 100)
+
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toEqual({ type: "number", value: 100 })
+
+    vm.popScope()
+
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+
+    expect(vm.scope.get("y")).toBeUndefined()
+  })
+})