Repository: syuilo/aiscript (ts)

Issue:
letで定義した変数が変更できてしまう
#311 から作成

# 起きていること
letで定義した変数が変更できてしまう
```ts
let hoge = 1234
<: hoge
hoge = 5678
<: hoge
```

# 提案
- letで定義した変数へ値を代入しようとしたときにランタイムエラーを発生させる

Pull request description:
Fix: letで定義した変数が上書きできてしまうのを修正
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# What
<!-- このPRで何をしたのか？ どう変わるのか？ -->
<!-- What did you do with this PR? How will it change things? -->
resolve #328
- letで定義した変数へ値を代入しようとした場合に `RuntimeError` を投げるように

# Why
<!-- なぜそうするのか？ どういう意図なのか？ 何が困っているのか？ -->
<!-- Why do you do it? What are your intentions? What is the problem? -->

# Additional info (optional)
<!-- テスト観点など -->
<!-- Test perspective, etc -->
テストも書いておきました。

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0df3fc6..5222b43 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 - オブジェクトを添字で参照できるように（`object['index']`のように）
 - 「エラー型（`error`）」を導入
 - `Json:parse`がパース失敗時にエラー型の値を返すように
+- `let` で定義した変数が上書きできてしまうのを修正
 
 # 0.15.0
 - Mathを強化
diff --git a/etc/aiscript.api.md b/etc/aiscript.api.md
index f5c4ff0..18e1608 100644
--- a/etc/aiscript.api.md
+++ b/etc/aiscript.api.md
@@ -528,7 +528,8 @@ type InfixOperator = '||' | '&&' | '==' | '!=' | '<=' | '>=' | '<' | '>' | '+' |
 
 // @public (undocumented)
 export class Interpreter {
-    constructor(vars: Record<string, Value>, opts?: {
+    // Warning: (ae-forgotten-export) The symbol "Variable" needs to be exported by the entry point index.d.ts
+    constructor(vars: Record<string, Variable>, opts?: {
         in?(q: string): Promise<string>;
         out?(value: Value): void;
         log?(type: string, params: Record<string, any>): void;
@@ -813,13 +814,13 @@ class RuntimeError extends AiScriptError {
 // @public (undocumented)
 export class Scope {
     constructor(layerdStates?: Scope['layerdStates'], parent?: Scope, name?: Scope['name']);
-    add(name: string, val: Value): void;
+    add(name: string, variable: Variable): void;
     assign(name: string, val: Value): void;
     // (undocumented)
-    createChildScope(states?: Map<string, Value>, name?: Scope['name']): Scope;
+    createChildScope(states?: Map<string, Variable>, name?: Scope['name']): Scope;
     exists(name: string): boolean;
     get(name: string): Value;
-    getAll(): Map<string, Value>;
+    getAll(): Map<string, Variable>;
     // (undocumented)
     name: string;
     // (undocumented)
diff --git a/src/interpreter/index.ts b/src/interpreter/index.ts
index 11cc4f7..67d4ea4 100644
--- a/src/interpreter/index.ts
+++ b/src/interpreter/index.ts
@@ -11,6 +11,7 @@ import { NULL, RETURN, unWrapRet, FN_NATIVE, BOOL, NUM, STR, ARR, OBJ, FN, BREAK
 import { getPrimProp } from './primitive-props.js';
 import type { Value, VFn } from './value.js';
 import type * as Ast from '../node.js';
+import { Variable } from './variable.js';
 
 const IRQ_RATE = 300;
 const IRQ_AT = IRQ_RATE - 1;
@@ -22,7 +23,7 @@ export class Interpreter {
 	private abortHandlers: (() => void)[] = [];
 
 	constructor(
-		private vars: Record<string, Value>,
+		private vars: Record<string, Variable>,
 		private opts: {
 			in?(q: string): Promise<string>;
 			out?(value: Value): void;
@@ -31,20 +32,27 @@ export class Interpreter {
 		} = {},
 	) {
 		const io = {
-			print: FN_NATIVE(([v]) => {
+			print: Variable.const(FN_NATIVE(([v]) => {
 				expectAny(v);
 				if (this.opts.out) this.opts.out(v);
-			}),
-			readline: FN_NATIVE(async args => {
+			})),
+			readline: Variable.const(FN_NATIVE(async args => {
 				const q = args[0];
 				assertString(q);
 				if (this.opts.in == null) return NULL;
 				const a = await this.opts.in!(q.value);
 				return STR(a);
-			}),
+			})),
 		};
 
-		this.vars = { ...vars, ...std, ...io };
+		this.vars = {
+			...vars,
+			...Object.fromEntries(
+				Object.entries(std)
+					.map(([k, v]) => [k, Variable.const(v)]),
+			),
+			...io,
+		};
 
 		this.scope = new Scope([new Map(Object.entries(this.vars))]);
 		this.scope.opts.log = (type, params): void => {
@@ -142,9 +150,13 @@ export class Interpreter {
 		for (const node of ns.members) {
 			switch (node.type) {
 				case 'def': {
-					const v = await this._eval(node.expr, scope);
-					scope.add(node.name, v);
-					this.scope.add(ns.name + ':' + node.name, v);
+					const variable: Variable = {
+						isMutable: node.mut,
+						value: await this._eval(node.expr, scope),
+					};
+					scope.add(node.name, variable);
+
+					this.scope.add(ns.name + ':' + node.name, variable);
 					break;
 				}
 
@@ -167,12 +179,15 @@ export class Interpreter {
 				registerAbortHandler: this.registerAbortHandler,
 				unregisterAbortHandler: this.unregisterAbortHandler,
 			});
-			// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition 
+			// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
 			return result ?? NULL;
 		} else {
-			const _args = new Map() as Map<string, Value>;
+			const _args = new Map<string, Variable>();
 			for (let i = 0; i < (fn.args ?? []).length; i++) {
-				_args.set(fn.args![i]!, args[i]!);
+				_args.set(fn.args![i]!, {
+					isMutable: true,
+					value: args[i]!,
+				});
 			}
 			const fnScope = fn.scope!.createChildScope(_args);
 			return unWrapRet(await this._run(fn.statements!, fnScope));
@@ -266,7 +281,10 @@ export class Interpreter {
 					assertNumber(to);
 					for (let i = from.value; i < from.value + to.value; i++) {
 						const v = await this._eval(node.for, scope.createChildScope(new Map([
-							[node.var!, NUM(i)],
+							[node.var!, {
+								isMutable: false,
+								value: NUM(i),
+							}],
 						])));
 						if (v.type === 'break') {
 							break;
@@ -283,7 +301,10 @@ export class Interpreter {
 				assertArray(items);
 				for (const item of items.value) {
 					const v = await this._eval(node.for, scope.createChildScope(new Map([
-						[node.var, item],
+						[node.var, {
+							isMutable: false,
+							value: item,
+						}],
 					])));
 					if (v.type === 'break') {
 						break;
@@ -306,7 +327,10 @@ export class Interpreter {
 					}
 					value.attr = attrs;
 				}
-				scope.add(node.name, value);
+				scope.add(node.name, {
+					isMutable: node.mut,
+					value: value,
+				});
 				return NULL;
 			}
 
diff --git a/src/interpreter/scope.ts b/src/interpreter/scope.ts
index 0ab8b79..9938252 100644
--- a/src/interpreter/scope.ts
+++ b/src/interpreter/scope.ts
@@ -1,10 +1,11 @@
 import { autobind } from '../utils/mini-autobind.js';
 import { RuntimeError } from '../error.js';
 import type { Value } from './value.js';
+import type { Variable } from './variable.js';
 
 export class Scope {
 	private parent?: Scope;
-	private layerdStates: Map<string, Value>[];
+	private layerdStates: Map<string, Variable>[];
 	public name: string;
 	public opts: {
 		log?(type: string, params: Record<string, any>): void;
@@ -36,7 +37,7 @@ export class Scope {
 	}
 
 	@autobind
-	public createChildScope(states: Map<string, Value> = new Map(), name?: Scope['name']): Scope {
+	public createChildScope(states: Map<string, Variable> = new Map(), name?: Scope['name']): Scope {
 		const layer = [states, ...this.layerdStates];
 		return new Scope(layer, this, name);
 	}
@@ -49,7 +50,7 @@ export class Scope {
 	public get(name: string): Value {
 		for (const layer of this.layerdStates) {
 			if (layer.has(name)) {
-				const state = layer.get(name)!;
+				const state = layer.get(name)!.value;
 				this.log('read', { var: name, val: state });
 				return state;
 			}
@@ -81,10 +82,10 @@ export class Scope {
 	 * 現在のスコープに存在する全ての変数を取得します
 	 */
 	@autobind
-	public getAll(): Map<string, Value> {
+	public getAll(): Map<string, Variable> {
 		const vars = this.layerdStates.reduce((arr, layer) => {
 			return [...arr, ...layer];
-		}, [] as [string, Value][]);
+		}, [] as [string, Variable][]);
 		return new Map(vars);
 	}
 
@@ -94,16 +95,16 @@ export class Scope {
 	 * @param val - 初期値
 	 */
 	@autobind
-	public add(name: string, val: Value): void {
-		this.log('add', { var: name, val: val });
+	public add(name: string, variable: Variable): void {
+		this.log('add', { var: name, val: variable });
 		const states = this.layerdStates[0]!;
 		if (states.has(name)) {
 			throw new RuntimeError(
 				`Variable '${name}' is alerady exists in scope '${this.name}'`,
 				{ scope: this.layerdStates });
 		}
-		states.set(name, val);
-		if (this.parent == null) this.onUpdated(name, val);
+		states.set(name, variable);
+		if (this.parent == null) this.onUpdated(name, variable.value);
 	}
 
 	/**
@@ -116,7 +117,13 @@ export class Scope {
 		let i = 1;
 		for (const layer of this.layerdStates) {
 			if (layer.has(name)) {
-				layer.set(name, val);
+				const variable = layer.get(name)!;
+				if (!variable.isMutable) {
+					throw new RuntimeError(`Cannot assign to an immutable variable ${name}.`);
+				}
+
+				variable.value = val;
+
 				this.log('assign', { var: name, val: val });
 				if (i === this.layerdStates.length) this.onUpdated(name, val);
 				return;
diff --git a/src/interpreter/variable.ts b/src/interpreter/variable.ts
new file mode 100644
index 0000000..30252b5
--- /dev/null
+++ b/src/interpreter/variable.ts
@@ -0,0 +1,26 @@
+import type { Value } from './value.js';
+
+export type Variable =
+  | {
+    isMutable: false
+    readonly value: Value
+  }
+  | {
+    isMutable: true
+    value: Value
+  }
+
+export const Variable = {
+	mut(value: Value): Variable {
+		return {
+			isMutable: true,
+			value,
+		};
+	},
+	const(value: Value): Variable {
+		return {
+			isMutable: false,
+			value,
+		};
+	},
+};


Test patch:
diff --git a/test/index.ts b/test/index.ts
index 2da9812..2e23691 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -2250,6 +2250,17 @@ describe('Location', () => {
 	});
 });
 
+describe('Variable declaration', () => {
+	test.concurrent('Do not assign to let (issue #328)', async () => {
+		const err = await exe(`
+			let hoge = 33
+			hoge = 4
+		`).then(() => undefined).catch(err => err);
+
+		assert.ok(err instanceof RuntimeError);
+	});
+})
+
 describe('primitive props', () => {
 	describe('num', () => {
 		test.concurrent('to_str', async () => {