diff --git a/TODO.md b/TODO.md
index 00e7bbf..c137fc3 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,5 +1,3 @@
-[lang0] `Env` should not be a class
-
 [lang1] 支持 `(import)`
 [lang1] 支持 `(assert-equal)` 与 `(assert-not-equal)`
 [lang1] 直接用 lang0 的测试
diff --git a/src/lang0/actions/doAp.ts b/src/lang0/actions/doAp.ts
index 84c5e94..62b88b1 100644
--- a/src/lang0/actions/doAp.ts
+++ b/src/lang0/actions/doAp.ts
@@ -1,3 +1,4 @@
+import { envExtend } from "../env/Env.js"
 import { evaluate } from "../evaluate/index.js"
 import * as Neutrals from "../neutral/index.js"
 import * as Values from "../value/index.js"
@@ -8,7 +9,7 @@ export function doAp(target: Value, arg: Value): Value {
     case "Fn": {
       return evaluate(
         target.mod,
-        target.env.extend(target.name, arg),
+        envExtend(target.env, target.name, arg),
         target.ret,
       )
     }
diff --git a/src/lang0/env/Env.ts b/src/lang0/env/Env.ts
index 55dd907..34dfb35 100644
--- a/src/lang0/env/Env.ts
+++ b/src/lang0/env/Env.ts
@@ -1,30 +1,19 @@
 import { type Value } from "../value/index.js"
 
-export class Env {
-  values: Map<string, Value>
+export type Env = Map<string, Value>
 
-  constructor(options: { values: Map<string, Value> }) {
-    this.values = options.values
-  }
-
-  static init(): Env {
-    return new Env({
-      values: new Map(),
-    })
-  }
+export function envEmpty(): Env {
+  return new Map()
+}
 
-  get names(): Array<string> {
-    return Array.from(this.values.keys())
-  }
+export function envNames(env: Env): Array<string> {
+  return Array.from(env.keys())
+}
 
-  findValue(name: string): undefined | Value {
-    return this.values.get(name)
-  }
+export function envFindValue(env: Env, name: string): undefined | Value {
+  return env.get(name)
+}
 
-  extend(name: string, value: Value): Env {
-    return new Env({
-      ...this,
-      values: new Map([...this.values, [name, value]]),
-    })
-  }
+export function envExtend(env: Env, name: string, value: Value): Env {
+  return new Map([...env, [name, value]])
 }
diff --git a/src/lang0/evaluate/evaluate.ts b/src/lang0/evaluate/evaluate.ts
index 401aa5d..fbe13df 100644
--- a/src/lang0/evaluate/evaluate.ts
+++ b/src/lang0/evaluate/evaluate.ts
@@ -1,5 +1,5 @@
 import * as Actions from "../actions/index.js"
-import { type Env } from "../env/index.js"
+import { envExtend, envFindValue, type Env } from "../env/index.js"
 import { type Exp } from "../exp/index.js"
 import { modFindValue, type Mod } from "../mod/index.js"
 import { substitutionBindings } from "../substitution/index.js"
@@ -11,7 +11,7 @@ export function evaluate(mod: Mod, env: Env, exp: Exp): Value {
     case "Var": {
       let value = undefined
 
-      value = env.findValue(exp.name)
+      value = envFindValue(env, exp.name)
       if (value !== undefined) return value
 
       value = modFindValue(mod, exp.name)
@@ -33,7 +33,11 @@ export function evaluate(mod: Mod, env: Env, exp: Exp): Value {
     case "Let": {
       let newEnv = env
       for (const binding of substitutionBindings(exp.substitution)) {
-        newEnv = newEnv.extend(binding.name, evaluate(mod, env, binding.exp))
+        newEnv = envExtend(
+          newEnv,
+          binding.name,
+          evaluate(mod, env, binding.exp),
+        )
       }
 
       return evaluate(mod, newEnv, exp.body)
diff --git a/src/lang0/evaluate/evaluateDefinition.ts b/src/lang0/evaluate/evaluateDefinition.ts
index 61b4095..fbcd02d 100644
--- a/src/lang0/evaluate/evaluateDefinition.ts
+++ b/src/lang0/evaluate/evaluateDefinition.ts
@@ -1,5 +1,5 @@
 import { type Definition } from "../definition/index.js"
-import { Env } from "../env/index.js"
+import { envEmpty } from "../env/index.js"
 import { evaluate } from "../evaluate/index.js"
 import { type Value } from "../value/index.js"
 
@@ -8,6 +8,6 @@ export function evaluateDefinition(definition: Definition): Value {
     return definition.cache
   }
 
-  definition.cache = evaluate(definition.mod, Env.init(), definition.exp)
+  definition.cache = evaluate(definition.mod, envEmpty(), definition.exp)
   return definition.cache
 }
diff --git a/src/lang0/run/execute.ts b/src/lang0/run/execute.ts
index 728b77a..8a83935 100644
--- a/src/lang0/run/execute.ts
+++ b/src/lang0/run/execute.ts
@@ -1,4 +1,4 @@
-import { Env } from "../env/index.js"
+import { envEmpty } from "../env/index.js"
 import { equivalent, EquivalentCtx } from "../equivalent/index.js"
 import { evaluate } from "../evaluate/index.js"
 import * as Exps from "../exp/index.js"
@@ -29,7 +29,7 @@ export function execute(mod: Mod, stmt: Stmt): void | string {
     }
 
     case "Compute": {
-      const value = evaluate(mod, Env.init(), stmt.exp)
+      const value = evaluate(mod, envEmpty(), stmt.exp)
       const exp = readback(ReadbackCtx.init(), value)
       return formatExp(exp)
     }
@@ -74,8 +74,8 @@ export function execute(mod: Mod, stmt: Stmt): void | string {
 }
 
 function assertEqual(mod: Mod, left: Exp, right: Exp): void {
-  const leftValue = evaluate(mod, Env.init(), left)
-  const rightValue = evaluate(mod, Env.init(), right)
+  const leftValue = evaluate(mod, envEmpty(), left)
+  const rightValue = evaluate(mod, envEmpty(), right)
   if (!equivalent(EquivalentCtx.init(), leftValue, rightValue)) {
     throw new Error(
       `((fail assert-equal) ${formatExp(left)} ${formatExp(right)})`,
@@ -84,8 +84,8 @@ function assertEqual(mod: Mod, left: Exp, right: Exp): void {
 }
 
 function assertNotEqual(mod: Mod, left: Exp, right: Exp): void {
-  const leftValue = evaluate(mod, Env.init(), left)
-  const rightValue = evaluate(mod, Env.init(), right)
+  const leftValue = evaluate(mod, envEmpty(), left)
+  const rightValue = evaluate(mod, envEmpty(), right)
   if (equivalent(EquivalentCtx.init(), leftValue, rightValue)) {
     throw new Error(
       `((fail assert-not-equal) ${formatExp(left)} ${formatExp(right)})`,