diff --git a/benchmark/demo.js b/benchmark/demo.js
index e049aad70..cb9f2f3c7 100755
--- a/benchmark/demo.js
+++ b/benchmark/demo.js
@@ -8,17 +8,6 @@ const engine = new Liquid({
   extname: '.liquid'
 })
 
-engine.registerTag('header', {
-  parse: function (token) {
-    const [key, val] = token.args.split(':')
-    this[key] = val
-  },
-  render: function (ctx) {
-    const title = this.liquid.evalValue(this.content, ctx)
-    return `<h1>${title}</h1>`
-  }
-})
-
 function demo () {
   console.log('         demo')
   console.log('------------------------')
diff --git a/demo/nodejs/index.js b/demo/nodejs/index.js
index 8849502bc..b1a45c9ad 100644
--- a/demo/nodejs/index.js
+++ b/demo/nodejs/index.js
@@ -1,4 +1,4 @@
-const { Liquid } = require('liquidjs')
+const { Liquid, Tag, Value } = require('liquidjs')
 
 const engine = new Liquid({
   extname: '.liquid',
@@ -11,13 +11,13 @@ const engine = new Liquid({
   partials: './partials'
 })
 
-engine.registerTag('header', {
-  parse: function (token) {
-    const [key, val] = token.args.split(':')
-    this[key] = val
-  },
-  render: async function (scope, emitter) {
-    const title = await this.liquid.evalValue(this.content, scope)
+engine.registerTag('header', class HeaderTag extends Tag {
+  constructor (token, remainTokens, liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  * render (ctx, emitter) {
+    const title = yield this.value.value(ctx)
     emitter.write(`<h1>${title}</h1>`)
   }
 })
diff --git a/demo/nodejs/todolist.liquid b/demo/nodejs/todolist.liquid
index e3156a889..51bbe4af9 100644
--- a/demo/nodejs/todolist.liquid
+++ b/demo/nodejs/todolist.liquid
@@ -1,5 +1,5 @@
 {% layout 'html.liquid' %}
-{%header content: title | capitalize%}
+{%header title | capitalize%}
 <ul>
   {% for todo in todos %}
     {% render 'todo.liquid' with todo, index: forloop.index%}
diff --git a/demo/typescript/index.ts b/demo/typescript/index.ts
index d0b5a2773..611f5361e 100644
--- a/demo/typescript/index.ts
+++ b/demo/typescript/index.ts
@@ -1,24 +1,25 @@
-import { Liquid, TagToken, Context, Emitter } from 'liquidjs'
+import { Value, Liquid, TagToken, Context, Emitter, Tag, TopLevelToken } from 'liquidjs'
 
 const engine = new Liquid({
   root: __dirname,
   extname: '.liquid'
 })
 
-engine.registerTag('header', {
-  parse: function (token: TagToken) {
-    const [key, val] = token.args.split(':')
-    this[key] = val
-  },
-  render: async function (context: Context, emitter: Emitter) {
-    const title = await this.liquid.evalValue(this['content'], context)
+engine.registerTag('header', class HeaderTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  * render (ctx: Context, emitter: Emitter) {
+    const title = yield this.value.value(ctx)
     emitter.write(`<h1>${title}</h1>`)
   }
 })
 
-const ctx = {
+const scope = {
   todos: ['fork and clone', 'make it better', 'make a pull request'],
   title: 'Welcome to liquidjs!'
 }
 
-engine.renderFile('todolist', ctx).then(console.log)
+engine.renderFile('todolist', scope).then(console.log)
diff --git a/demo/typescript/todolist.liquid b/demo/typescript/todolist.liquid
index cc7ef4972..d452e1fc8 100644
--- a/demo/typescript/todolist.liquid
+++ b/demo/typescript/todolist.liquid
@@ -1,4 +1,4 @@
-{%header content: title | capitalize%}
+{%header title | capitalize%}
 
 <ul>
   {% for todo in todos %}
diff --git a/demo/typescript/tsconfig.json b/demo/typescript/tsconfig.json
index 07f29e72e..273db946d 100644
--- a/demo/typescript/tsconfig.json
+++ b/demo/typescript/tsconfig.json
@@ -1,7 +1,9 @@
 {
   "compilerOptions": {
-      "types": [
-        "node"
-      ]
+    "target": "es6",
+    "moduleResolution": "node",
+    "types": [
+      "node"
+    ]
   }
 }
\ No newline at end of file
diff --git a/docs/source/tutorials/register-filters-tags.md b/docs/source/tutorials/register-filters-tags.md
index 3ceb12d0d..906973e8f 100644
--- a/docs/source/tutorials/register-filters-tags.md
+++ b/docs/source/tutorials/register-filters-tags.md
@@ -6,14 +6,14 @@ title: Register Filters/Tags
 
 ```typescript
 // Usage: {% upper name %}
-import { TagToken, Context, Emitter, TopLevelToken } from 'liquidjs'
+import { Value, TagToken, Context, Emitter, TopLevelToken } from 'liquidjs'
 
 engine.registerTag('upper', {
     parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
-        this.str = tagToken.args; // name
+        this.value = new Value(token.args, liquid)
     },
     render: function*(ctx: Context) {
-        const str = yield this.liquid.evalValue(this.str, ctx); // 'alice'
+        const str = yield this.value.value(ctx); // 'alice'
         return str.toUpperCase() // 'ALICE'
     }
 });
@@ -41,7 +41,7 @@ engine.registerTag('upper', class UpperTag extends Tag {
 });
 ```
 
-See existing tag implementations here: <https://github.com/harttle/liquidjs/tree/master/src/builtin/tags>
+See existing tag implementations here: <https://github.com/harttle/liquidjs/tree/master/src/tags>
 See demo example here: https://github.com/harttle/liquidjs/blob/master/demo/typescript/index.ts
 
 ## Register Filters
@@ -58,7 +58,7 @@ Filter arguments will be passed to the registered filter function, for example:
 engine.registerFilter('add', (initial, arg1, arg2) => initial + arg1 + arg2)
 ```
 
-See existing filter implementations here: <https://github.com/harttle/liquidjs/tree/master/src/builtin/filters>
+See existing filter implementations here: <https://github.com/harttle/liquidjs/tree/master/src/filters>
 
 ## Unregister Tags/Filters
 
diff --git a/docs/source/tutorials/sync-and-async.md b/docs/source/tutorials/sync-and-async.md
index 1929dd9a9..dbca383b4 100644
--- a/docs/source/tutorials/sync-and-async.md
+++ b/docs/source/tutorials/sync-and-async.md
@@ -24,67 +24,93 @@ The synchronous version of methods contains a `Sync` suffix:
 
 ## Implement Sync-Compatible Tags 
 
-### Requirements
+LiquidJS uses a generator-based async implementation to support both async and sync in one piece of tag implementation. For example, below `UpperTag` can be used in both `engine.renderSync()` and `engine.render()`.
 
-All builtin tags are *sync-compatible* and safe to use for both sync and async APIs. To make your custom tag *sync-compatible*, you'll need to avoid return a `Promise`. That means the `render(context, emitter)`:
+```typescript
+import { TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid } from 'liquidjs'
 
-- Should not directly `return <Promise>`, and
-- Should not be declared as `async`.
+// Usage: {% upper "alice" %}
+// Output: ALICE
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  * render (ctx: Context, emitter: Emitter) {
+    const title = yield this.value.value(ctx)
+    emitter.write(title.toUpperCase())
+  }
+})
+```
+
+All builtin tags are implemented this way and safe to use in both sync and async (I'll call it *sync-compatible*). To make your custom tag *sync-compatible*, you'll need to:
+
+- declare render function as `* render()`, in which
+- do not directly `return <Promise>`, and
+- do not call any APIs that returns a Promise.
+
+## Call APIs that return a Promise
+
+But LiquidJS is Promise-friendly, right? You can still call Promise-based functions and wait for that Promise within tag implementations. Just replace `await` with `yield`. e.g. we're calling `fs.readFile()` which returns a `Promise`:
+
+```typescript
+  * render (ctx: Context, emitter: Emitter) {
+    const file = yield this.value.value(ctx)
+    const title = yield fs.readFile(file, 'utf8')
+    emitter.write(title.toUpperCase())
+  }
+```
+
+Now that this `* render()` calls an API that returns a Promise, so it's no longer *sync-compatible*.
 
 {% note info Non Sync-Compatible Tags %}
 Non <em>sync-compatible</em> tags are also valid tags, will work just fine for asynchronous API calls. When called synchronously, tags that return a <code>Promise</code> will be rendered as <code>[object Promise]</code>.
 {% endnote %}
 
-### Await Promises
-But LiquidJS is Promise-friendly, right? You can still call Promise-based functions and wait for that Promise within tag implementations. Just replace `await` with `yield` and keep `* render()` instead of `async render()`. e.g.
+## Convert LiquidJS async Generator to Promise
+
+You can convert a Generator to Promise by [toPromise][toPromise], for example:
 
 ```typescript
-import { TagToken, Context, Emitter, TopLevelToken } from 'liquidjs'
+import { TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid, toPromise } from 'liquidjs'
 
 // Usage: {% upper "alice" %}
 // Output: ALICE
-engine.registerTag('upper', {
-    parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
-        this.str = tagToken.args
-    },
-    * render: function(ctx: Context) {
-        // _evalValue will behave synchronously when called by synchronous API
-        // in which case `ctx.sync == true`
-        var str = yield this.liquid._evalValue(this.str, ctx)
-        return str.toUpperCase()
-    }
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  async render (ctx: Context, emitter: Emitter) {
+    const title = await toPromise(this.value.value(ctx))
+    emitter.write(title.toUpperCase())
+  }
 })
 ```
 
-See this JSFiddle: <http://jsfiddle.net/ctj364up/6/>.
+## Async only Tags
 
-## Async-only Tags
-
-For tags that intended to be used only by async API, or those cannot be implemented synchronously, there's no difference between using generator-base syntax or async syntax. I'll call them *async-only tags*.
-
-For example, if the above `this.liquid._evalValue()` doesn't respect `ctx.sync` and always returns a Promise, even if the tag is implemented using `* render()` and `yield this.liquid._evalValue()`, it will be rendered as `<object Promise>` anyway.
-
-For *async-only tags*, you can use async syntax at will. Be careful some APIs in LiquidJS return Promises and others return Generators. You'll need [toPromise][toPromise] API to convert a Generator to a Promise, for example:
+If your tag is intend to be used only asynchronously, it can be declared as `async render()` so you can use `await` in its implementation directly:
 
 ```typescript
-import { TagToken, Context, Emitter, TopLevelToken, toPromise } from 'liquidjs'
+import { toPromise, TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid } from 'liquidjs'
 
 // Usage: {% upper "alice" %}
 // Output: ALICE
-engine.registerTag('upper', {
-    parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
-        this.str = tagToken.args; // name
-    },
-    render: async function(ctx: Context) {
-        var str = await toPromise(this.liquid._evalValue(this.str, ctx));
-        // Or use the alternate API that returns a Promise
-        // var str = await this.liquid.evalValue(this.str, ctx);
-        return str.toUpperCase()
-    }
-});
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  async render (ctx: Context, emitter: Emitter) {
+    const title = await toPromise(this.value.value(ctx))
+    emitter.write(`<h1>${title}</h1>`)
+  }
+})
 ```
 
-See this JSFiddle: <http://jsfiddle.net/ctj364up/5/>.
-
 [Liquid]: /api/classes/liquid_.liquid.html
 [toPromise]: /api/modules/liquid_.html#toPromise
diff --git a/docs/source/zh-cn/tutorials/register-filters-tags.md b/docs/source/zh-cn/tutorials/register-filters-tags.md
index 6ff712144..f7ff29230 100644
--- a/docs/source/zh-cn/tutorials/register-filters-tags.md
+++ b/docs/source/zh-cn/tutorials/register-filters-tags.md
@@ -8,10 +8,10 @@ title: 注册标签和过滤器
 // 使用方式: {% upper name %}
 engine.registerTag('upper', {
     parse: function(tagToken, remainTokens) {
-        this.str = tagToken.args; // name
+        this.value = new Value(token.args, liquid)
     },
-    render: async function(scope, hash) {
-        var str = await this.liquid.evalValue(this.str, scope); // 'alice'
+    render: function*(scope, hash) {
+        const str = yield this.value.value(ctx); // 'alice'
         return str.toUpperCase()  // 'Alice'
     }
 });
@@ -20,7 +20,26 @@ engine.registerTag('upper', {
 * `parse`: 从 `remainTokens` 中读取后续的标签/输出/HTML，直到找到你期望的结束标签。
 * `render`: 把 scope 数据和此前解析得到的 Token 结合，输出 HTML 字符串。
 
-查看已有的标签实现：<https://github.com/harttle/liquidjs/tree/master/src/builtin/tags>
+对于更复杂的标签实现，可以提供一个继承自 `Tag` 的类：
+
+```typescript
+// Usage: {% upper name:"alice" %}
+import { Hash, Tag, TagToken, Context, Emitter, TopLevelToken, Liquid } from 'liquidjs'
+
+engine.registerTag('upper', class UpperTag extends Tag {
+    private hash: Hash
+    constructor(tagToken: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+        super(tagToken, remainTokens, liquid)
+        this.hash = new Hash(tagToken.args)
+    }
+    * render(ctx: Context) {
+        const hash = yield this.hash.render();
+        return hash.name.toUpperCase() // 'ALICE'
+    }
+});
+```
+
+可以参考已有的标签实现：<https://github.com/harttle/liquidjs/tree/master/src/tags>
 
 ## 注册过滤器
 
@@ -36,7 +55,7 @@ engine.registerFilter('upper', v => v.toUpperCase())
 engine.registerFilter('add', (initial, arg1, arg2) => initial + arg1 + arg2)
 ```
 
-查看已有的过滤器实现：<https://github.com/harttle/liquidjs/tree/master/src/builtin/filters>。对于复杂的标签，也可以用一个类来实现：
+查看已有的过滤器实现：<https://github.com/harttle/liquidjs/tree/master/src/filters>。对于复杂的标签，也可以用一个类来实现：
 
 ```typescript
 // Usage: {% upper name:"alice" %}
diff --git a/docs/source/zh-cn/tutorials/sync-and-async.md b/docs/source/zh-cn/tutorials/sync-and-async.md
index b90f2917b..44792a926 100644
--- a/docs/source/zh-cn/tutorials/sync-and-async.md
+++ b/docs/source/zh-cn/tutorials/sync-and-async.md
@@ -24,66 +24,89 @@ LiquidJS 支持同步调用也支持异步调用，支持 Promise。为了同异
 
 ## 如何实现兼容同步的标签
 
-### 要求
-
-所有内置标签都兼容同步，可以安全地用于同步或异步 API。为了让你的自定义标签页支持同步，你的标签不能返回 `Promise`，这意味着你的 `render(context, emitter)` 函数：
-
-- 不能直接 `return <Promise>`，
-- 也不能声明为 `async`。
-
-{% note info 不兼容同步的标签 %}
-不兼容同步的标签也仍然是合法标签，在异步 API 下也会正常运行。被同步调用时，返回 <code>Promise</code> 的标签会被渲染成 <code>[object Promise]</code>。
-{% endnote %}
-
-### 等待 Promise
-但 LiquidJS 是支持 `Promise` 的，你仍然可以调用返回 `Promise` 的方法并等它 resolve。只需要把 `await` 换成 `yield` 并保留 `* render()` 不要改成 `async render()`。例如：
+LiquidJS 使用基于生成器的异步实现，来让同一份代码支持同步和异步调用。例如下面的 `UpperTag` 既可以用于 `engine.renderSync()` 也可以用于 `engine.render()`：
 
 ```typescript
-import { TagToken, Context, Emitter, TopLevelToken } from 'liquidjs'
+import { TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid } from 'liquidjs'
 
 // Usage: {% upper "alice" %}
 // Output: ALICE
-engine.registerTag('upper', {
-    parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
-        this.str = tagToken.args
-    },
-    * render: function(ctx: Context) {
-        // 同步调用时 `ctx.sync == true`，`_evalValue()` 会同步地执行
-        var str = yield this.liquid._evalValue(this.str, ctx)
-        return str.toUpperCase()
-    }
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  * render (ctx: Context, emitter: Emitter) {
+    const title = yield this.value.value(ctx)
+    emitter.write(title.toUpperCase())
+  }
 })
 ```
 
-见这个 JSFiddle：<http://jsfiddle.net/ctj364up/6/>。
+所有内置标签都兼容同步，可以安全地用于同步或异步 API。实现同时支持同异步的标签，需要：
 
-## 只支持异步的标签
+- render 函数声明成 `* render()`，并且在里面
+- 不能直接 `return <Promise>`，
+- 不能调用会返回 Promise 的函数。
 
-对于只用于异步 API 的标签，或者只能实现为异步的标签，使用生成器语法和 async 语法并没有区别。
+## 调用返回 Promise 的函数
 
-例如，如果上面的 `this.liquid._evalValue()` 不会检查 `ctx.sync` 而且总是返回一个 `Promise`，那么即使这个标签用 `* render()` 和 `yield this.liquid._evalValue()` 实现，最终也会渲染成 `<object Promise>`。
-
-这时可以直接使用 async 语法。注意有些 LiquidJS API 会返回 `Promise`，有些会返回生成器。你需要用 [toPromise][toPromise] API 来把生成器转换为 `Promise`，比如：
+但 LiquidJS 是支持 `Promise` 的，你仍然可以调用返回 `Promise` 的方法并等它 resolve。只需要把 `await` 换成 `yield`。例如：
 
 ```typescript
-import { TagToken, Context, Emitter, TopLevelToken, toPromise } from 'liquidjs'
+  * render (ctx: Context, emitter: Emitter) {
+    const file = yield this.value.value(ctx)
+    const title = yield fs.readFile(file, 'utf8')
+    emitter.write(title.toUpperCase())
+  }
+```
+
+现在 `* render()` 调用了一个返回 Promise 的 API，它就不再兼容同步了。不兼容同步的标签也仍然是合法标签，在异步 API 下也会正常运行。被同步调用时，返回 <code>Promise</code> 的标签会被渲染成 <code>[object Promise]</code>。
+
+## 把 LiquidJS 生成器转换成 Promise
+
+有些 LiquidJS API 会返回 `Promise`，有些会返回生成器。你可以用 [toPromise][toPromise] 来把生成器转换为 `Promise`，比如：
+
+```typescript
+import { TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid, toPromise } from 'liquidjs'
 
 // Usage: {% upper "alice" %}
 // Output: ALICE
-engine.registerTag('upper', {
-    parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
-        this.str = tagToken.args; // name
-    },
-    render: async function(ctx: Context) {
-        var str = await toPromise(this.liquid._evalValue(this.str, ctx));
-        // Or use the alternate API that returns a Promise
-        // var str = await this.liquid.evalValue(this.str, ctx);
-        return str.toUpperCase()
-    }
-});
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  async render (ctx: Context, emitter: Emitter) {
+    const title = await toPromise(this.value.value(ctx))
+    emitter.write(title.toUpperCase())
+  }
+})
 ```
 
-见这个 JSFiddle：<http://jsfiddle.net/ctj364up/5/>。
+## 纯异步标签
+
+如果你的标签就不打算支持同步，可以干脆实现成 `async render()`，这样就可以使用更熟悉的 `await` 了：
+
+```typescript
+import { toPromise, TagToken, Context, Emitter, TopLevelToken, Value, Tag, Liquid } from 'liquidjs'
+
+// Usage: {% upper "alice" %}
+// Output: ALICE
+engine.registerTag('upper', class UpperTag extends Tag {
+  private value: Value
+  constructor (token: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+    super(token, remainTokens, liquid)
+    this.value = new Value(token.args, liquid)
+  }
+  async render (ctx: Context, emitter: Emitter) {
+    const title = await toPromise(this.value.value(ctx))
+    emitter.write(`<h1>${title}</h1>`)
+  }
+})
+```
 
 [Liquid]: /api/classes/liquid_.liquid.html
 [toPromise]: /api/modules/liquid_.html#toPromise
diff --git a/src/liquid.ts b/src/liquid.ts
index 0e306d888..30f199632 100644
--- a/src/liquid.ts
+++ b/src/liquid.ts
@@ -79,7 +79,7 @@ export class Liquid {
   public _evalValue (str: string, scope?: object | Context): IterableIterator<any> {
     const value = new Value(str, this)
     const ctx = scope instanceof Context ? scope : new Context(scope, this.options)
-    return value.value(ctx, false)
+    return value.value(ctx)
   }
   public async evalValue (str: string, scope?: object | Context): Promise<any> {
     return toPromise(this._evalValue(str, scope))
diff --git a/src/template/value.ts b/src/template/value.ts
index 902ffb2a4..e0f6da125 100644
--- a/src/template/value.ts
+++ b/src/template/value.ts
@@ -17,7 +17,7 @@ export class Value {
     this.initial = tokenizer.readExpression()
     this.filters = tokenizer.readFilters().map(({ name, args }) => new Filter(name, this.getFilter(liquid, name), args, liquid))
   }
-  public * value (ctx: Context, lenient: boolean): Generator<unknown, unknown, unknown> {
+  public * value (ctx: Context, lenient?: boolean): Generator<unknown, unknown, unknown> {
     lenient = lenient || (ctx.opts.lenientIf && this.filters.length > 0 && this.filters[0].name === 'default')
     let val = yield this.initial.evaluate(ctx, lenient)