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+<h1 align="center">Declare a Chain</h1>
+<p align="center">
+    Use the <code>chain</code> macro to declare a chain and handle Entry input
+</p>
+
+In the previous section, we declared `dispatcher!("greet", CMDGreet => EntryGreet)`.
+
+Now when a user types `greet`, it gets matched and wrapped into `EntryGreet`.
+
+But what happens after we get the Entry?
+
+We need a Chain to process it.
+
+## The `#[chain]` Macro
+
+`#[chain]` marks a handler function. The format is straightforward:
+
+```rust
+@@@dispatcher!("greet", CMDGreet => EntryGreet);
+pack!(ResultName = String);
+ 
+#[chain]
+fn handle_greet(args: EntryGreet) -> Next {
+    // args contains the remaining params after matching user input
+    let name = args.inner.first().cloned().unwrap_or_else(|| "World".to_string());
+    // Wrap the result into Next, telling the dispatcher where to go next
+    ResultName::new(name)
+}
+```
+ 
+Notice anything?
+
+The Chain function signature declares what it needs — `args: EntryGreet`.
+
+Then it returns a newtype via `ResultName::new(name)`.
+
+This returned `Next` expands into `impl Into<ChainProcess<ThisProgram>>`.
+
+> [!TIP]
+> Wondering how `Into<ChainProcess<G>>` works?
+>
+> Check out the [Any Output Mechanism](pages/concepts/3-any-output) chapter to learn about `ChainProcess`.
+
+## The `pack!` Macro
+
+You've probably guessed it — `pack!(ResultName = String)` defines a type that flows through the pipeline:
+
+```rust
+// pack!(ResultName = String) generates code roughly like this
+ 
+#[derive(Groupped)]
+pub struct ResultName {
+    pub inner: String,
+}
+```
+ 
+Think of it as a **tagged** `String`.
+
+The dispatcher uses this tag for precise routing, ensuring data doesn't get mixed up — e.g., data sent to `RenderGreet` won't be misdelivered to `RenderError`.
+
+> [!NOTE]
+> Unlike a simple type alias (`type`), `pack!` generates a completely new type with its own `TypeId`.
+
+Here's a recommended naming convention:
+
+| Role         | Naming Pattern         | Example              |
+| ------------ | ---------------------- | -------------------- |
+| Entry        | `Entry` + command      | `EntryGreet`         |
+| Intermediate | `State` + description  | `StateParsedArgs`    |
+| Result       | `Result` + description | `ResultGreetSomeone` |
+| Error        | `Error` + description  | `ErrorUserNotFound`  |
+
+See [Naming Convention](pages/other/naming_rule) for details, but for now just remember: **use `pack!` to give your data a meaningful name**.
+
+## Extracting Params from Entry
+
+`EntryGreet`'s `inner` is a `Vec<String>`, which you can freely process inside a Chain:
+
+```rust
+@@@dispatcher!("greet", CMDGreet => EntryGreet);
+@@@pack!(ResultName = String);
+#[chain]
+fn handle_greet(args: EntryGreet) -> Next {
+    // Take the first param, or use a default
+    let name = args
+        .inner
+        .first()
+        .cloned()
+        .unwrap_or_else(|| "World".to_string());
+ 
+    ResultName::new(name)
+}
+```
+ 
+If you enable the `parser` feature, you can also use `Picker` for more flexible param extraction — but that's a topic for later.
+
+## Putting It Together
+
+Now let's connect the Dispatcher and Chain:
+
+```rust
+// 1. Declare the command
+dispatcher!("greet", CMDGreet => EntryGreet);
+ 
+// 2. Declare the pipeline data type
+pack!(ResultName = String);
+ 
+// 3. Processing logic
+#[chain]
+fn handle_greet(args: EntryGreet) -> Next {
+    let name = args.inner
+        .first()
+        .cloned()
+        .unwrap_or_else(|| "World".to_string());
+    ResultName::new(name)
+}
+ 
+fn main() {
+    let mut program = ThisProgram::new();
+    program.with_dispatcher(CMDGreet);
+    program.exec_and_exit();
+}
+ 
+gen_program!();
+```
+ 
+But this code isn't complete yet — we only have the Dispatcher and Chain. One last step remains: **rendering the result**. That's what the next chapter, Renderer, covers.
+
+<p align="center" style="font-size: 0.85em; color: gray;">
+    Written by @Weicao-CatilGrass
+</p>