add doc for question operator

README.md

@@ -837,6 +837,49 @@ fn init {
 }
 ```
 
+## The question operator
+MoonBit features a convenient `?` operator for error handling.
+The `?` postfix operator can be applied to expressions of type `Option` or `Result`.
+When applied to expression `t : Option[T]`, `t?` is equivalent to:
+
+```rust
+match t {
+  None => { return None }
+  Some(x) => x
+}
+```
+
+When applied to expression `t: Result[T, E]`, `t?` is equivalent to:
+
+```rust
+match t {
+  Err(err) => { return Err(err) }
+  Ok(x) => x
+}
+```
+
+The question operator can be used to combine codes that may fail or error elegantly:
+
+```rust
+fn may_fail() -> Option[Int] { ... }
+
+fn f() -> Option[Int] {
+  let x = may_fail()?
+  let y = may_fail()?.lsr(1) + 1
+  if y == 0 { return None }
+  Some(x / y)
+}
+
+fn may_error() -> Result[Int, String] { ... }
+
+fn g() -> Result[Int, String] {
+  let x = may_error()?
+  let y = may_error()? * 2
+  if y == 0 { return Err("divide by zero") }
+  Ok(x / y)
+}
+```
+
 ## MoonBit's build system
 
 The introduction to the build system is available at [MoonBit's Build System Tutorial](./build-system-tutorial.md).

zh-docs/README.md

@@ -796,6 +796,49 @@ fn init {
 }
 ```
 
+## 问号操作符
+MoonBit 提供一个便捷的 `?` 操作符，用于错误处理。
+`?` 是一个后缀运算符。它可以作用于类型为 `Option` 或 `Result` 的表达式。
+被应用在表达式 `t : Option[T]` 上时，`t?` 等价于：
+
+```rust
+match t {
+  None => { return None }
+  Some(x) => x
+}
+```
+
+被应用在表达式 `t : Result[T, E]` 上时，`t?` 等价于：
+
+```rust
+match t {
+  Err(err) => { return Err(err) }
+  Ok(x) => x
+}
+```
+
+问号操作符可以用于优雅地组合多段可能失败或产生错误的程序：
+
+```rust
+fn may_fail() -> Option[Int] { ... }
+
+fn f() -> Option[Int] {
+  let x = may_fail()?
+  let y = may_fail()?.lsr(1) + 1
+  if y == 0 { return None }
+  Some(x / y)
+}
+
+fn may_error() -> Result[Int, String] { ... }
+
+fn g() -> Result[Int, String] {
+  let x = may_error()?
+  let y = may_error()? * 2
+  if y == 0 { return Err("divide by zero") }
+  Ok(x / y)
+}
+```
+
 ## MoonBit 的构建系统
 
 构建系统的介绍可以在 [MoonBit 的构建系统教程](./build-system-tutorial.md) 中找到。