add doc for map pattern

README.md

@@ -846,6 +846,24 @@ match expr {
 }
 ```
 
+### Map Pattern
+MoonBit allows convenient matching on map-like data structures:
+```rust
+match map {
+  // matches if any only if "b" exists in `map`
+  { "b": Some(_) } => ..
+  // matches if and only if "b" does not exist in `map` and "a" exists in `map`.
+  // When matches, bind the value of "a" in `map` to `x`
+  { "b": None, "a": Some(x) } => ..
+  // compiler reports missing case: { "b": None, "a": None }
+}
+```
+
+- To match a data type `T` using map pattern, `T` must have a method `op_get(Self, K) -> Option[V]` for some type `K` and `V`.
+- Currently, the key part of map pattern must be a constant
+- Map patterns are always open: unmatched keys are silently ignored
+- Map pattern will be compiled to efficient code: every key will be fetched at most once
+
 ## Generics
 
 Generics are supported in top-level function and data type definitions. Type parameters can be introduced within square brackets. We can rewrite the aforementioned data type `List` to add a type parameter `T` to obtain a generic version of lists. We can then define generic functions over lists like `map` and `reduce`.

zh-docs/README.md

@@ -840,6 +840,24 @@ match expr {
 }
 ```
 
+### 键值对模式匹配
+MoonBit 允许模式匹配字典等具有键值对结构的数据结构：
+```rust
+match map {
+  // 匹配键 "b" 存在的情况
+  { "b": Some(_) } => ..
+  // 匹配 "b" 不存在且 "a" 存在的情况，
+  // 匹配成功时，把 "a" 的值绑定到 `x`
+  { "b": None, "a": Some(x) } => ..
+  // 编译器指出下列情况没有被匹配到：{ "b": None, "a": None }
+}
+```
+
+- 如果要用字典模式匹配类型 `T`，必须给类型 `T` 实现方法 `op_get`。`op_get` 的类型必须是 `(Self, K) -> Option[V]`，其中 `K` 是键的类型，`V` 是值的类型
+- 目前，字典模式的键部分必须是一个常量
+- 字典模式永远是开放的：未被匹配到的键会被忽略
+- 字典模式会被编译成高效的代码：每个键至多被查询一次
+
 ## 泛型
 
 您可以在顶层的函数和数据结构定义中使用泛型。类型参数可以由方括号引入。