E4118

Cannot match type with map pattern.

You can use map patterns on custom types, as long as they have a method op_get that returns an optional value for a given key.

错误示例

type MyMap[K, V] Map[K, V]

fn main {
  let map : MyMap[String, Int] = { "a": 1, "b": 2, "c": 3 }
  match map {
    { "a": a, .. } => println("a: \{a}")
  //^~~~~~~~~~~~~~
  // Error: Please implement method `op_get` for type MyMap[String, Int] to match it with map pattern.
  }
}

建议

Implement the op_get method as suggested in the error message.

pub fn MyMap::op_get[K: Eq + Hash, V](self : MyMap[K, V], key : K) -> V? {
  self._[key]
}