枚舉可以以一個自然的方式通過它們的構造器進行匹配： > Enums can be matched by their constructors in a natural way: ~~~ var myTree = Node(Leaf("foo"), Node(Leaf("bar"), Leaf("foobar"))); var match = switch(myTree) { // matches any Leaf case Leaf(_): "0"; // matches any Node that has r = Leaf case Node(_, Leaf(_)): "1"; // matches any Node that has has // r = another Node, which has // l = Leaf("bar") case Node(_, Node(Leaf("bar"), _)): "2"; // matches anything 12 case _: "3"; } trace(match); // 2 ~~~ 模式匹配器會檢查每個 case ，從頭至尾，采用第一個匹配輸入值的 case 。下面關于case規則的說明解釋幫助你理解該過程： > The pattern matcher will check each case from top to bottom and pick the ?rst one that matches the input value. The following manual interpretation of each case rule helps understanding the process: case Leaf(\_)：匹配失敗，因為 myTree 是一個i額 Node case Node(\_,Leaf(\_))：匹配失敗，因為myTree右側的子樹不是一個Leaf，而是另一個 Node **case Node(\_,Node(Leaf("bar"),\_))**：匹配成功 **case \_**：不被檢查，因為前一行已經匹配成功 > **case Leaf(\_)**: matching fails because myTree is a Node > **case Node(\_, Leaf(\_))**: matching fails because the right sub-tree of myTree is not a Leaf, but another Node > **case Node(\_, Node(Leaf("bar"), \_))**: matching succeeds > **case \_**: this is not checked here because the previous line matched