计算数据类中所有元素的数量

Question

需要统计组件gui中的所有按钮。无法理解使用此数据对象的正确方法

data Component = TextBox {name :: String, text :: String}
                | Button {name :: String, value :: String}
                | Container {name :: String, children :: [Component]}

gui :: Component 
gui = Container "My App" [
        Container "Menu" [
          Button "btn_new" "New",
          Button "btn_open" "Open",
          Button "btn_close" "Close"
        ],
        Container "Body" [TextBox "textbox_1" "Some text does here"],
        Container "Footer" []]


countButtons :: Component -> Int
countButtons (TextBox []) = 0 
countButtons (Container _ Button) = 1 + countButtons Container

Answer 1

aComponent可以有三个值。如果用countButtons文字描述的话就是：

如果我的组件是 a TextBox，它将有 0 个按钮；如果它是 a Button，它将恰好有 1 个，如果它是 a ，Container它将具有与其子级一样多的按钮

在这种情况下，它几乎可以逐字翻译成 Haskell

countButtons :: Component -> Int
countButtons (TextBox _ _) = 0
countButtons (Button _ _) = 1         
countButtons (Container _ child) = sum (fmap countButtons child)
                              --   |- this transform children into integers, and sum them all.