如何部分创建类型

Question

我正在尝试创建Btree单子，但是我不知道如何为此定义return，因为我想将值放在Node (Leaf value)中，而将另一个保留分支为空。

module Tree where 
    import Control.Monad

    data Btree a=Node (Btree a)(Btree a)|Leaf a 

如果我想在Monad中插入一个值，则会出现问题：

    instance Monad Btree  where 
      (>>=) (Leaf x) f = f x
      (>>=) (Node a b) f=Node (a>>=f) (b>>=f)
      return t=Node (Leaf t)  # i want to partially apply `Node` to not have to specify the other branch . Do i need an additional ADT  ?  `Leaf/Node/Root` ? for single elements ?

PS 当尝试使用return=Leaf或return t=Node (Leaf t)(Leaf t)运行此程序时，我也收到错误消息：

* No instance for (Applicative Btree)
        arising from the superclasses of an instance declaration
    * In the instance declaration for `Monad Btree'