相关疑难解决方法(0)

我正试图XRankNTypes在GHCi中启用.我该怎么做呢？

我的树是由

data Tree a = Leaf a | Node (Tree a) (Tree a) 
        deriving (Show)

我还宣布了一个测试树.

myTree = Node (Node (Leaf 1) (Leaf 2)) (Leaf 3)

我想要做的是创建一个函数maptree f,它将作用于Leaf.更具体地说f x = x +1,

然后maptree f myTree会回来

Node (Node (Leaf 2) (Leaf 3)) (Leaf 4)

我的解决方案是

maptree f (Leaf a)= Leaf (f a)
maptree f (Node xl xr ) = Node (maptree xl) (maptree xr)

但它会返回以下错误

Couldn't match expected type `Tree a'
       against inferred type `Tree t -> …