我按照本书来定义树数据类型,但show无法正常工作.为什么？
data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show)
test = show EmptyTree
给出错误消息:
No instance for (Show a0) arising from a use of ???show???
The type variable ???a0??? is ambiguous
Note: there are several potential instances:
instance Show a => Show (Tree a)
-- Defined at /Users/gzhao/Documents/workspace/hsTest2/src/Tree.hs:3:62
instance Show Double -- Defined in ???GHC.Float???
instance Show Float -- Defined in ???GHC.Float???
...plus 25 others
In the expression: show EmptyTree
In an equation for …haskell ×1