递归显示数据类型

Question

我有以下递归数据类型：

data Person boss action = Person{
  salary  :: Float,
  subordinates :: [Person boss action],
  act :: action
  b :: boss
 }

我需要一个函数来打印当前人员的所有字段，然后重复调用该函数。据我所见，秀只能有一个论点。我试过这样的方法：

instance (Show boss, Show action) => Show (Person boss action) where
    show  p =  (show (salary p) (act p) (b p)) map (\x -> show x) (subordinates p)

Answer 1

由于show返回String，您可以通过`++连接show的多个调用：

instance (Show boss, Show action) => Show (Person boss action) where
    show  p =  "salary: " ++ show (salary p) ++ " act " ++ show (act p) ++ " b " ++ show (b p) ++ " subordinates " ++ intercalate "i " (map show (subordinates p))

例如，这段代码

import Data.List                                                                                                                                                                                        

data Person boss action = Person {
salary  :: Float,
subordinates :: [Person boss action],
act :: action,
b :: boss
}

instance (Show boss, Show action) => Show (Person boss action) where
    show  p =  "salary: " ++ show (salary p) ++ " act " ++ show (act p) ++ " b " ++ show (b p) ++ " subordinates " ++ intercalate "i " (map show (subordinates p))

main =
    let
        a = Person { salary=1, subordinates=[], act=1, b=1 }
        aa = Person { salary=2, subordinates=[a], act=2, b=2 }
    in
        do
            putStrLn $ show aa

输出

salary: 2.0 act 2 b 2 subordinates salary: 1.0 act 1 b 1 subordinates

顺便提一句，注意

map (\x -> show x) ...

可以写成

map show ...