haskell - 与预期类型不匹配

Question

如何解决此错误并使我的代码正常工作？

我的数据类型：

   data ID x = ID ( ( x, Mark ), [ ID x ] ) 
   data Mark = Marked | Unmarked

显示实例：

  instance  Show a  => Show ( ID a )  where

           show t  = go " " t   where 

                  go aP ( ID ( (x, Marked ), z ) ) =

                           aP ++ x ++ "\n" 

错误：

Couldn't match expected type `[Char]' against inferred type `a'
  `a' is a rigid type variable bound by
      the instance declaration at Dictionary.hs:117:23
  Expected type: ID [Char]
  Inferred type: ID a
In the second argument of `go', namely `t'
In the expression: go "" t
Failed, modules loaded: none.

Answer 1

编辑：

我发现您的意图难以破译，但我想您可能想将树结构表示为字符串？这是一个演示实现，向您展示了它是如何完成的（以一种快速、笨拙的方式。）

data ID x = ID ((x, Mark), [ID x]) 
data Mark = Marked | Unmarked

instance Show a => Show (ID a) where
    show (ID ((x, _), ids)) = "Val: " ++ show x ++
                              ", Children: " ++ 
                              (show $ map show ids)