Haskell：为自定义类型派生 Show

Question

我有这个类型定义：

data Operace = Op (Int->Int->Int) String (Int->Int->Int) deriving Show

我想将此类型打印到交互式外壳 (GHCi) 中。应该打印的只是String 字段。

我试过了：

instance Show Operace where
    show (Op op str inv) = show str

但我还是不断地得到

No instance for (Show (Int -> Int -> Int))
  arising from the 'deriving' clause of a data type declaration
Possible fix:
  add an instance declaration for (Show (Int -> Int -> Int))
  or use a standalone 'deriving instance' declaration,
       so you can specify the instance context yourself
When deriving the instance for (Show Operace)

我不想为(Int->Int->Int) 添加Show，我只想打印字符串。

感谢您的帮助！

编辑：

供以后参考，固定版本为：

data Operace = Op (Int->Int->Int) String (Int->Int->Int)

instance Show Operace where
    show (Op _ str _) = str

Answer 1

您所做的实例声明是正确的方法。您似乎忘记从原始 data 声明中删除错误的 deriving 子句。

data Operace = Op (Int->Int->Int) String (Int->Int->Int)

instance Show Operace where
   show (Op op str inv) = show str

Answer 2

你可以导出Show，只需先导入Text.Show.Functions。