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。所有应该打印的是 String field 。

我试过这个:

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

但我仍然不断得到

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)

我不想添加 Show对于 (Int->Int->Int) ，我只想打印字符串。

感谢帮助!

编辑:

为了将来引用，固定版本是:

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。看来你忘了删除那个有缺陷的 deriving来自原始 data 的条款宣言。

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

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