haskell - 如何在实现 `instance`时隐藏辅助函数

Question

我有data RegEx并且我想实现instance Show a => Show RegEx a。这是我的代码:

showAtom :: Show a => RegEx a -> String
showAtom (Lit x) = show x
showAtom r       = "(" ++ (show r) ++ ")"

instance Show a => Show (RegEx a) where
    show (Lit x)   = show [x]
    show (Opt r)   = (showAtom r) ++ "?"
    show (Alt p q) = (showAtom p) ++ "|" ++ (showAtom q)
    show (Seq p q) = (show p) ++ (show q)
    show (Rep r)   = (showAtom r) ++ "*"

showAtom 函数只是一个实现细节。有什么方法可以隐藏它，使其仅在 instance 定义中可见吗？或者更好的是，使其仅在 show 中可见。

Answer 1

这样的事情应该有效:

instance Show a => Show (RegEx a) where
    show x = show' x
      where
        showAtom :: Show a => RegEx a -> String
        showAtom (Lit x) = show x
        showAtom r       = "(" ++ (show r) ++ ")"

        show' (Lit x)   = show [x]
        show' (Opt r)   = (showAtom r) ++ "?"
        show' (Alt p q) = (showAtom p) ++ "|" ++ (showAtom q)
        show' (Seq p q) = (show p) ++ (show q)
        show' (Rep r)   = (showAtom r) ++ "*"

或者，您可以从模块的导出列表中排除 showAtom。