haskell - 在类型同义词声明中循环

Question

我想用下面的代码试试 Haskell type 和 data

type Program a = Maybe a -> (Operation a, Maybe Program a)
data Operation a = Read a | Write a

Program 遵循延续传递风格 (CPS)，Nothing 表示终止。 a 可以用 Int 或其他东西来实例化。

但是，GHC 提示说:

main.hs:1:1:
    Cycle in type synonym declarations:
      main.hs:1:1-58: type Program a =
                          Maybe a -> (Operation a, Maybe Program a)

我是 Haskell 的新手，所以我不明白为什么不允许这样做。我如何在 Haskell 中表达这种类型？

Answer 1

这里有两个问题:

• 应该是Maybe (Program a)，而不是Maybe Program a
• 类型同义词不能递归

一个工作版本是:

newtype Program a = Program { runProgram :: Maybe a -> (Operation a, Maybe (Program a)) }
data Operation a = Read a | Write a