循环类型同义词声明

Question

我想尝试Haskell type并data使用以下代码

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

Program遵循延续传递方式(CPS)并Nothing指示终止.a可以用Int其他东西实例化.

然而,GHC抱怨说:

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.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

一个工作版本将是:

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