haskell - Haskell 代数数据类型中的常量

Question

在 Haskell 代数数据类型中声明常量值是否合法？换句话说，我可以定义这样的东西吗:

data HttpStatusCodes = BadRequest "Bad request" 400
                     | NotAuthorized "Not authorized" 401
                     | -- ...

我试图从规范中弄清楚，但我不清楚语法。

Answer 1

不，你不能这样做(数据类型是类型，而不是数据)。相反，您可以执行以下操作:

data HTTPStatus = HTTPStatus Int String

badRequest, notAuthorized, notFound :: HTTPStatus
badRequest    = HTTPStatus 400 "Bad Request"
notAuthorized = HTTPStatus 401 "Not Authorized"
notFound      = HTTPStatus 404 "Not Found"

或者，类似的:

data HTTPStatus = BadRequest | NotFound

code :: HTTPStatus -> Int
code BadRequest = 400
code NotFound = 404

message :: HTTPStatus -> String
message BadRequest = "Bad Request"
message NotFound = "Not Found"