常量在Haskell代數數據類型

Question

它是合法的Haskell的代數數據類型有在聲明常數值？換句話說，我可以定義是這樣的：

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

我試圖從規範看着辦吧，但語法不是很清楚，我。

Answer 1

不，你不能做到這一點（數據類型是類型，不數據）。相反，你可以做到以下幾點：

data HTTPStatus = HTTPStatus Int String 

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

，或者類似：

data HTTPStatus = BadRequest | NotFound 

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

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