Haskell代数数据类型中的常量

Question

Is it legal in a Haskell algebraic datatype to have constant values in the declaration? In other words, can I define something like this:

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

I tried to figure it out from the specification, but the grammar is not clear to me.

Answer 1

No, you are not allowed to do that (data types are types , not data ). Instead you can do the following:

data HTTPStatus = HTTPStatus Int String

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

or, similar:

data HTTPStatus = BadRequest | NotFound

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

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