独自のデータ型の定義、Eitherの使い方を勉強。HTTPステータスコードと状態の対応表を導き出すコードを簡単な応用で書いてみた。 #CodePiece

CodePiece.hs

data LockerState = Taken | Free deriving (Show, Eq)
type Code = String
type LockerMap = Map.Map Int (LockerState, Code)

lockerLookup :: Int -> LockerMap -> Either String Code
lockerLookup lockerNumber map = case Map.lookup lockerNumber map of 
     Nothing -> Left $ "Locker " ++ show lockerNumber ++ " doesn't exist!" 
     Just (state, code) -> if state /= Taken
                              then Right code 
                              else Left $ "Locker " ++ show lockerNumber ++ " is already taken!!" 

lockers :: LockerMap
lockers = Map.fromList
    [(100, (Taken, "ZD391")),
     (101, (Free, "ZD392")),
     (118, (Taken, "ZD432")),
     (202, (Free, "KU392"))
    ]

-- 200を成功, 400を失敗ということにする
data HTTPStatus = Success | Failure deriving (Show, Eq)
type FailureDescription = String
type HTTPStatusMap = Map.Map Int (HTTPStatus, FailureDescription)

httpStatusLookup :: Int -> HTTPStatusMap -> Either String FailureDescription
httpStatusLookup httpStatus map = case Map.lookup httpStatus map of 
    Nothing -> Left $ "Unrecognized statusCode"
    Just (statusCode, description) -> if httpStatus < 400
                                      then Right $ description
                                      else Left $ description

httpStatusMap :: HTTPStatusMap
httpStatusMap = Map.fromList
    [(200, (Success, "Success")),
     (201, (Success, "Created")),
     (202, (Success, "Accepted")),
     (400, (Failure, "Bad Request")),
     (401, (Failure, "Unauthorized")),
     (404, (Failure, "Not Found")), 
     (500, (Failure, "Internal Server Error"))     
    ]