isNothing 在解析 JSON 文件时抛出异常

Question

我有以下函数可以使用 Data.Aeson 库解码 JSON 文件：

data SearchResult = SearchResult {
  items :: [Item]
} deriving (Show)

instance FromJSON SearchResult where
  parseJSON :: Value -> Parser SearchResult
  parseJSON (Object v) = SearchResult <data SearchResult = SearchResult {
  items :: [Item]
} deriving (Show)

instance FromJSON SearchResult where
  parseJSON :: Value -> Parser SearchResult
  parseJSON (Object v) = SearchResult <$>
    parseJSON (fromJust $ HM.lookup "items" v)
  parseJSON _ = mzero

data Item = Item {
  volumeInfo :: VolumeInfo
} deriving (Show)

instance FromJSON Item where
  parseJSON :: Value -> Parser Item
  parseJSON (Object v) = Item <$>
    parseJSON (fromJust $ HM.lookup "volumeInfo" v)
  parseJSON _ = mzero

data VolumeInfo = VolumeInfo {
  title :: String,
  authors :: [String],
  publisher :: String,
  publishedDate :: String,
  industryIdentifiers :: [IndustryIdentifier],
  pageCount :: Int,
  categories :: [String]
} deriving (Show)

instance FromJSON VolumeInfo where
  parseJSON :: Value -> Parser VolumeInfo
  parseJSON (Object v) = VolumeInfo <$>
    v .: "title" <*>
    v .: "authors" <*>
    v .: "publisher" <*>
    v .: "publishedDate" <*>
    parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>
    v .: "pageCount" <*>
    v .: "categories"
  parseJSON _ = mzero

data IndustryIdentifier = IndustryIdentifier {
  identifierType :: String,
  identifier :: String
} deriving (Show)

instance FromJSON IndustryIdentifier where
  parseJSON :: Value -> Parser IndustryIdentifier
  parseJSON (Object v) = IndustryIdentifier <$>
    v .: "type" <*>
    v .: "identifier"
  parseJSON _ = mzero
gt;
    parseJSON (fromJust $ HM.lookup "items" v)
  parseJSON _ = mzero

data Item = Item {
  volumeInfo :: VolumeInfo
} deriving (Show)

instance FromJSON Item where
  parseJSON :: Value -> Parser Item
  parseJSON (Object v) = Item <data SearchResult = SearchResult {
  items :: [Item]
} deriving (Show)

instance FromJSON SearchResult where
  parseJSON :: Value -> Parser SearchResult
  parseJSON (Object v) = SearchResult <$>
    parseJSON (fromJust $ HM.lookup "items" v)
  parseJSON _ = mzero

data Item = Item {
  volumeInfo :: VolumeInfo
} deriving (Show)

instance FromJSON Item where
  parseJSON :: Value -> Parser Item
  parseJSON (Object v) = Item <$>
    parseJSON (fromJust $ HM.lookup "volumeInfo" v)
  parseJSON _ = mzero

data VolumeInfo = VolumeInfo {
  title :: String,
  authors :: [String],
  publisher :: String,
  publishedDate :: String,
  industryIdentifiers :: [IndustryIdentifier],
  pageCount :: Int,
  categories :: [String]
} deriving (Show)

instance FromJSON VolumeInfo where
  parseJSON :: Value -> Parser VolumeInfo
  parseJSON (Object v) = VolumeInfo <$>
    v .: "title" <*>
    v .: "authors" <*>
    v .: "publisher" <*>
    v .: "publishedDate" <*>
    parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>
    v .: "pageCount" <*>
    v .: "categories"
  parseJSON _ = mzero

data IndustryIdentifier = IndustryIdentifier {
  identifierType :: String,
  identifier :: String
} deriving (Show)

instance FromJSON IndustryIdentifier where
  parseJSON :: Value -> Parser IndustryIdentifier
  parseJSON (Object v) = IndustryIdentifier <$>
    v .: "type" <*>
    v .: "identifier"
  parseJSON _ = mzero
gt;
    parseJSON (fromJust $ HM.lookup "volumeInfo" v)
  parseJSON _ = mzero

data VolumeInfo = VolumeInfo {
  title :: String,
  authors :: [String],
  publisher :: String,
  publishedDate :: String,
  industryIdentifiers :: [IndustryIdentifier],
  pageCount :: Int,
  categories :: [String]
} deriving (Show)

instance FromJSON VolumeInfo where
  parseJSON :: Value -> Parser VolumeInfo
  parseJSON (Object v) = VolumeInfo <data SearchResult = SearchResult {
  items :: [Item]
} deriving (Show)

instance FromJSON SearchResult where
  parseJSON :: Value -> Parser SearchResult
  parseJSON (Object v) = SearchResult <$>
    parseJSON (fromJust $ HM.lookup "items" v)
  parseJSON _ = mzero

data Item = Item {
  volumeInfo :: VolumeInfo
} deriving (Show)

instance FromJSON Item where
  parseJSON :: Value -> Parser Item
  parseJSON (Object v) = Item <$>
    parseJSON (fromJust $ HM.lookup "volumeInfo" v)
  parseJSON _ = mzero

data VolumeInfo = VolumeInfo {
  title :: String,
  authors :: [String],
  publisher :: String,
  publishedDate :: String,
  industryIdentifiers :: [IndustryIdentifier],
  pageCount :: Int,
  categories :: [String]
} deriving (Show)

instance FromJSON VolumeInfo where
  parseJSON :: Value -> Parser VolumeInfo
  parseJSON (Object v) = VolumeInfo <$>
    v .: "title" <*>
    v .: "authors" <*>
    v .: "publisher" <*>
    v .: "publishedDate" <*>
    parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>
    v .: "pageCount" <*>
    v .: "categories"
  parseJSON _ = mzero

data IndustryIdentifier = IndustryIdentifier {
  identifierType :: String,
  identifier :: String
} deriving (Show)

instance FromJSON IndustryIdentifier where
  parseJSON :: Value -> Parser IndustryIdentifier
  parseJSON (Object v) = IndustryIdentifier <$>
    v .: "type" <*>
    v .: "identifier"
  parseJSON _ = mzero
gt;
    v .: "title" <*>
    v .: "authors" <*>
    v .: "publisher" <*>
    v .: "publishedDate" <*>
    parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>
    v .: "pageCount" <*>
    v .: "categories"
  parseJSON _ = mzero

data IndustryIdentifier = IndustryIdentifier {
  identifierType :: String,
  identifier :: String
} deriving (Show)

instance FromJSON IndustryIdentifier where
  parseJSON :: Value -> Parser IndustryIdentifier
  parseJSON (Object v) = IndustryIdentifier <data SearchResult = SearchResult {
  items :: [Item]
} deriving (Show)

instance FromJSON SearchResult where
  parseJSON :: Value -> Parser SearchResult
  parseJSON (Object v) = SearchResult <$>
    parseJSON (fromJust $ HM.lookup "items" v)
  parseJSON _ = mzero

data Item = Item {
  volumeInfo :: VolumeInfo
} deriving (Show)

instance FromJSON Item where
  parseJSON :: Value -> Parser Item
  parseJSON (Object v) = Item <$>
    parseJSON (fromJust $ HM.lookup "volumeInfo" v)
  parseJSON _ = mzero

data VolumeInfo = VolumeInfo {
  title :: String,
  authors :: [String],
  publisher :: String,
  publishedDate :: String,
  industryIdentifiers :: [IndustryIdentifier],
  pageCount :: Int,
  categories :: [String]
} deriving (Show)

instance FromJSON VolumeInfo where
  parseJSON :: Value -> Parser VolumeInfo
  parseJSON (Object v) = VolumeInfo <$>
    v .: "title" <*>
    v .: "authors" <*>
    v .: "publisher" <*>
    v .: "publishedDate" <*>
    parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>
    v .: "pageCount" <*>
    v .: "categories"
  parseJSON _ = mzero

data IndustryIdentifier = IndustryIdentifier {
  identifierType :: String,
  identifier :: String
} deriving (Show)

instance FromJSON IndustryIdentifier where
  parseJSON :: Value -> Parser IndustryIdentifier
  parseJSON (Object v) = IndustryIdentifier <$>
    v .: "type" <*>
    v .: "identifier"
  parseJSON _ = mzero
gt;
    v .: "type" <*>
    v .: "identifier"
  parseJSON _ = mzero

还有这个函数：

getBook content = do
  putStrLn (Data.ByteString.Lazy.Char8.unpack content)
  let searchResult = decode content :: Maybe SearchResult
  print (isNothing searchResult)
  print searchResult

函数 getBook 适用于许多 JSON 文件。这是一个例子：

False
Just (SearchResult {items = [Item {volumeInfo = VolumeInfo {title = "A Memoir of Jane Austen", authors = ["James Edward Austen-Leigh","Jane Austen, James Austen-Leigh"], publisher = "Wordsworth Editions", publishedDate = "2007", industryIdentifiers = [IndustryIdentifier {identifierType = "ISBN_10", identifier = "1840225602"},IndustryIdentifier {identifierType = "ISBN_13", identifier = "9781840225600"}], pageCount = 256, categories = ["Novelists, English"]}}]})

JSON内容解码成功，因此第一行isNothingreturnsFalse，后面是解码后的内容。如果我使用 this JSON file 作为 content 再次运行该函数，我会得到以下输出：

True
Nothing

无法解码文件（因为 JSON 文件中没有字段 categories），因此 isNothing returns True , Nothing 打印在屏幕上。现在的问题是当我使用 this JSON file 作为 content 运行它时。我明白了：

*** Exception: Maybe.fromJust: Nothing

执行print (isNothing searchResult)时抛出异常，不明白为什么没有像上一个例子那样返回True（因为这里没有字段industryIdentifiers，例如）。我错过了什么或做错了什么？

编辑：

我发现每次 JSON 文件不包含字段 industryIdentifiers 时都会出现问题。它在这一行中失败：

parseJSON (fromJust $ HM.lookup "industryIdentifiers" v) <*>

Answer 1

github包定义了一个方便的操作符来定义数组字段：

-- | A slightly more generic version of Aeson's @(.:?)@, using `mzero' instead
-- of `Nothing'.
(.:<) :: (FromJSON a) => Object -> T.Text -> Parser [a]
obj .:< key = case Map.lookup key obj of
                   Nothing -> pure mzero
                   Just v  -> parseJSON v

（这里Map是Data.HashMap.Lazy的别名）

那么 VolumeInfo 的 FromJSON 实例将定义如下：

instance FromJSON VolumeInfo
  v .: "title" <*>
  ...
  v .:< "industryIdentifiers" <*>
  ...

isNothing 在解析 JSON 文件时抛出异常

isNothing throws an exception when parsing JSON file

json

exception

haskell

aeson