haskell – 使用Aeson嵌套JSON中的Parse Array

我正在为Aeson写一个From JSON 函数.

{
  "total": 1,"movies": [
    {
      "id": "771315522","title": "Harry Potter and the Philosophers Stone (Wizard's Collection)","posters": {
        "thumbnail": "http://content7.flixster.com/movie/11/16/66/11166609_mob.jpg","profile": "http://content7.flixster.com/movie/11/16/66/11166609_pro.jpg","detailed": "http://content7.flixster.com/movie/11/16/66/11166609_det.jpg","original": "http://content7.flixster.com/movie/11/16/66/11166609_ori.jpg"
      }
    }
  ]
}

ADT：data Movie = Movie {id :: String,title :: String}

我的尝试：

instance FromJSON Movie where
    parseJSON (Object o) = do
       movies <- parseJSON =<< (o .: "movies") :: Parser Array
       v <- head $decode movies
       return $Movie <$>
           (v .: "movies" >>= (.: "id") ) <*>
           (v .: "movies" >>= (.: "title") )
    parseJSON _ = mzero

这给出了无法匹配预期类型’Parser t0’与实际类型’也许a0’在’头’的第一个参数.

正如你所看到的,我正在尝试选择Array中的第一部电影,但是我也不会介意获得电影列表(如果Array中有几个).

解决方法@H_502_18@

如果您真的想从JSON数组电影解析单个影片,您可以执行以下操作：

instance FromJSON Movie where
    parseJSON (Object o) = do
        movieValue <- head <$> o .: "movies"
        Movie <$> movieValue .: "id" <*> movieValue .: "title"
    parseJSON _ = mzero

但更安全的路线是通过newtype包装解析[Movie]：

main = print $movieList <$> decode "{\"total\":1,\"movies\":[ {\"id\":\"771315522\",\"title\":\"Harry Potter and the Philosophers Stone (Wizard's Collection)\",\"posters\":{\"thumbnail\":\"http://content7.flixster.com/movie/11/16/66/11166609_mob.jpg\",\"profile\":\"http://content7.flixster.com/movie/11/16/66/11166609_pro.jpg\",\"detailed\":\"http://content7.flixster.com/movie/11/16/66/11166609_det.jpg\",\"original\":\"http://content7.flixster.com/movie/11/16/66/11166609_ori.jpg\"}}]}"

newtype MovieList = MovieList {movieList :: [Movie]}

instance FromJSON MovieList where
    parseJSON (Object o) = MovieList <$> o .: "movies"
    parseJSON _ = mzero

data Movie = Movie {id :: String,title :: String}

instance FromJSON Movie where
    parseJSON (Object o) = Movie <$> o .: "id" <*> o .: "title"
    parseJSON _ = mzero

haskell – 使用Aeson嵌套JSON中的Parse Array

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