找不到一个好例子.感谢任何帮助.
JSON如下：
[{
"EXIF:Make": "Canon","EXIF:Model": "Canon PowerShot S95","EXIF:Orientation": "Horizontal (normal)","EXIF:XResolution": 180,"EXIF:YResolution": 180,"EXIF:ResolutionUnit": "inches"
}]
我使用的代码如下：
import Data.Aeson import Data.Attoparsec import Data.ByteString x <- fmap (parse json) (Data.ByteString.readFile "json.txt")
我该如何定义&使用FromJSON类型从x转换为：
data Exif = Exif [[(String,String)]]
或类似的数据结构？注意[[]] – 我希望JSON有多个顶级条目.
解决方法
这是一个惯用的解决方案：
{-# LANGUAGE OverloadedStrings #-}
module Main
where
import Control.Applicative
import Control.Monad
import Data.Aeson
import Data.Attoparsec
import qualified Data.ByteString as B
import qualified Data.Text as T
data ExifEntry = ExifEntry { exifMake :: T.Text,exifModel :: T.Text,exifOrientation :: T.Text,exifXResolution :: Int,exifYResolution :: Int,exifResolutionUnit :: T.Text
} deriving (Eq,Show)
instance FromJSON ExifEntry
where
parseJSON (Object v) = ExifEntry <$>
v .: "EXIF:Make" <*>
v .: "EXIF:Model" <*>
v .: "EXIF:Orientation" <*>
v .: "EXIF:XResolution" <*>
v .: "EXIF:YResolution" <*>
v .: "EXIF:ResolutionUnit"
parseJSON _ = mzero
parseAll :: B.ByteString -> [ExifEntry]
parseAll s = case (parse (fromJSON <$> json) s) of
Done _ (Error err) -> error err
Done ss (Success e) -> e:(parseAll ss)
_ -> []
main :: IO ()
main = do s <- B.readFile "json.txt"
let p = parseAll s
print p
测试：
$cat json.txt
{
"EXIF:Make": "Canon","EXIF:ResolutionUnit": "inches"
}
{
"EXIF:Make": "Canon","EXIF:Model": "Canon PowerShot S995","EXIF:ResolutionUnit": "inches"
}
$./dist/build/test/test
[ExifEntry {exifMake = "Canon",exifModel = "Canon PowerShot S95",exifOrientation = "Horizontal (normal)",exifXResolution = 180,exifYResolution = 180,exifResolutionUnit = "inches"},ExifEntry {exifMake = "Canon",exifModel = "Canon PowerShot S995",exifResolutionUnit = "inches"}]
或者,这是一个slightly more ugly solution,它为您提供所请求的数据类型([[(Text,Text)]]).
