Haskell Aeson：如何将Value转换为自定义类型？

本文介绍了Haskell Aeson：如何将Value转换为自定义类型？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 29岁程序员，3月因学历无情被辞！ 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：水平（普通），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.readFilejson.txt） 如何定义&使用 FromJSON 类型从 x 转换为： data Exif = Exif [[（String，String）]] [[]] - 我期待JSON有多个顶级条目。 解决方案以下是一个惯用的解决方案： { - ＃LANGUAGE OverloadedStrings＃ - } 模块主要其中 导入Control.Applicative 导入Control.Monad 导入Data.Aeson 导入Data.Attoparsec 将合格的Data.ByteString导入为B 将合格的Data.Text导入为T 数据ExifEntry = ExifEntry {exifMake :: T.Text， exifModel： ：T.Text， exifOrientation :: T.Text， exifXResolution :: Int， exifYResolution :: Int， exifResolutionUnit :: T.Text }派生（Eq，Show） 实例FromJSON ExifEntry 其中 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（解析（fromJSON< $> json）s）完成_（错误错误） - >错误错误完成ss（成功e） - > e：（parseAll ss） _ - > [] main :: IO（） main = do s let p = parseAll s print p 测试： {EXIF：Make：Canon，EXIF：Model：Canon PowerShot S95， EXIF：Orientation：Horizo​​ntal（normal），EXIF：XResolution：180，EXIF：YResolution：180，EXIF：ResolutionUnit：inches $ bEXIF：Make：Canon，EXIF：Model：Canon PowerShot S995，EXIF：方向：水平（普通），EXIF：X分辨率：180，EXIF：Y分辨率：180，EXIF：ResolutionUnit：英寸$ b $ [ExifEntry {exifMake =Canon，exifModel =Canon PowerShot S95，exifOrientation =Horizo​​ntal（normal），exifXResolution = 180， exifYResolution = 180，exifResolutionUnit =inches}，ExifEntry {exifMake =Canon，exifModel =Canon PowerShot S995，exifOrient ation =Horizo​​ntal（normal），exifXResolution = 180，exifYResolution = 180，exifResolutionUnit =inches}] 或者，这里有一个稍微更难看的解决方案，它为您提供了您请求的数据类型（ [[ （Text，Text）]] ）。 Can't find a good example. Appreciate any help. The JSON is as follows:[{ "EXIF:Make": "Canon", "EXIF:Model": "Canon PowerShot S95", "EXIF:Orientation": "Horizontal (normal)", "EXIF:XResolution": 180, "EXIF:YResolution": 180, "EXIF:ResolutionUnit": "inches"}]The code I used is as follows:import Data.Aesonimport Data.Attoparsecimport Data.ByteStringx <- fmap (parse json) (Data.ByteString.readFile "json.txt")How do I define & use the FromJSON type to convert from x into:data Exif = Exif [[(String, String)]]or similar data structure? Note the [[]] -- I'm expecting the JSON to have multiple top-level entries. 解决方案 Here's an idiomatic solution:{-# LANGUAGE OverloadedStrings #-}module Main whereimport Control.Applicativeimport Control.Monadimport Data.Aesonimport Data.Attoparsecimport qualified Data.ByteString as Bimport qualified Data.Text as Tdata 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 <$> v .: "EXIF:Make" <*> v .: "EXIF:Model" <*> v .: "EXIF:Orientation" <*> v .: "EXIF:XResolution" <*> v .: "EXIF:YResolution" <*> v .: "EXIF:ResolutionUnit" parseJSON _ = mzeroparseAll :: B.ByteString -> [ExifEntry]parseAll s = case (parse (fromJSON <$> 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 pTesting:$ cat json.txt{ "EXIF:Make": "Canon", "EXIF:Model": "Canon PowerShot S95", "EXIF:Orientation": "Horizontal (normal)", "EXIF:XResolution": 180, "EXIF:YResolution": 180, "EXIF:ResolutionUnit": "inches"}{ "EXIF:Make": "Canon", "EXIF:Model": "Canon PowerShot S995", "EXIF:Orientation": "Horizontal (normal)", "EXIF:XResolution": 180, "EXIF:YResolution": 180, "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", exifOrientation = "Horizontal (normal)", exifXResolution = 180, exifYResolution = 180, exifResolutionUnit = "inches"}]Alternatively, here's a slightly more ugly solution that gives you the data type you requested ([[(Text,Text)]]). 这篇关于Haskell Aeson：如何将Value转换为自定义类型？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！