CODEWITHSUNDEEP
jsonhaskell
ManfredP
ManfredP

Reputation: 1057

Convert JSON back to Data in Haskell

I have the following Haskell to call my WCF Service:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric #-}

module Main where

import Data.Aeson
import Data.Dynamic
import Data.Aeson.Lens
import Data.ByteString.Lazy as BS
import GHC.Generics
import Network.Wreq
import Control.Lens

data Point = Point { x :: Int, y :: Int } deriving (Generic, Show)
instance ToJSON Point
instance FromJSON Point

data Rectangle = Rectangle { width :: Int, height :: Int, point :: Point } deriving (Generic, Show)
instance ToJSON Rectangle
instance FromJSON Rectangle

main = do        
    let p = Point 1 2
    let r = Rectangle 10 20 p 
    let url = "http://localhost:8000/Rectangle"   
    let opts = defaults & header "Content-Type" .~ ["application/json"]
    r <- postWith opts url (encode r)    
    let returnData = r ^? responseBody
    case (decode returnData) of
        Nothing       -> BS.putStrLn "Error decoding JSON"
        Just json     -> BS.putStrLn $ show $ decode json

The output in this case is:

Just "{\"height\":20,\"point\":{\"x\":1,\"y\":2},\"width\":10}"

I already tried it with fromJSON:

print $ fromJSON returnData

and got this error:

Couldn't match expected type `Value'
                with actual type `Maybe
                                    bytestring-0.10.6.0:Data.ByteString.Lazy.Internal.ByteString'
    In the first argument of `fromJSON', namely `returnData'
    In the second argument of `($)', namely `fromJSON returnData'
Failed, modules loaded: none.

My question is now how to convert this JSON string back to an object of type "Rectangle"?

EDIT 1: I changed my code due to Janos Potecki's answer and get now the following error:

Couldn't match type `[Char]' with `ByteString'
    Expected type: ByteString
      Actual type: String
    In the second argument of `($)', namely `show $ decode json'
    In the expression: BS.putStrLn $ show $ decode json
    In a case alternative:
        Just json -> BS.putStrLn $ show $ decode json
Failed, modules loaded: none.

EDIT 2: i changed it to:

main = do        
    let point = Point 1 2
    let rectangle = Rectangle 10 20 point
    let url = "http://localhost:8000/Rectangle/Move/100,200"   
    let opts = defaults & header "Content-Type" .~ ["application/json"]
    r <- postWith opts url (encode rectangle)    
    let returnData = (r ^? responseBody) >>= decode
    case returnData of
     Nothing       -> BS.putStrLn "Error decoding JSON"
     Just json     -> BS.putStrLn json

and now i get:

No instance for (FromJSON ByteString)
      arising from a use of `decode'
    In the second argument of `(>>=)', namely `decode'
    In the expression: (r ^? responseBody) >>= decode
    In an equation for `returnData':
        returnData = (r ^? responseBody) >>= decode

Upvotes: 1

Views: 336

Answers (1)

Janos Potecki
Janos Potecki

Reputation: 162

working solution

r' <- asJSON =<< postWith opts url (encode rectangle) :: IO Res
case r' of
     Nothing        -> print "Error decoding JSON"
     Just x         -> print x

For performance I'd suggest you add the following to your instance ToJSON:

instance ToJSON Point where
       toEncoding = genericToEncoding defaultOptions

and the same for Rectangle

Upvotes: 3

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