Correctly Parsing a JSON Array as Custom Data Type Object

Question

I defined the following datatype object in Haskell:

import GHC.Generics
import Data.Aeson
import qualified Data.ByteString.Lazy.Char8 as C
import Data.Maybe (fromJust)

data DLA = DLA {
                a::String,
                b::Int,
                c::Int,
                d::String,
                e::[Int]
                } deriving (Show, Generic)

instance FromJSON DLA
instance ToJSON DLA

And receive a json array from some frontend-application in the following format

[\"hello\",3,2,\"world\",[1,3,5]]

When I tried to parse this like

decode $ C.pack "[\"hello\",3,2,\"world\",[1,3,5]]"::Maybe DLA

it simply returned Nothing. Trying to pass it more general like

decode $ C.pack "[\"hello\",3,2,\"world\",[1,3,5]]"::Maybe Value

returned the following output:

Just (Array [String "hello",Number 3.0,Number 2.0,String "world",Array [Number 1.0,Number 3.0,Number 5.0]])

So the problem seems to be that Haskell interprets the string as an array, not an object, and thus rightly throws an exception. Any ideas how to fix this?

I am using the current version of ghc and ghci, and Data.Aeson for this.

That string is a JSON array. It's not an object. A JSON object would look like this: { "a": "hello", "b": 3, "c": 2, "d": "world", "e": [1, 3, 5] } — Mark Seemann
– Mark Seemann, Commented Jul 1, 2019 at 12:44

aru_bdd · Accepted Answer · 2019-07-01 14:52:31Z

1

I found a solution which is quite simple using pattern matching but more direct approaches are also welcome, as I believe there should be some functionality for this.

raw = fromJust (decode $ C.pack "[\"hello\",3,2,\"world\",[1,3,5]]"::Maybe (String, Int, Int,   String, [Int]))

func::(String, Int, Int, String, [Int]) -> DLA
func (a,b,c,d,e) = DLA a b c d e

answered Jul 1, 2019 at 14:52

aru_bdd

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