Dfyjkt

I'm a Haskell beginner trying to learn JSON parsing by going through USDA database.

I want to get the value of "ndbno" key from this link
https://api.nal.usda.gov/ndb/search/?format=json&q=potato+salad&sort=n&max=25&offset=0&api_key=DEMO_KEY

The JSON file from that link looks like this:

{
 "list": {
 "q": "potato salad",
 "sr": "1",
 "ds": "any",
 "start": 0,
 "end": 25,
 "total": 251,
 "group": "",
 "sort": "n",
 "item": [
 
 "offset": 0,
 "group": "Branded Food Products Database",
 "name": "AHOLD, DEVILED EGG POTATO SALAD, UPC: 688267141584",
 "ndbno": "45044170",
 "ds": "LI",
 "manu": "Ahold USA, Inc."
 ,
 
 "offset": 1,
 "group": "Branded Food Products Database",
 "name": "AHOLD, REDSKIN POTATO SALAD, UPC: 688267141591",
 "ndbno": "45044169",
 "ds": "LI",
 "manu": "Ahold USA, Inc."
 ,
...

"nbdno" key is 4th in "item"

-# LANGUAGE DeriveGeneric #-

module Lib
 ( someFunc
 ) where


import Data.Aeson
import Data.HashMap.Strict as HS
import Data.List as L
import Data.Maybe
import Data.Text
import Data.Vector as V
import GHC.Generics
import Network.HTTP.Conduit
import Text.Pretty.Simple


data Food = Name (String, Text)
 | Stuff (String, Int)
 deriving (Eq, Read, Show, Generic)



someFunc :: IO ()
someFunc = putStrLn "Under Consruction!"


apiKey = "https://api.nal.usda.gov/ndb/search/?format=json&q=chana+dal&sort=n&max=25&offset=0&api_key=jJN9dsKFe507qjLVqbecijtrYB4AiojXVIt1EaEZ"


instance FromJSON Food


unString x = case x of
 String x -> x

unObject x = case x of
 Object x -> x

unArray x = case x of
 Array x -> x


getParsed :: IO ()
getParsed = do
 x <- simpleHttp apiKey
 let y = (unObject . snd . L.head . HS.toList . fromJust) (decode x :: Maybe Object)
 z = (unArray . snd . L.last . L.init) (HS.toList y)
 a = V.map (unpack . unString . snd . L.head . L.drop 5 . HS.toList . unObject) z
 b = V.map (x -> read x :: Int) a
 pPrint b

The way I implemented it is quite messy, but it gets the job done.

It's just that the code looks really bad and almost impossible to read. So I was wondering if there's a better way of doing that.

Define the types you'd like to deserialise into, for example:

-# LANGUAGE DeriveGeneric #-
module Q53251928 where

import Data.Aeson
import GHC.Generics

data Item = Item ndbno :: String deriving (Eq, Show, Generic)
instance FromJSON Item

data FoodStuff = FoodStuff item :: [Item] deriving (Eq, Show, Generic)
instance FromJSON FoodStuff

data RootJSON = RootJSON list :: FoodStuff deriving (Eq, Show, Generic)
instance FromJSON RootJSON

These types are the minimal types you'll need to access all the ndbno values. You can add other labels to the types to capture other data, e.g. group, start, end, etc.

I downloaded the file in the link and attempted to decode it from GHCi, which worked fine:

*Q53251928 Q53251928> decodeFileStrict "download.json" :: IO (Maybe RootJSON)
Just (RootJSON list = FoodStuff item = [Item ndbno = "45044170",
Item ndbno = "45044169",Item ndbno = "45287938",Item ndbno = "45314045",
Item ndbno = "45036851",Item ndbno = "45036824",Item ndbno = "45198766",
Item ndbno = "45270526",Item ndbno = "45270516",Item ndbno = "45374956",
Item ndbno = "45215370",Item ndbno = "45225694",Item ndbno = "45225696",
Item ndbno = "45217133",Item ndbno = "45053881",Item ndbno = "45270521",
Item ndbno = "45226407",Item ndbno = "45218344",Item ndbno = "45225703",
Item ndbno = "45335760",Item ndbno = "45221588",Item ndbno = "45193027",
Item ndbno = "45194338",Item ndbno = "45194331",Item ndbno = "45256801"])