Nrthugu

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"}]}})