I have a Haskell query function to get latest token price using
https://coinmarketcap.com/api/documentation/v1/#operation/getV1CryptocurrencyQuotesLatest
The function takes token id as arg, say 2010 for ADA.
import Data.Aeson
import Network.HTTP.Req
newtype Rate = Rate Double
query :: Int -> IO (Either Text Rate)
query tokenId =
let
url = https queryPrefix /: "v1" /: "cryptocurrency" /: "quotes" /: "latest"
idParam = "id" =: tokenId
options = standardHeader <> idParam
in
runReq defaultHttpConfig $ do
v <- req GET url NoReqBody jsonResponse options
let responseCode = responseStatusCode v
if isValidHttpResponse responseCode then do
case fromJSON $ responseBody v of
Success x -> pure $ Right x
Error e -> pure $ Left $ pack $ "Error decoding state: " <> e
else
pure $ Left $ pack ("Error with CoinMarketCap query 'Quotes Latest': " <> show responseCode <> ". " <> show (responseStatusMessage v))
The Json output though has "2010" as a key:
{"status":
{"timestamp":"2021-10-24T03:35:01.583Z","error_code":0,"error_message":null,"elapsed":163,"credit_count":1,"notice":null}
,"data":
{"2010":
{"id":2010
,"name":"Cardano"
,"symbol":"ADA"
,"slug":"cardano"
,"num_market_pairs":302,"date_added":"2017-10-01T00:00:00.000Z"
,"tags":["mineable","dpos","pos","platform","research","smart-contracts","staking","binance-smart-chain","cardano-ecosystem"]
,"max_supply":45000000000
,"circulating_supply":32904527668.666
,"total_supply":33250650235.236,"is_active":1
,"platform":null
,"cmc_rank":4
,"is_fiat":0
,"last_updated":"2021-10-24T03:33:31.000Z"
,"quote":
{"USD":
{"price":2.16109553945978
,"volume_24h":2048006882.386299
,"volume_change_24h":-24.06,"percent_change_1h":0.24896227
,"percent_change_24h":0.38920394
,"percent_change_7d":-0.97094597
,"percent_change_30d":-6.13245906
,"percent_change_60d":-21.94246757
,"percent_change_90d":63.56901345
,"market_cap":71109827972.785
,"market_cap_dominance":2.7813
,"fully_diluted_market_cap":97249299275.69,"last_updated":"2021-10-24T03:33:31.000Z"}}}}}
Being that 2010 is an arg to query, I clearly do not want to drill in as data.2010.quote.USD.price with something like this:
instance FromJSON Rate where
parseJSON = withObject "Rate" $ \o -> do
dataO <- o .: "data"
_2010O <- dataO .: "2010" -- #############
quoteO <- _2010O .: "quote"
usdO <- quoteO .: "USD"
price <- usdO .: "price"
pure $ Rate price
Question: How can I achieve the flexibility I want? Can I somehow pass in the token id to parseJSON? Or is there perhaps a Lens-Aeson technique to use a wildcard? ...
I you are completely sure that the object inside "data" will only ever have a single key, we can take the object, convert it into a list of values, fail if the list is empty or has more than one value, and otherwise continue parsing. Like this:
instance FromJSON Rate where
parseJSON = withObject "Rate" $ \o -> do
Object dataO <- o .: "data" -- we expect an Object
-- get the single value, it should be an Object itself
[Object _2010O] <- pure $ Data.Foldable.toList dataO
quoteO <- _2010O .: "quote"
usdO <- quoteO .: "USD"
price <- usdO .: "price"
pure $ Rate price
When there's no key, more than one key, or the value is not an aeson Object, the pattern [Object _2010O] <- fails to match and gives an parsing error through the MonadFail instance of aeson's Parser.
We could also be a bit more explicit:
instance FromJSON Rate where
parseJSON = withObject "Rate" $ \o -> do
Object dataO <- o .: "data"
let objects = Data.Foldable.toList dataO
case objects of
[Object _2010O] -> do
quoteO <- _2010O .: "quote"
usdO <- quoteO .: "USD"
price <- usdO .: "price"
pure $ Rate price
[_] -> fail "value is not Object"
_ -> fail "zero or more than one key"