gistfile1.hs

parseInvoices :: L.ByteString -> Either String [Invoice]
parseInvoices s = do
    result <- eitherDecode s
    flip parseEither result $ \obj -> do
        d <- (obj .: "invoices") >>= (.: "invoice")
        case d of
            Object v -> return [v]
            Array v  -> return v

The structure of the json looks like this:

{"invoices":
     {"invoice": {...} or [{...}] }
}

where "invoice" will be a Array of Objects if there are more than 1 entry, or a single Object if there is only one.

Solution:

import qualified Data.Vector as V

parseObjOrArray :: (FromJSON a) => Value -> Parser a
parseObjOrArray obj = case obj of
               ob@(Object v) -> parseJSON $ Array (V.singleton ob)
               arr@(Array v) -> parseJSON arr
               _ -> error "Expected {} or []"

Use like this:

parseInvoices :: L.ByteString -> Either String [Invoice]
parseInvoices s = do
    result <- eitherDecode s
    flip parseEither result $ \obj -> (obj .: "invoices") >>= (.: "invoice") >>= parseObjOrArray