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@jyrimatti jyrimatti/Main.hs
Created Sep 22, 2015

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Aeson example
-- build with cabal file:
-- name: aeson-ex
-- version: 0.1
-- build-type: Simple
--
-- executable aeson_ex
-- hs-source-dirs: src
-- main-is: Main.hs
-- build-depends: base >= 4,
-- aeson,
-- time,
-- bytestring
-- ghc-options: -Wall
-- run with:
-- curl http://data.itsfactory.fi/siriaccess/vm/json | cabal run
{-# LANGUAGE DeriveGeneric #-}
module Main where
import GHC.Generics
import Data.Time.Clock (UTCTime)
import Data.Time.Clock.POSIX
import Data.Aeson
import Data.Aeson.Types (Options(..),defaultOptions)
import Data.Char (toUpper)
import qualified Data.ByteString.Lazy as BS
-- define some domain specific types
data Ref = Ref { _value :: String } deriving (Generic, Show)
data Degrees = Degrees Int deriving (Generic, Show)
data LocalizedString = LocalizedString { lang :: String, value :: String } deriving (Generic, Show)
data Coordinate = Coordinate { latitude :: Degrees, longitude :: Degrees } deriving (Generic, Show)
data Timestamp = Timestamp UTCTime deriving (Generic, Show)
data Duration = Duration String deriving (Generic, Show) -- TODO: actual impl...
-- define the data structure
data Root = Root {
siri :: Siri
} deriving (Generic, Show)
data Siri = Siri {
serviceDelivery :: ServiceDelivery,
version :: String
} deriving (Generic, Show)
data ServiceDelivery = ServiceDelivery {
moreData :: Bool,
producerRef :: Ref,
responseTimestamp :: Timestamp,
status :: Bool,
vehicleMonitoringDelivery :: [VehicleMonitoringDelivery]
} deriving (Generic, Show)
data VehicleMonitoringDelivery = VehicleMonitoringDelivery {
_responseTimestamp :: Timestamp,
_status :: Bool,
vehicleActivity :: [VehicleActivity],
_version :: String
} deriving (Generic, Show)
data VehicleActivity = VehicleActivity {
monitoredVehicleJourney :: MonitoredVehicleJourney,
recordedAtTime :: Timestamp,
validUntilTime :: Timestamp
} deriving (Generic, Show)
data MonitoredVehicleJourney = MonitoredVehicleJourney {
bearing :: Degrees,
delay :: Duration,
destinationName :: LocalizedString,
directionRef :: Ref,
framedVehicleJourneyRef :: FramedVehicleJourneyRef,
lineRef :: Ref,
monitored :: Bool,
operatorRef :: Ref,
originName :: LocalizedString,
vehicleLocation :: Coordinate,
vehicleRef :: Ref
} deriving (Generic, Show)
data FramedVehicleJourneyRef = FramedVehicleJourneyRef {
dataFrameRef :: Ref,
datedVehicleJourneyRef :: String
} deriving (Generic, Show)
-- some hacks to
-- 1) andle equal record field names
-- 2) handle the fact that the ITSFactory Siri-json-format has some capitalized field names
unPrefix = dropWhile (== '_')
capitalize s@"version" = s
capitalize s@"value" = s
capitalize s@"lang" = s
capitalize str = toUpper (head str) : tail str
options = defaultOptions { fieldLabelModifier = capitalize . unPrefix }
-- JSON deserialization definitions
instance FromJSON Ref where parseJSON = genericParseJSON options
instance FromJSON Degrees where parseJSON = genericParseJSON options
instance FromJSON LocalizedString where parseJSON = genericParseJSON options
instance FromJSON Coordinate where parseJSON = genericParseJSON options
instance FromJSON Duration where parseJSON = genericParseJSON options
instance FromJSON Timestamp where parseJSON i = Timestamp . posixSecondsToUTCTime . fromInteger <$> parseJSON i
instance FromJSON Root where parseJSON = genericParseJSON options
instance FromJSON Siri where parseJSON = genericParseJSON options
instance FromJSON ServiceDelivery where parseJSON = genericParseJSON options
instance FromJSON VehicleMonitoringDelivery where parseJSON = genericParseJSON options
instance FromJSON VehicleActivity where parseJSON = genericParseJSON options
instance FromJSON MonitoredVehicleJourney where parseJSON = genericParseJSON options
instance FromJSON FramedVehicleJourneyRef where parseJSON = genericParseJSON options
main = do
jsonData <- BS.getContents
case eitherDecode jsonData of
Left err ->
print err
-- ah, blessed pattern matching...
Right (Root (Siri (ServiceDelivery _ _ _ _ vehicleMonitoringDeliveries) _)) ->
print vehicleRefs
where
getVehicleActivities (VehicleMonitoringDelivery _ _ vehicleActivities _) = vehicleActivities
getVehicleRef (VehicleActivity (MonitoredVehicleJourney _ _ _ _ _ _ _ _ _ _ (Ref vehicleRef)) _ _) = vehicleRef
vehicleRefs = map getVehicleRef (concatMap getVehicleActivities vehicleMonitoringDeliveries)
@jyrimatti

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jyrimatti commented Sep 22, 2015

In a static language:

  • data is really well defined upfront, whereas in JavaScript or Clojure it's more ad-hoc within the application logic.
  • serialization/deserialization is completely separated from the data definitions and the application using the data. Thus you can add e.g. XML or binary serialization without messing with the data definitions.
  • no need to handle low-level data (lists, maps, strings...) when there exists a more structural or "sensible" format, for example with timestamps.
  • and one can always use just maps and lists if it's a simple use case or the app is interested in only a small part of the data.

Perhaps everything is possible with (Typed) Clojure, but is it still idiomatic?

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