EventType.hs

{-- This instance does a lot with a small amount of code so this is a
short explanation of what is going on. Once we know we have a JSON
Object the next thing to determine before more parsing is the type of
event we are dealing with. To determine this we use some handy Lens
helpers for dealing with JSON from Data.Aeson.Lens to traverse the
document to where the type should be and return a `Maybe Text`. We can
then pass this result right to the `toEventType` function using the
Monad bind operator. If all goes well we now have the information to
proceed with parsing the event and if something has gone wrong we have
`Nothing`. We use `fmap` to apply `parseJSON v` to the `eventType`
result. This is done inside a call to `maybe` to avoid having to write
a `if` or `case` statement to handle the possible states of
`eventType`.  --}
instance FromJSON EventType where
    parseJSON v@(Object _) =
        maybe (fail "Unknown event type") (flip fmap (parseJSON v)) eventType
            where typeField x = x ^? _Value.key "contents"._Value.key "type"._String
                  eventType = typeField v >>= toEventType
    parseJSON _ = fail "Invalid event format"

toEventType :: T.Text -> Maybe (ElementDetails -> EventType)
toEventType "element_connected" =  Just ElementConnect
toEventType "element_connection_failure" = Just ElementConnectFail
toEventType _ = Nothing