Question

I’ve been working on a very small program to grab details about Half Life 2 servers (using the protocol-srcds library). The workflow is pretty straightforward; it takes a list of servers from a file, queries each of them, and writes the output out to another file (which is read by a PHP script for display, as I’m tied to vBulletin). Would be nice if it was done in SQL or something, but seeing as I’m still just learning, that’s a step too far for now!

Anyway, my question relates to serialization, namely, serializing to JSON. For now, I’ve written a scrappy helper function jsonify, such that:

jsonify (Just (SRCDS.GameServerInfo serverVersion
                           serverName   
                           serverMap    
                           serverMod    
                           serverModDesc
                           serverAppId  
                           serverPlayers
                           serverMaxPlayers 
                           serverBots
                           serverType
                           serverOS  
                           serverPassword
                           serverSecure  
                           serverGameVersioning)) = 
                            toJSObject [ ("serverName", serverName)
                                       , ("serverMap", serverMap)
                                       , ("serverPlayers", show serverPlayers)
                                       , ("serverMaxPlayers", show serverMaxPlayers) ]

(I’m using the Text.JSON package). This is obviously not ideal. At this stage, however, I don’t understand using instances to define serializers for records, and my attempts to do so met a wall of frustration in the type system.

Could someone please walk me through the “correct” way of doing this? How would I go about defining an instance that serializes the record? What functions should I use in the instance (showJSON?).

Thanks in advance for any help.

Answer 1

You might want to consider using Data.Aeson instead which might be regarded as the successor to Text.JSON.

With aeson you define separate instances for serialize/deserializing (with Text.JSON you have to define both directions even if you need only one, otherwise the compile will annoy you — unless you silence the warning somehow), and it provides a few operators making defining instances a bit more compact, e.g. the example from @hammar’s answer can be written a little bit less noisy as shown below with the aeson API:

instance ToJSON SRCDS.GameServerInfo where
    toJSON (SRCDS.GameServerInfo {..}) = object
        [ "serverName"       .= serverName
        , "serverMap"        .= serverMap
        , "serverPlayers"    .= serverPlayers
        , "serverMaxPlayers" .= serverMaxPlayers
        ]