Parsing JSON

If you've ever used the built in Dart SDK jsonDecode function, you'll know that your JSON (de)serialization code becomes littered with the dreaded dynamic type. Dynamic bring along with a lot of responsibility for the developer to check what type of data exactly they're dealing with. Then you also have to deal with the potential thrown exception, adding yet another responsibility which could instead be encoded as a type.

By contrast Ribs uses it's own parser (heavily derived from Scala's jawn), to build typed JSON data! No more type checks or casts using is or as which pollute your code and reduce the readability.

Let's begin with a very simple example to get aquainted with the Ribs JSON API:

Simple Parse

final Either<ParsingFailure, Json> json =
    Json.parse('[ null, 1, true, "hi!", { "distance": 3.14 } ]');

You can see that we can simply pass a string to the Json.parse function and get our value back (emphasis on value because an exception will never be raised!). With our value, it's easy to check if the parse succeeded or failed from the provided JSON string. The developer will need to explicitly handle a failure because the Either type dictates it!

Handling Potentially Invalid JSON

json.fold(
  (err) => notifyUserOfError(err),
  (json) => proceedToUseValidJson(json),
);

Assuming that we've got a valid string to parse, as we do in this example, what exactly is the Json type and what do we do with it? Json is a sealed class that can be one of a few possible types:

• JNull
• JBoolean
• JNumber
• JString
• JArray
• JObject

These represent all the possible types of JSON as defined by the spec. To confirm this, print out the result of parsing the string from above:

Typed JSON

void printIt() {
  print(json);
  // Right(JArray([JNull, JNumber(1), JBoolean(true), JString("hi!"), JObject({ "distance": JNumber(3.14) })]))
}

You can see that each element of the top-level array has successfully been parsed and has the proper type.

Naturally if we feed an invalid JSON string into the parser, we'll get an error. For illustrative purposes:

Typed JSON

// Removed the ',' between 1 and true...
final badJson = Json.parse('[ null, 1 true, "hi!", { "distance": 3.14 } ]');

// Left(ParsingFailure(ParseException: expected ] or , got 'true, ... (line 1, column 11) [index: 10, line: 1, col: 11]))