Smile - JSON

JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate. JSON is a text format that is completely language independent but uses conventions that are familiar to programmers of the C-family of languages,

JSON has only types of string, number, boolean, object, array, and null. This library includes additional types such as date, int, long, double, counter, binary, UUID, ObjectId (as in BSON), etc.

It is very easy to parse a JSON object:

val doc =
  json"""
  {
    "store": {
      "book": [
        {
          "category": "reference",
          "author": "Nigel Rees",
          "title": "Sayings of the Century",
          "price": 8.95
        },
        {
          "category": "fiction",
          "author": "Evelyn Waugh",
          "title": "Sword of Honour",
          "price": 12.99
        },
        {
          "category": "fiction",
          "author": "Herman Melville",
          "title": "Moby Dick",
          "isbn": "0-553-21311-3",
          "price": 8.99
        },
        {
          "category": "fiction",
          "author": "J. R. R. Tolkien",
          "title": "The Lord of the Rings",
          "isbn": "0-395-19395-8",
          "price": 22.99
        }
      ],
      "bicycle": {
        "color": "red",
        "price": 19.95
      }
    }
  }
  """

The interpolator json parse a string to JsObject. To parse an array, use the interpolator jsan to JsArray. It is also okay to embed variable references directly in processed string literals.

val x = 1
json"""
  {
    "x": $x
  }
"""

If the string is not a JSON object but any other valid JSON expression, one may use parseJson method to convert the string to a JsValue.

"1".parseJson

The json interpolator can only be applied to string literals. If you want to parse a string variable, the parseJson method can always be employed. If you know the string contains a JSON object, you may also use the method parseJsObject.

val s = """{"x":1}"""
s.parseJsObject

To serialize a JSON value (of type JsValue) in compact mode, you can just use toString. To pretty print, use the method prettyPrint.

doc.toString
doc.prettyPrint

With a JsObject or JsArray, you can refer to the individual elements with a variation of array syntax, like this:

doc("store")("bicycle")("color")
doc("store")("book")(0)("author")

Note that we follow Scala's array access convention by () rather than [] in JavaScript.

Besides, you can use the dot notation to access its fields/elements just like in JavaScript:

doc.store.bicycle.color
doc.store.book(0).author

It is worth noting that we didn't define the type/schema of the document while Scala is a strong type language. In other words, we have both the type safe features of strong type language and the flexibility of dynamic language in this JSON library.

If you try to access a non-exist field, JsUndefined is returned.

scala> doc.book
res11: unicorn.json.JsValue = undefined

Although there are already several nice JSON libraries for Scala, the JSON objects are immutable by design, which is a natural choice for a functional language. For database, however, data mutation is necessary. Therefore, JsObject and JsArray are mutable data structures. You can set/add a field just like in JavaScript:

json.store.bicycle.color = "green"

To delete a field from JsObject, use remove method:

doc.store.book(0) remove "price"

It is same as setting it JsUndefined:

doc.store.book(0).price = `JsUndefined`

To delete an element from JsArray, the remove method will effectively remove it from the array. However, setting an element to undefined doesn't reduce the array size.

// delete the first element and array size is smaller
doc.store.book.remove(0)
// set the first element to undefined but array size keeps same
doc.store.book(0) = JsUndefined

It is also possible to append an element or another array to JsArray:

val a = JsArray(1, 2, 3, 4)
a += 5

a ++= JsArray(5, 6)

Common iterative operations such as foreach, map, reduce can be applied to JsArray too.

doc.store.book.asInstanceOf[JsArray].foreach { book =>
 println(book.price)
}

Because Scala is a static language, it is impossible to know doc.store.book is an array at compile time. So it is typed as generic JsValue, which is the parent type of specific JSON data types. Therefore, we use asInstanceOf[JsArray] to convert it to JsArray in order to use foreach.