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JSON-MOP

Introduction

JSON-MOP is a small library aiming to cut down time spent moving data between CLOS and JSON objects. It depends on YASON and it should be possible to use it alongside straight calls to functions from YASON.

Quick Start

To use JSON-MOP, define your classes with the class option (:metaclass json-serializable-class). For slots that you want to appear in the JSON representation of your class, add the slot option :json-key with the string to use as the attribute name. The option :json-type defaults to :any, but you can control how each slot value is transformed to and from JSON with one of the following:

JSON type specifiers

Type	Remarks
`:any`	Guesses the way to encode and decode the value
`:string`	Enforces a string value
`:number`	Enforces a number value
`:integer`	Enforces an integer value
`:hash-table`	Enforces a hash table value
`:vector`	Enforces a vector value
`:list`	Enforces a list value
`:bool`	Maps `T` and `NIL` with `true` and `false`
`<symbol>`	Uses a `(:metaclass json-serializable-class)` class definition to direct the transformation of the value

Homogeneous sequences and objects

In addition, the type specifier may be a list of two elements, first element is one of :list, :vector, :hash-table; the second is any JSON type specifier that is to be applied to the elements of the list or the values of the hash-table.

NIL and null semantics

JSON null is treated as an unbound slot in CLOS. Unbound slots are ignored when encoding objects, unless *encode-unbound-slots* is bound to T, in which case they are represented as JSON null.

Slots bound to NIL with JSON types other :bool will signal an error, but this may change in the future.

Encoding and decoding JSON

Turning an object into JSON is done with the yason:encode generic function. Turning it back into an object is slightly more involved, using json-to-clos on a stream, string or hash table; a class name; and optional initargs for the class. Values decoded from the JSON will override values specified in the initargs.

Example

First, define your classes:

(defclass book ()
  ((title :initarg :title
          :json-type :string
          :json-key "title")
   (published-year :initarg :year
         :json-type :number
         :json-key "year_published")
   (fiction :initarg :fiction
            :json-type :bool
            :json-key "is_fiction"))
  (:metaclass json-serializable-class))

(defclass author ()
  ((name :initarg :name
         :json-type :string
         :json-key "name")
   (birth-year :initarg :year
               :json-type :number
               :json-key "year_birth")
   (bibliography :initarg :bibliography
                 :json-type (:list book)
                 :json-key "bibliography"))
  (:metaclass json-serializable-class))

Let's try creating an instance:

(defparameter *author*
  (make-instance 'author
                 :name "Mark Twain"
                 :year 1835
                 :bibliography
                 (list
                  (make-instance 'book
                                 :title "The Gilded Age: A Tale of Today"
                                 :year 1873
                                 :fiction t)
                  (make-instance 'book
                                 :title "Life on the Mississippi"
                                 :year 1883
                                 :fiction nil)
                  (make-instance 'book
                                 :title "Adventures of Huckleberry Finn"
                                 :year 1884
                                 :fiction t))))

To turn it into JSON, encode it:

(encode *author*)

This will print the following:

{"name":"Mark Twain","year_birth":1835,"bibliography":[{"title":"The Gilded Age: A Tale of Today","year_published":1873,"is_fiction":true},{"title":"Life on the Mississippi","year_published":1883,"is_fiction":false},{"title":"Adventures of Huckleberry Finn","year_published":1884,"is_fiction":true}]}

The same can be turned back into a CLOS object with (json-to-clos input 'author)

Licence

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.