Joysen JSON encoding library for Common Lisp

Frode Fjeld <frodevf@gmail.com>

A mechanism for flexible and composable encoding of (nested) lisp values into strings in JSON syntax.

This library operates in one of two modes: Normal or Implicit mode.

Normal (explicit) mode:

Encoding happens by the (lisp) value and a schema as two separate entities, as provided to JOYSEN:ENCODE. The value is any lisp object, and the schema (a list tree structure) describes how that value is mapped to JSON syntax. The lisp value and the schema are traversed in parallell while generating JSON output.

The schema object is a (lisp list) tree structure where the nodes name JSON encoder functions that map a lisp value to a (JSON) string. Each such function takes (implicitly) the lisp value as its first argument, and optionally more parameters. In the schema tree, an encoder function is named either by its symbol (which is applied to a single argument: the lisp value) or it is named by a list (<symbol> <arg>*), in which case the symbol is applied to the lisp value and the provided ARGS. The ARGS can hold further sub-schemas, thus creating the tree structure.

The Joysen library provides encoder functions for typical mappings, e.g. integers (joysen:json-integer), plists to JSON 'objects' (joysen:json-object), lists or vectors to arrays (joysen:json-array), etc. However you can easily provide and use your own encoder functions that map your objects either to calls to Joysen encoder functions or directly to (JSON) strings.

Normal mode is useful for encoding values used otherwise in lisp code, and to give those a JSON representation.

Usage:

(JOYSEN:ENCODE <value> :schema <schema>)

Basic examples:

> (encode 42 :schema 'json-integer) ; basic schema
"42"
> (encode 42 :schema 'json-decimal) ; same value, different schema
"42.00"
> (encode 42 :schema '(json-decimal 5)) ; optional precision parameter
"42.00000"
> (joysen:encode 42) ; A NULL schema denotes a (configurable) mapping...
"42"                 ; ...the default will map integers to JSON-INTEGER.

Composite schema example:

> (defparameter *example* (list :bar 'whatever :foo 42))
> (princ
    (encode *example*
            :schema '(json-object
                      :foo json-integer
                      :bar json-bool)
                      :keyword :camel))

Output (notice ordering wrt. schema):

{
    "foo": 42,
    "bar": true
}

In this example, the plist (:bar whatever :foo 42) is the lisp value that is encoded according to the schema (json-object :foo json-integer :bar json-bool). Notice that the bar property is mapped to a JSON boolean based on the schema, meaning its lisp value is only relevant in terms of its boolean status: Only NIL would yield a JSON output of false.

The schema (and lisp value) can be nested to arbitrary depth. In fact, the schema is a fairly expressive language. For example, this is an example where the schema specifies that a symbol should be encoded as an array of the individual characters in the symbol's name:

> (princ
    (encode 'hello
            :schema '(json-map string (json-array json-string)))

Output:

[
    "H",
    "E",
    "L",
    "L",
    "O"
]

Here, first the symbol HELLO is mapped through the function CL:STRING. Then the array takes a CL:SEQUENCE (i.e. the symbol's name) and applies the json-string encoder to each element.

Implicit mode:

A lisp form is used to construct JSON output directly. There is no explicit 'schema' object, rather the JSON mapping declarations (i.e. function calls) are intertwined with the lisp code that generate the content. That is, the JSON encoder functions are called as you normally would, with no schema traversal. The WITH-IMPLICIT-JSON macro will then prettify the resulting output.

Implicit mode is useful for generating one-off JSON syntax that doesn't correspond to any particular lisp objects/values.

Usage:

(JOYSEN:WITH-IMPLICIT-JSON ([keyword options]*) <forms>)

Example:

> (princ
    (with-implicit-json (:keyword :camel)
      (json-object
        (list :bar (json-bool (getf *example* :bar))
              :foo-zap (getf *example* :foo)))))

Output:

{
    "bar": true,
    "fooZap": 42
}

In this example, the json-object form will output the JSON object according to the layout given by its argument, e.g. the result of the list form.

Encoder functions

defun json-object (plist &rest properties-schema &key &allow-other-keys)

Format plist as a JSON object with properties-schema [<key> <sub-schema>]*, where each key corresponds to a plist indicator and identifies the sub-schema for that object property. Encoding follows properties-schema. Entries in plist but not in properties-schema are ignored. Entries in properties-schema but not in plist are taken as NIL, with two specially-interpreted exceptions for the propertie's schema: json-optional means a missing property is not encoded at all, while json-required means a missing property is an error. A null plist is encoded as "null".

defun json-array (sequence &optional element-schema)

A list or vector of elements with the same element-schema encoded as a JSON array.

defun json-decimal (value &optional (precision 2))

Format value as a decimal with precision.

defun json-integer (value &optional min max)

value is output as a basic integer (in decimal), optinally min and max are checked.."

defun json-string (value)

value is printed into a quoted string. The value NIL designates the empty string.

defun json-dict (plist &optional element-schema)

This is essentially the same as an JSON-OBJECT, except the keys are exact strings, the keys are arbitrary (not to any schema) and all the values are formatted to the optional ELEMENT-SCHEMA.

defun json-bool (value)

True if VALUE is non-NIL, otherwise false.

License

Unlicense