Implementation of JSON schema for pharo
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This is an implementation of JSON Schema for the pharo language. It is used to define the structure and values of a JSON string and to validate it. The schema itself can be externalized for being consumed by a third party.

DISCLAIMER: This is not a full implementation of the json schema spec. Basic types and constraints should work. If you need support for something not implemented you are invited to provide a pull request for it. If you cannot develop it you sill can open a ticket in this repository

It can be loaded by downloading it in pharo via

  Metacello new
    repository: 'github://zweidenker/JSONSchema';
    baseline: #JSONSchema;

Defining a schema

These are the expression to create a schema model inside pharo.

schema := {
  #name -> JSONSchema string.
  #dateAndTime -> (JSONSchema stringWithFormat: 'date-time').
  #numberOfPets -> JSONSchema number } asJSONSchema.

defines as schema that can parse the following JSON:

jsonString := '{
  "name" : "John Doe",
  "dateAndTime" : "1970-01-01T14:00:00",
  "numberOfPets" : 3

Reading/Writing a value using a schema

To parse the value from JSON we only need to invoke:

value := schema read: jsonString

The object in value will have name as a string, dateAndTime as a DateAndTime object and numberOfPets as a SmallInteger object.

The schema can also be used to write out the value as JSON. This is especially useful if we want to ensure that only valid JSON is written. For this invoke

jsonString := schema write: value.

Serialize/Materialize a schema

Addtionally to reading and writing objects a schema can be serialized to string.

schemaString := NeoJSONWriter toStringPretty: schema.


	"type" : "object",
	"properties" : {
		"name" : {
			"type" : "string"
		"numberOfPets" : {
			"type" : "number"
		"dateAndTime" : {
			"type" : "string",
			"format" : "date-time"

If we would get a schema as string we can instantiate by invoking

schema := JSONSchema fromString: schemaString.

Nested schemas

Schemas can be nested in any depth. And it can be specified by using the literal Array syntax.

schema := {
  #name -> JSONSchema string.
  #address -> {
    #street -> JSONSchema string.
    #number -> JSONSchema number
  } } asJSONSchema


JSON Schema has a defined set of constraints that can be specified. E.g. for a number the inerval of the value can be specified by

numberSchema := JSONSchema number.
numberSchema interval
  minimum: 1;
  exclusiveMaximum: 100

constraining the number value to be greater or equal to 1 and smaller than 100.