`json_schema`

The json_schema library provides predicates for parsing JSON Schema documents and validating JSON data against schemas. It is based on the JSON Schema specification found at:

https://json-schema.org/

This library builds on top of the json library for JSON parsing and uses the same representation choices for JSON data.

API documentation

Open the ../../apis/library_index.html#json_schema link in a web browser.

Loading

To load all entities in this library, load the loader.lgt file:

| ?- logtalk_load(json_schema(loader)).

Testing

To test this library predicates, load the tester.lgt file:

| ?- logtalk_load(json_schema(tester)).

Schema Parsing

Schemas can be parsed from various sources using the parse/2 predicate:

| ?- json_schema::parse(file('schema.json'), Schema).

| ?- json_schema::parse(atom('{"type": "string"}'), Schema).

| ?- json_schema::parse(codes(Codes), Schema).

Validation

JSON data can be validated against a schema using the validate/2 or validate/3 predicates:

% Success/failure validation
| ?- json_schema::parse(atom('{"type": "string"}'), Schema),
     json_schema::validate(Schema, hello).
yes

| ?- json_schema::parse(atom('{"type": "string"}'), Schema),
     json_schema::validate(Schema, 42).
no

% Validation with error collection
| ?- json_schema::parse(atom('{"type": "string"}'), Schema),
     json_schema::validate(Schema, 42, Errors).
Errors = [error([], expected_type(string))]

Supported Keywords

The library supports the following JSON Schema keywords:

Type validation:

type (string, number, integer, boolean, array, object, null)
Multiple types: {"type": ["string", "number"]}

Generic validation:

enum - value must be one of the specified values
const - value must be exactly the specified value

String validation:

minLength - minimum string length
maxLength - maximum string length

Numeric validation:

minimum - minimum value (inclusive)
maximum - maximum value (inclusive)
exclusiveMinimum - minimum value (exclusive)
exclusiveMaximum - maximum value (exclusive)
multipleOf - value must be a multiple of this number

Array validation:

items - schema for array items
prefixItems - schemas for array items by position
minItems - minimum number of items
maxItems - maximum number of items
uniqueItems - all items must be unique
contains - at least one item must match the schema

Object validation:

properties - schemas for object properties
required - list of required property names
minProperties - minimum number of properties
maxProperties - maximum number of properties
propertyNames - schema for property names

Composition:

allOf - value must match all schemas
anyOf - value must match at least one schema
oneOf - value must match exactly one schema
not - value must not match the schema

Conditional:

if/then/else - conditional schema application

Schema References:

$defs/definitions - schema definitions
$ref - local JSON Pointer references (e.g., #/$defs/name)

Format validation:

The format keyword validates string formats. Non-string values pass format validation. Unknown formats are ignored per JSON Schema specification. Supported formats:

email - basic email format (local@domain)
date - ISO 8601 date (YYYY-MM-DD)
time - ISO 8601 time (HH:MM:SS) with optional fractional seconds and timezone
date-time - ISO 8601 combined date and time
uri - URI with scheme (e.g., http://example.com)
uri-reference - URI or relative reference
ipv4 - IPv4 address (dotted decimal notation)
ipv6 - IPv6 address (hex groups with colons, supports :: compression)
uuid - UUID format (8-4-4-4-12 hex pattern)

Boolean schemas:

true - accepts any value
false - rejects all values

Known Limitations

The following JSON Schema features are not yet supported:

pattern - regular expression validation (requires regex support)
patternProperties - property matching by pattern
format - formats not listed above (e.g., hostname, regex, json-pointer)
$ref with remote URLs - external schema references require HTTP support

json_schema