OCaml JSON Schema

A JSON Schema validator for OCaml supporting drafts 4, 6, 7, 2019-09, and 2020-12.

Installation

opam install jsonschema

Usage

High-level API

open Jsonschema

(* Validate JSON string against schema string *)
let schema = {|{"type": "object", "required": ["name"]}|}
let json = {|{"name": "John", "age": 30}|}

match validate_strings ~schema ~json with
| Ok () -> print_endline "Valid!"
| Error err -> print_endline (Validation_error.to_string err)

(* Validate from files *)
let json = Yojson.Basic.from_file "data.json" in
match validate_file ~schema:"schema.json" json with
| Ok () -> print_endline "Valid!"
| Error err -> print_endline (Validation_error.to_string err)

(* Create a reusable validator *)
match create_validator "schema.json" with
| Ok validator ->
    let json = Yojson.Basic.from_file "data.json" in
    (match validate validator json with
     | Ok () -> print_endline "Valid!"
     | Error err -> print_endline (Validation_error.to_string err))
| Error err -> pp_compile_error Format.std_formatter err

JSON Pointer

let json = `Assoc [("foo", `Assoc [("bar", `Int 42)])] in
match Json_pointer.of_string "/foo/bar" with
| Ok ptr ->
    (match Json_pointer.lookup ptr json with
     | Some (`Int n) -> Printf.printf "Found: %d\n" n
     | _ -> print_endline "Not found")
| Error e -> Printf.printf "Invalid pointer: %s\n" e

Format Validators

The library includes validators for common formats:

• email, idn_email
• date, time, date_time
• ipv4, ipv6
• hostname, idn_hostname
• uri, iri, uri_reference, iri_reference
• uuid
• json_pointer, relative_json_pointer
• regex

Implementation Status

Test Coverage by Draft Version

• Draft 4: 153/159 test groups (96%)
• Draft 6: 209/231 test groups (90%)
• Draft 7: 228/254 test groups (89%)
• Draft 2019-09: 309/361 test groups (85%)
• Draft 2020-12: 309/368 test groups (83%)

Core Features (Fully Implemented)

• ✅ Type validation (null, boolean, number, integer, string, array, object)
• ✅ Basic constraints (enum, const, type)
• ✅ Numeric constraints (minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf)
• ✅ String constraints (minLength, maxLength, pattern)
• ✅ Array constraints (minItems, maxItems, uniqueItems)
• ✅ Object constraints (required, minProperties, maxProperties)
• ✅ Format validation (with extensible format registry)
• ✅ JSON Pointer (RFC 6901)
• ✅ Error reporting with instance locations
• ✅ Boolean schemas (true/false)
• ✅ Empty schema validation

Schema Composition (Partially Implemented)

• ✅ allOf - validates against all schemas
• ✅ anyOf - validates against at least one schema
• ✅ oneOf - validates against exactly one schema
• ✅ not - validates if schema does not match
• ✅ if/then/else - conditional validation

Object Validation (Fully Implemented)

• ✅ properties - validates specific properties
• ✅ patternProperties - validates properties matching regex patterns
• ✅ additionalProperties - validates/restricts additional properties
• ✅ propertyNames - validates property name format
• ✅ dependencies - property and schema dependencies (draft 4-7)
• ✅ dependentRequired - property dependencies (draft 2019-09+)
• ✅ dependentSchemas - schema dependencies (draft 2019-09+)

Array Validation (Fully Implemented)

• ✅ items - validates array items (uniform or tuple validation)
• ✅ additionalItems - validates/restricts additional items beyond tuple
• ✅ prefixItems - validates initial items (draft 2020-12)
• ✅ contains - validates at least one item matches
• ✅ minContains/maxContains - number of items matching contains

References and Schema Identification

• ✅ $ref - basic JSON pointer and fragment resolution
• ✅ $ref with sibling keywords - validates both $ref and siblings
• ✅ $id - schema identification and base URI changes
• ✅ $anchor/$dynamicAnchor - schema anchors for references
• ✅ $recursiveRef/$dynamicRef - basic resolution (full dynamic scope pending)
• ⚠️ Remote schema loading - basic file:// support only

Advanced Validation

• ✅ unevaluatedProperties - validates properties not covered by other keywords
• ✅ unevaluatedItems - validates items not covered by other keywords
• ⚠️ Vocabulary support - basic structure, not fully implemented

Content Validation

• ✅ contentEncoding (base64)
• ✅ contentMediaType (application/json)
• ✅ contentSchema - validates decoded content against schema

Output Formats

• ✅ Flag output - simple boolean result
• ✅ Basic output - error with instance location
• ✅ Detailed output - includes schema location and error details

Known Limitations

• Remote schema loading only supports file:// URLs
• Dynamic scope resolution for $recursiveRef/$dynamicRef is simplified
• Some complex interactions between unevaluatedProperties/Items and $ref
• Vocabulary support is minimal

License

ocaml-jsonschema is available under the ISC License.