package validate

  1. Overview
  2. Docs
OCaml library enabling efficient data validation through PPX derivers and a suite of annotation-based validators


Dune Dependency






Validate is an OCaml library that focuses on data validation using PPX derivers and a range of annotations for different data types. It allows developers to apply annotations for various validation rules, such as string length, numeric values, and format constraints like URLs and UUIDs. This functionality makes it suitable for a wide array of applications in OCaml development where data integrity is crucial.


validation ppx

Published: 01 Feb 2024




validate is an OCaml library designed to streamline the process of validating records, lists, or values. It primarily operates through a PPX deriver that automatically generates validators using annotations, utilizing an underlying library of helper validation functions.


  • OCaml version 5.0 or higher.


Installing validate

To install the validate library, use OPAM with the following command:

opam install validate

After installation, you need to add validate as a library in your project's dune file and specify validate.ppx_derive_validate as a preprocessor. Here is an example of how to set up the dune file:

  (name something)
  (preprocess (pps validate.ppx_derive_validate))
  (libraries validate))

Annotations and Usage

The validate library in OCaml allows for precise validation of data through a variety of annotations. For each type, the library generates a function named validate_[type_name] which can be used to perform the validation.

Example Usage

Validating record
type test_record = {
  min : string; [@min_length 2]
  max : string; [@max_length 5]
  numeric_list : (int [@less_than 10]) list; [@min_length 2] 
[@@deriving validate, show, eq]

let example_record = { min = "ab"; max = "hello"; numeric_list = [1, 2, 3] }
let validation_result = validate_test_record example_record

In this example:

  • min is a string field validated for a minimum length of 2.

  • max is validated for a maximum length of 5.

  • numeric_list is an integer list, validated for a minimum length of 2, and each element is validated to be less than 10.

Validating simple types
type list_type = ((string[@min_length 1]) list[@min_length 2]) [@@deriving validate]

let example_list = ["a"; "bc"]
let validation_result = validate_list_type example_list

In this example, validate_list_type function will validate that each string in the list has a minimum length of 1 and that the list itself has a minimum length of 2.

Validating tuples
type tuple_type = (string[@email]) * (int[@greater_than 1]) [@@deriving validate]

let example_tuple = ("", 2)
let validation_result = validate_tuple_type example_tuple

Here, the validate_tuple_type function ensures the first element of the tuple is a valid email address and the second element is an integer greater than 1.

Validating tuples

Variants in OCaml can also be validated using validate. Here's an example of how to use annotations with variants:

type tuple_variant =
  | EmailToId of (string[@email]) * (int[@greater_than_or_equal 0])
  | Email of (string[@email])
  | Profile of {
    username : string [@min_length 3];
    email : string [@email];
[@@deriving validate]

(* Example usage *)
let email_to_id_variant = EmailToId ("", 0)
let validation_result = validate_tuple_variant email_to_id_variant

In this example:

  • EmailToId is a variant that takes a tuple. The first element is validated as an email, and the second as an integer greater than or equal to 0.

  • Email is a single-element variant validated as an email.

  • Profile is a record variant with username validated for a minimum length of 3, and email validated as a valid email address.

Validating Recursive Types

validate also supports recursive types, allowing for the validation of nested, self-referential data structures. Here is an example demonstrating the validation of a recursive type representing a binary tree:

type tree =
  | Leaf of (int[@greater_than 0])
  | Node of { left : tree; [@dive] right : (tree[@dive]) }
[@@deriving validate, show, eq]

(* Example usage *)
let my_tree = Node { left = Leaf 1; right = Leaf 2 }
let validation_result = validate_tree my_tree

In this example:

  • Leaf is a variant that takes an integer, validated to be greater than 0.

  • Node is a variant representing a binary tree node with left and right branches, both of which are recursively validated as tree instances.

Validating Circular Recursive Types

validate also handles circular recursive types, which are useful for defining structures where two types refer to each other recursively. This feature is particularly useful for complex data models. Here's an example:

type a = { 
  a_id : int; [@greater_than 0] 
  b : (b[@dive]) option 
[@@deriving validate, show, eq]

and b = { 
  b_id : int; [@greater_than 0] 
  a : (a[@dive]) option 
[@@deriving validate, show, eq]

(* Example usage *)
let rec a_instance = { a_id = 1; b = Some { b_id = 2; a = Some a_instance } }
let validation_result_a = validate_a a_instance

let rec b_instance = { b_id = 1; a = Some { a_id = 2; b = Some b_instance } }
let validation_result_b = validate_b b_instance

In this example:

  • Type a has an integer field a_id validated to be greater than 0, and an optional field b of type b.

  • Type b similarly has a b_id field and an optional field a of type a.

  • Both types use the [@dive] annotation to indicate recursive validation within their optional fields.

Categorized Annotations

String/List Annotations

  • @min_length: Validates minimum length of a string/list.

  • @max_length: Validates maximum length of a string/list.

  • @length_equals: Validates length of a string/list.

String Annotations

  • @url: Checks if a string is a valid URL.

  • @uuid: Validates a string as a UUID.

  • @numeric: Ensures a string contains only numeric characters.

  • @alpha: Checks for alphabetic characters in a string.

  • @alphanumeric: Requires a string to be alphanumeric.

  • @lowercase: Validates a string to be in lowercase.

  • @uppercase: Ensures a string is in uppercase.

  • @lowercase_alphanumeric: Validates a lowercase alphanumeric string.

  • @uppercase_alphanumeric: Validates an uppercase alphanumeric string.

  • @email: Checks if a string is a valid email.

  • @regex: Checks if string matches given regex. Note: Under the hood it uses ocaml-re which does not support back-references and look-ahead/look-behind assertions.

  • @ulid: Ensures a string is a valid Universally Unique Lexicographically Sortable Identifier (ULID).

  • @ipv4: Validates that a string is a valid IPv4 address.

  • @ipv6: Ensures a string is a valid IPv6 address.

  • @phone: Validates that a string conforms to the E.164 international phone number format.

  • @mac_address: Ensures a string is a valid MAC address.

Integer/Float Annotations

  • @less_than: Validates an integer/float to be less than a specified value.

  • @less_than_or_equal: Validates an integer/float to be less than or equal to a specified value.

  • @greater_than: Validates an integer/float to be greater than a specified value.

  • @greater_than_or_equal: Validates an integer/float to be greater than or equal to a specified value.

  • @equal_to: Validates an integer/float to be equal to a specified value.

  • @not_equal_to: Validates an integer/float to not be equal to a specified value.

Annotations for Other Types

  • @dive: Used for nested record validation, allowing validation of each element within a composite structure like records or lists.

Option Type Annotations

  • @some: Ensures that an option type is Some, indicating a value is present.

  • @none: Ensures that an option type is None, indicating no value is present.

Advanced Annotations

  • @custom: This annotation allows you to define custom validation logic. You provide a function that takes one argument and returns a result, which is either Ok () for successful validation or Error validation_error for a validation failure.

let custom_validator str =
  if String.length str > 1 then Ok ()
  else Error (Validate.BaseError { code = "custom_validator"; params = [] })

type custom_validator_record = {
  custom_validator : string; [@custom custom_validator]
  custom_inline_validator : int;
        fun i ->
          if i > 1 then Ok ()
              (Validate.BaseError { code = "custom_validator"; params = [] })]
[@@deriving validate]
  • @ignore_if: This annotation accepts a function that takes the type itself as an argument and returns a boolean. If the function returns true, other validators for the field are ignored.

type cond_record = {
  unit : string;
  temperature : int; [@greater_than_or_equal 0] [@ignore_if fun r -> r.unit <> "K"]
[@@deriving validate]
  • @some_if and @none_if: These annotations are applicable only to option types. @some_if requires the option to be Some if the provided function returns true, and @none_if requires it to be None under the specified condition.

type username_or_email = {
  username : string option; [@some_if fun r -> = None]
  email : string option; [@none_if fun r -> Option.is_some r.username]
[@@deriving validate]

Error Handling

In validate, the validation function returns a result type, which includes an Ok value equal to the input parameter, or an error parameter. The error types are defined as follows:

  • base_validation_error: Represents a basic validation error with a code and a list of parameters.

type base_validation_error = { 
  code : string; 
  params : (string * string) list 
  • keyed_validation_errors: Represents errors associated with specific keys e.g. for records key equal to field name and for tuples key is position in tuple.

type keyed_validation_errors = string * validation_error list
  • index_validation_error: Represents errors indexed by an integer, typically for list validations.

type index_validation_error = int * validation_error list
  • validation_error: The main error type which can be one of the following:

    • BaseError: Basic validation error.

    • KeyedError: Errors associated with specific keys.

    • IterableError: Errors in iterable structures like lists.

    • GroupError: A group of nested validation errors.

type validation_error =
  | BaseError of base_validation_error
  | KeyedError of keyed_validation_errors list
  | IterableError of index_validation_error list
  | GroupError of validation_error list


Contributions to validate are warmly welcomed and appreciated. Whether it's opening issues for bug reports, suggesting new features, or submitting pull requests, all forms of contribution help in making validate better.

Dependencies (5)

  1. uri
  2. re
  3. ppx_deriving
  4. dune >= "3.12"
  5. ocaml >= "5.0.0"

Dev Dependencies (3)

  1. odoc with-doc
  2. bisect_ppx with-test
  3. alcotest with-test

Used by





Innovation. Community. Security.