Brr FFI cookbook

This cookbook has a few tips and off-the-shelf design answers to common JavaScript binding scenarios. You are likely in a hurry but if you haven't read the FFI manual yet, it's a good idea to do it before.

Most of the examples in this manual can be cut and pasted directly into the OCaml console.

JavaScript and Web APIs documentation

These are useful references for browser programming:

OCaml identifier convention

Oddly (but luckily) idiomatic OCaml code uses the snake_case identifier convention, not the CamlCase one.

To map JavaScript identifiers to OCaml ones, introduce a _ in front of non initial upper case letters and down case these. If an identifier name clashes with an OCaml keyword, simply prime' it. A few examples:

JavaScript        OCaml
maxTouchPoints    max_touch_points
parseInt          parse_int
FetchEvent        Fetch_event
new               new'
class             class'
type              type'
for               for'

Documentation strings

Make your APIs productive to use: devise a proper doc string and have direct links to the exact functionality you bind to. What you find painful to do now will result in incomensurate time savings in the future, for yourself and anyone using your binding.

For standard APIs link either on MDN which is sometimes incomplete or, if unavailable, on the appropriate standards which are sometimes a bit dry – but better than nothing.


Give me a complete example!

A reasonably sized complete example is the binding to Blob objects via Brr.Blob. It shows how to deal with enums, initalization dictionaries, method calls and promises.

Bindings classes or mixin APIs

The basic pattern to expose an API exposed by a JavaScript class or mixin is to create a module with an abstract type equal to Jv.t and expose its properties and methods in this module.

Since these abstract values may need to be used by other APIs, always provide hidden conversion function to Jv.t. One way of quickly doing this is to use Jv.CONV and Jv.Id as follows:

(** [Object_kind] objects. *)
module Object_kind : sig
  type t
  (** The type for {{:}[ObjectKind]s}. *)
  include Jv.CONV with type t := t
end = struct
  type t = Jv.t
  include (Jv.Id : Jv.CONV with type t := t)

For each property p of the object have a function named p modulo the naming convention to get the property. If the property is mutable have another function set_p to set it – that may seem painful but in practice, at least on web APIs, most object properties are read only.

For each method m of the object have a function of the same name modulo the naming convention to call the method on the object.

See also Create an object of a given class, Deal with initialisation dictionaries and Call a method.

Call a JavaScript function

Look for the function name in the global object (or any another object), construct an array of Jv.t values for the function arguments and call Jv.apply.

The following is a binding to the parseInt JavaScript function. We handle JavaScript exceptions and map them on the OCaml result type (in practice parseInt never raises, this is for demonstration purpose only).

let parse_int' = Jv.get "parseInt"
let parse_int ?radix s =
  let r = Jv.of_option ~none:Jv.undefined Jv.of_int radix in
  match Jv.apply parse_int' Jv.[| of_jstr s; r |] with
  | exception Jv.Error e -> Error e
  | v -> Ok (Jv.to_int v)

Use Jv.get' if you have to deal with full Unicode identifiers:

(* function nπ (n) { return n * Math.PI; } *)
let npi' = Jv.get' (Jstr.v "nπ")
let npi n = Jv.to_float @@ Jv.apply npi' Jv.[| of_float n |]

Note that js_of_ocaml dead codes away these toplevel gets on the object if you end up not using these.

Create an object

See this section of the FFI manual.

Create an object of a given class

Lookup the constructor in the global object (or any other object) and call' with the constructor and its arguments.

let url = Jv.get "URL" (* the URL JavaScript constructor *)
let url_of_jstr s =              (* 'new URL(s);' and handle error *)
  match' url Jv.[| of_jstr s |] with
  | exception Jv.Error e -> Error e
  | v -> Ok v

Call a method

Construct an array of Jv.t values for the method arguments an invoke on the object with the method name:

let to_string o = Jv.to_jstr @@ o "toString" [||]
let set_length o l = ignore @@ o "setLength" Jv.[| of_int l |]

If you hit a Unicode method name, use'.

See also How to return unit ?

How to return unit ?

JavaScript's unit is undefined, when you call functions and methods for side effects they return the Jv.undefined value. Simply use OCaml's ignore to ignore the value:

let log s =
  ignore @@ (Jv.get "console") "log" Jv.[| of_jstr s |]

How to test for class membership ?

Get a hand on the class constructor and test using Jv.instanceof.

let array = Jv.get "Array"
let is_array_class a = Jv.instanceof (Jv.repr a) array

Deal with initialisation dictionaries

Lots of JavaScript constructors take an optional initialisation dictionary to specify parameters for the resulting object.

To deal with this pattern:

  1. Create an abstract data type for the dictionary. Call that type init or opts according to the terminology used by the constructor.
  2. Make a function of the same name with optional arguments for each of the dictionary field and that returns a dictionary.
  3. In the constructor have an optional parameter with the same name for the dictionary.

Sample code:

module Jobj : sig
 type opts
 (** The the type for [Jobj] options. *)

 val opts : ?param1:int -> ?param2:Jstr.t -> unit -> opts
 (** [opts ()] are options for [Jojb] with given parameters. *)

 type t
 (** The type for [Jobj] objects. *)

 val create : ?opts:opts -> unit -> t
 (** [create () ~opts] is a [Jobj] with options [opts]. *)

end = struct
  type opts = Jv.t
  let opts ?param1 ?param2 () =
    let o = Jv.obj [||] in
    Jv.Int.set_if_some o "param1" param1;
    Jv.Jstr.set_if_some o "param2" param2;

  type t = Jv.t
  let jobj = Jv.get "Jobj" (* constructor *)
  let create ?(opts = Jv.undefined) () =' jobj [| opts |]

Deal with the iterator protocol

APIs returning sequences of values via the iterator protocol can be dealt with using the functions provided in Jv.It. Convenience functions are provided to turn them directly into folds.

The following folds over the Unicode characters of strings returned as strings. The function Jv.It.iterator accesses the JavaScript string iterator (formally the symbolic property Symbol.iterator). The Jv.It.fold combinator takes care of folding over the values it produces.

let fold_uchar_jstrs f s acc =
  Jv.It.fold Jv.to_jstr f (Jv.It.iterator (Jv.of_jstr s)) acc

Deal with enums

In browser APIs enums are strings most of the time used to specify options. Pattern matching is of little use in this case so do not bother to translate them to OCaml variants.

Use the naming convention to define a dedicated module for the enum with:

  1. A type t equal to Jstr.t.
  2. Constants for each value.
  3. At least one link in a doc string that explains the semantics of values.

For example for this Blob object EndingType enum:

(** The line ending type enum. *)
module Ending_type : sig
  type t = Jstr.t
  (** The type for line
    [EndingType] values}. *)

  val native : Jstr.t
  val transparent : Jstr.t
end = struct
  type t = Jstr.t
  let native = Jstr.v "native"
  let transparent = Jstr.v "transparent"

js_of_ocaml dead codes away these constants if they are not used by your program.

Deal with JavaScript arrays

JavaScript APIs arrays are often used for lists (indexability does not really matter). In this case make the OCaml programmer happy and expose them as such. Use Jv.to_list and Jv.of_list to convert JavaScript arrays, there are also a few specialized conversion functions which may be faster.

let navigator_languages : Navigator.t -> Jstr.t list =
fun n -> Jv.to_jstr_list @@ Jv.get (Navigator.to_jv n) "languages"

Deal with exceptions

JavaScript exceptions/errors are thrown in your face as the OCaml Jv.Error exception. To handle it simply pattern match on it like you do with any other OCaml exception:

let atob : Jstr.t -> (Jstr.t, Jv.Error.t) result =
fun a ->
  match Jv.apply (Jv.get "atob") Jv.[| of_jstr a |] with
  | exception Jv.Error e -> Error e
  | v -> Ok (Jv.to_jstr v)

To throw your own JavaScript exception use Jv.throw.

When a function or method throws exceptions distinguish between:

In the first case simply bind the function and mention in the documentation that it may raise. In the second case catch the JavaScript exception and turn it into a Stdlib.result type, see the example above.

See also Deal with promises.

Deal with promises

Brr represents JavaScript promises as Fut.result values which are Fut.t values that determine to a standard OCaml Stdlib.result type using the Error case for rejections.

In JavaScript APIs most promises reject by returning a JavaScript error object Jv.Error.t and the Fut.or_error type abbreviation represents exactly that. The Fut.of_promise function converts a promise directly to this type.

let read_text c = Fut.of_promise ~ok:Jv.to_jstr @@ c "readText" [||]
let fullscreen () =
  Fut.of_promise ~ok:ignore @@ (El.to_jv (Document.body G.document)) "requestFullscreen" [||]

Deal with BufferSources

Some APIs deal with BufferSources which makes things a bit more complicated than they could be.

Any Brr.Tarray.Buffer.t can be turned into into a Brr.Tarray.t if needed so:

Deal with callbacks to OCaml

You need to convert the callback to a JavaScript Jv.t value with the Jv.callback function.

let set_timeout : ms:int -> (unit -> unit) -> unit =
  let set_timeout = Jv.get "setTimeout" in
  fun ~ms f ->
    let f = Jv.callback ~arity:1 f in
    ignore @@ Jv.apply set_timeout Jv.[| f; of_int ms |]

let () =
  let alert = Jv.get "alert" in
  let alert v = ignore @@ Jv.apply alert Jv.[| of_string v |] in
  set_timeout ~ms:1000 @@ fun () -> alert "Iiiiiik!"

You need to make sure the representations of the function arguments and the result type are compatible with those JavaScript expect. See here for more discussion.

Exposing an OCaml function to JavaScript

Just name its JavaScript representation in the object (or any other object). You need to make sure the representations of the function arguments and the result type are compatible with those JavaScript expect, see the FFI manual for discussions on this. So for example to expose an OCaml factorial function in the global object as fact you could write:

let rec fact n = if n <= 0 then 1 else n * fact (n - 1)
let fact' n = Jv.of_int (fact (Jv.to_int n))
let () = Jv.set "fact" (Jv.callback ~arity:1 fact')

The JavaScript code or console can now call:

fact (3);

Note that technically since number conversions are nops you could write:

(* Do not do this *)
let () = Jv.set "fact" (Jv.callback ~arity:1 fact)

But it's not advised to do and may break if js_of_ocaml changes the representation of OCaml values in the future.

Testing for features

Browser discrepancies do still exist. To quickly test for features Jv.has takes a value of any type and checks whether it has the given property or method name:

let has_text_method : Blob.t -> bool = fun b -> Jv.has "text" b

Jv.defined v tests that v is neither null nor undefined – this is simply a shortcut for the longer Jv.is_some (J.repr v).