Docs / BuckleScript / Function


Modeling a JS function is like modeling a normal value:

[@bs.val] external encodeURI: string => string = "encodeURI"; let result = encodeURI("hello");

We also expose a few special features, described below.

Labeled Arguments

OCaml has labeled arguments (that are potentially optional). These work on an external too! You'd use them to fix a JS function's unclear usage. Assuming we're modeling this:

function draw(x, y, border) { /* border is optional, defaults to false */ } draw(10, 20) draw(20, 20, true)

It'd be nice if on the BS side, we can bind & call draw while labeling things a bit:

[@bs.val] external draw: (~x: int, ~y: int, ~border: bool=?, unit) => unit = "draw"; draw(~x=10, ~y=20, ~border=true, ()); draw(~x=10, ~y=20, ());


draw(10, 20, true); draw(10, 20, undefined);

We've compiled to the same function, but now the usage is much clearer on the BS side thanks to labels!

Note: in this particular case, you need a unit, () after border, since border is an optional argument at the last position. Not having a unit to indicate you've finished applying the function would generate a warning.

Note that you can change the order of labeled arguments on the Reason side and BuckleScript will ensure that they appear the right way in the JavaScript output:

[@bs.val] external draw: (~x: int, ~y: int, ~border: bool=?, unit) => unit = "draw"; draw(~x=10, ~y=20, ()); draw(~y=20, ~x=10, ());


draw(10, 20, undefined); draw(10, 20, undefined);

Object Method

Functions attached to a JS objects require a special way of binding to them, using bs.send:

type document; /* abstract type for a document object */ [@bs.send] external getElementById: (document, string) => Dom.element = "getElementById"; [@bs.val] external doc : document = "document"; let el = getElementById(doc, "myId");


var el = document.getElementById("myId");

In a bs.send, the object is always the first argument. Actual arguments of the method follow (this is a bit what modern OOP objects are really).


Ever used foo().bar().baz() chaining ("fluent api") in JS OOP? We can model that in BuckleScript too, through the Pipe First operator described in the next section.

Variadic Function Arguments

You might have JS functions that take an arbitrary amount of arguments. BuckleScript supports modeling those, under the condition that the arbitrary arguments part is homogenous (aka of the same type). If so, add bs.variadic (was bs.splice prior to version 4.08) to your external.

[@bs.module "path"] [@bs.variadic] external join: array(string) => string = "join"; let v = join([|"a", "b"|]);


var Path = require("path"); var v = Path.join("a", "b");

bs.module will be explained in the Import & Export section next.

Modeling Polymorphic Function

Apart from the above special-case, JS function in general are often arbitrary overloaded in terms of argument types and number. How would you bind to those?

Trick 1: Multiple externals

If you can exhaustively enumerate the many forms an overloaded JS function can take, simply bind to each differently:

[@bs.module "Drawing"] external drawCat: unit => unit = "draw"; [@bs.module "Drawing"] external drawDog: (~giveName: string) => unit = "draw"; [@bs.module "Drawing"] external draw: (string, ~useRandomAnimal: bool) => unit = "draw";

Note how all three externals bind to the same JS function, draw.

Trick 2: Polymorphic Variant + bs.unwrap

If you have the irresistible urge of saying "if only this JS function argument was a variant instead of informally being either string or int", then good news: we do provide such external features through annotating a parameter as a polymorphic variant! Assuming you have the following JS function you'd like to bind to:

function padLeft(value, padding) { if (typeof padding === "number") { return Array(padding + 1).join(" ") + value; } if (typeof padding === "string") { return padding + value; } throw new Error(`Expected string or number, got '${padding}'.`); }

Here, padding is really conceptually a variant. Let's model it as such.

[@bs.val] external padLeft: ( string, [@bs.unwrap] [ | `Str(string) | `Int(int) ]) => string = "padLeft"; padLeft("Hello World", `Int(4)); padLeft("Hello World", `Str("Message from BS: "));

Obviously, the JS side couldn't have an argument that's a polymorphic variant! But here, we're just piggy backing on poly variants' type checking and syntax. The secret is the [@bs.unwrap] annotation on the type. It strips the variant constructors and compile to just the payload's value. Output:

padLeft("Hello World", 4); padLeft("Hello World", "Message from BS: ");

Constrain Arguments Better

Consider the Node fs.readFileSync's second argument. It can take a string, but really only a defined set: "ascii", "utf8", etc. You can still bind it as a string, but we can use poly variants + bs.string to ensure that our usage's more correct:

[@bs.module "fs"] external readFileSync: ( ~name: string, [@bs.string] [ | `utf8 | [ "ascii"] `useAscii ]) => string = "fs"; readFileSync(~name="xx.txt", `useAscii);


var Fs = require("fs"); Fs.readFileSync("xx.txt", "ascii");
  • Attaching [@bs.string] to the whole poly variant type makes its constructor compile to a string of the same name.

  • Attaching a [ "foo"] to a constructor lets you customize the final string.

And now, passing something like "myOwnUnicode" or other variant constructor names to readFileSync would correctly error.

Aside from string, you can also compile an argument to an int, using instead of bs.string in a similar way:

[@bs.val] external test_int_type: ( [] [ | `on_closed | [ 20] `on_open | `in_bin ]) => int = "test_int_type"; test_int_type(`in_bin);

on_closed will compile to 0, on_open to 20 and in_bin to 21.

Special-case: Event Listeners

One last trick with polymorphic variants:

type readline; [@bs.send] external on: ( readline, [@bs.string] [ | `close(unit => unit) | `line(string => unit)] ) => readline = "on"; let register = rl => rl ->on(`close(event => ())) ->on(`line(line => print_endline(line)));


function register(rl) { return rl.on("close", (function () { return /* () */0; })) .on("line", (function (line) { console.log(line); return /* () */0; })); }

Fixed Arguments

Sometimes it's convenient to bind to a function using an external, while passing predetermined argument values to the JS function:

[@bs.val] external process_on_exit: ( [ "exit"] _, int => unit ) => unit = "process.on"; let () = process_on_exit(exit_code => Js.log("error code: " ++ string_of_int(exit_code)) );


process.on("exit", function (exit_code) { console.log("error code: " + exit_code); return /* () */0; });

The [ "exit"] and the placeholder _ argument together indicates that you want the first argument to compile to the string "exit". You can also use any JSON literal with [ {json|true|json}], [ {json|{"name": "John"}|json}], etc.

Curry & Uncurry

Curry is a delicious Indian dish. More importantly, in the context of BuckleScript (and functional programming in general), currying means that function taking multiple arguments can be applied a few arguments at time, until all the arguments are applied.

See the addFive intermediate function? add takes in 3 arguments but received only 1. It's interpreted as "currying" the argument 5 and waiting for the next 2 arguments to be applied later on. Type signatures:

let add: (int, int, int) => int; let addFive: (int, int) => int; let twelve: int;

(In a dynamic language such as JS, currying would be dangerous, since accidentally forgetting to pass an argument doesn't error at compile time).


Unfortunately, due to JS not having currying because of the aforementioned reason, it's hard for BS multi-argument functions to map cleanly to JS functions 100% of the time:

  1. When all the arguments of a function are supplied (aka no currying), BS does its best to to compile e.g. a 3-arguments call into a plain JS call with 3 arguments.

  2. If it's too hard to detect whether a function application is complete*, BS will use a runtime mechanism (the Curry module) to curry as many args as we can and check whether the result is fully applied.

  3. Some JS APIs like throttle, debounce and promise might mess with context, aka use the function bind mechanism, carry around this, etc. Such implementation clashes with the previous currying logic.

* If the call site is typed as having 3 arguments, we sometimes don't know whether it's a function that's being curried, or if the original one indeed has only 3 arguments.

BS tries to do #1 as much as it can. Even when it bails and uses #2's currying mechanism, it's usually harmless.

However, if you encounter #3, heuristics are not good enough: you need a guaranteed way of fully applying a function, without intermediate currying steps. We provide such guarantee through the use of the [@bs] "uncurrying" annotation on a function declaration & call site.

Solution: Guaranteed Uncurrying

If you annotate a function declaration signature on an external or let with a [@bs] (or, in Reason syntax, annotate the start of the parameters with a dot), you'll turn that function into an similar-looking one that's guaranteed to be uncurried:

type timerId; [@bs.val] external setTimeout: ((. unit) => unit, int) => timerId = "setTimeout"; let id = setTimeout((.) => Js.log("hello"), 1000);

Note: both the declaration site and the call site need to have the uncurry annotation. That's part of the guarantee/requirement.

When you try to curry such a function, you'll get a type error:

let add = (. x, y, z) => x + y + z; let addFiveOops = add(5);


This is an uncurried BuckleScript function. It must be applied with a dot.

Extra Solution

The above solution is safe, guaranteed, and performant, but sometimes visually a little burdensome. We provide an alternative solution if:

  • you're using external

  • the external function takes in an argument that's another function

  • you want the user not to need to annotate the call sites with [@bs] or the dot in Reason

Then try [@bs.uncurry]:

[@bs.send] external map: (array('a), [@bs.uncurry] ('a => 'b)) => array('b) = "map"; map([|1, 2, 3|], x => x + 1);


If you try to do this:

let id: (. 'a) => 'a = (. v) => v;

You’ll get this cryptic error message:

Error: The type of this expression, ('_a -> '_a [@bs]), contains type variables that cannot be generalized

The issue here isn’t that the function is polymorphic. You can use polymorphic uncurried functions as inline callbacks, but you can’t export them (and lets are exposed by default unless you hide it with an interface file). The issue here is a combination of the uncurried call, polymorphism and exporting the function. It’s an unfortunate limitation of how OCaml’s type system incorporates side-effects, and how BS handles uncurrying.

The simplest solution is in most cases to just not export it, by adding an interface to the module. Alternatively, if you really need to export it, you can do so in its curried form, and then wrap it in an uncurried lambda at the call site. E.g.:

map(v => id(. v));
Design Decisions

In general, bs.uncurry is recommended; the compiler will do lots of optimizations to resolve the currying to uncurrying at compile time. However, there are some cases the compiler can't optimize it. In these cases, it will be converted to a runtime check.

This means [@bs] are completely static behavior (no runtime cost), while [@bs.uncurry] is more convenient for end users but, in some rare cases, might be slower than [@bs].

Modeling this-based Callbacks

Many JS libraries have callbacks which rely on this (the source), for example:

x.onload = function(v) { console.log(this.response + v) }

Here, this would point to x (actually, it depends on how onload is called, but we digress). It's not correct to declare x.onload of type unit → unit [@bs]. Instead, we introduced a special attribute, bs.this, which allows us to type x as so:

type x; [@bs.val] external x: x = "x"; [@bs.set] external set_onload: (x, [@bs.this] ((x, int) => unit)) => unit = "onload"; [@bs.get] external resp: x => int = "response"; set_onload(x, [@bs.this] ((o, v) => Js.log(resp(o) + v)));


x.onload = (function (v) { var o = this; console.log(o.response + v | 0); return /* () */0; });

bs.this is the same as bs, except that its first parameter is reserved for this and for arity of 0, there is no need for a redundant unit type.