External function

Gleam is just one of many languages on the Erlang virtual machine and JavaScript runtimes. At times we may want to use functions from these other languages in our Gleam programs. To enable this Gleam allows the importing of external functions, which may be written in any language on the same runtime.

External functions are typically written in a different language with a different type system, so the compiler is unable to determine the type of the function and instead the programmer must inform the compiler the type.

Gleam trusts that the type given is correct so an inaccurate type annotation can result in unexpected behaviour and crashes at runtime. Be careful!

Erlang external functions

The Erlang rand module has a function named uniform that takes no arguments and returns a Float.

The Elixir module IO has a function named inspect that takes any value, prints it, and returns the same value.

If we want to import these functions and use them in our program we would do so like this:

@external(erlang, "rand", "uniform")
pub fn random_float() -> Float

// Elixir modules start with `Elixir.`
@external(erlang, "Elixir.IO", "inspect")
pub fn inspect(value: a) -> a

JavaScript external functions

When importing a JavaScript function the path to the module is given instead of the module name.

// In src/my-module.mjs
export function run() {
  return 0;
// In src/my_program.gleam
@external(javascript, "./my-module.js" "run")
pub fn run() -> Int

Gleam uses the JavaScript import syntax, so any module imported must use the esmodule export syntax, and if you are using the NodeJS runtime the file extension must be .mjs.

Multi-target external functions

An external implementation can be provided for multiple targets by given the @external attribute multiple times.

@external(erlang, "rand" "uniform")
@external(javascript, "./my-module.js" "random")
pub fn random() -> Float

The appropriate implementation will be chosen based on the target the program is being compiled for.

Gleam fallbacks

A Gleam implementation can be given as a fallback for when no external implementation has been specified for the current target.

@external(erlang, "lists" "reverse")
pub fn reverse(items: List(a)) -> List(a) {
  do_reverse(items, [])

fn do_reverse(items: List(a), accumulator: List(a)) -> List(a) {
  case items {
    [] -> accumulator
    [first, ..rest] -> do_reverse(rest, [first, ..accumulator])