External function

Gleam is just one of many languages on the Erlang virtual machine and 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 BEAM language.

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!


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:

pub external fn random_float() -> Float = "rand" "uniform"

// Elixir modules start with `Elixir.`
pub external fn inspect(a) -> a = "Elixir.IO" "inspect"

Labelled arguments

Like regular functions, external functions can have labelled arguments.

pub external fn any(in: List(a), satisfying: fn(a) -> Bool) =
  "my_external_module" "any"

This function has the labelled arguments in and satisfying, and can be called like so:

any(in: my_list, satisfying: is_even)
any(satisfying: is_even, in: my_list)