External gleam fallbacks

It's possible for a function to have both a Gleam implementation and an external implementation. If there exists an external implementation for the currently compiled-for target then it will be used, otherwise the Gleam implementation is used.

This may be useful if you have a function that can be implemented in Gleam, but there is an optimised implementation that can be used for one target. For example, the Erlang virtual machine has a built-in list reverse function that is implemented in native code. The code here uses this implementation when running on Erlang, as it is then available.

For further information on externals see the externals guide on the Gleam website.