FeatureFlag provides a macro that allows for conditional branching at the function level via configuration values.

In other words, you can change what a function does by setting/modifying a config value.

Here's a simple example:

defmodule MyApp use FeatureFlag def get(key), feature_flag do :cache -> get_from_cache(key) :database -> get_from_database(key) end end

The function MyApp.get/1 will perform different procedures depending on a config value you can set via:

config FeatureFlag, {MyApp, :get, 1}, :cache

or, you can set/change this value at runtime via:

FeatureFlag.set({MyApp, :get, 1}, :database)

If your function is only going to do one of two things based on a boolean feature flag, you can simplify your function like so:

def get(key), feature_flag do get_from_cache(key) else get_from_database(key) end

The first block will get called if Application.get_env(FeatureFlag, {MyApp, :get, 1}) == true, and the else block will get called if it's false.