# FeatureFlag
[](http://travis-ci.org/MainShayne233/feature_flag)
[](https://hex.pm/packages/feature_flag)
`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 at runtime by setting/modifying a config value.
## Use Case
The goal of this library was to provide an elegant and consistent mechanism for changing what a function does depending on a value that can easily be modified (i.e. a configuration value).
This could very easily be done in plain Elixir via a simple `case` statement:
```elixir
defmodule MyApp do
def math(x, y) do
case Application.fetch_env!(:my_app, :math) do
:add -> x + y
:multiply -> x * y
:subtract x - y
end
end
end
```
There's nothing wrong with this approach, and really no need to reach for anything else.
However, the same code can be rewritten as such using `FeatureFlag`
```elixir
defmodule MyApp do
def math(x, y), feature_flag do
:add -> x + y
:multiply -> x * y
:subtract x - y
end
end
```
When called, each case will attempt to match on the current value of `Application.fetch_env!(:feature_flag, :flags)[{MyApp, :math, 2}])`.
Beyond removing a marginal amount of code, `FeatureFlag` provides a consistent interface for defining functions with config-based branching.
## Usage
Add FeatureFlag as a dependency in your `mix.exs` file.
```elixir
def deps do
[
{:feature_flag, "~> 0.1.2"}
]
end
```
After that's done, run `mix deps.get`, and then you can define a feature flag'd function!
Here's a simple example:
```elixir
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:
```elixir
# config/{dev,test,prod}.exs
config FeatureFlag, :flags, %{{MyApp, :get, 1} => :cache}
```
or, you can set/change this value at runtime via:
```elixir
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:
```elixir
def get(key), feature_flag do
get_from_cache(key)
else
get_from_database(key)
end
```
The first block will get called if `Application.fetch_env!(FeatureFlag, {MyApp, :get, 1}) == true`, and the `else` block will get called if it's `false`.
## Mentions
I'd like to thank the following people who contributed to this project either via code and/or good ideas:
- [@evuez](https://github.com/evuez)
- [@zph](https://github.com/zph)
I'd also like to thank [@Packlane](https://github.com/Packlane) for giving me time to work on and share this software.
