# Defining custom feature flags

# Introduction

In this document the process of implementing custom feature flags is presented. In the subsequent sections we will create a feature flag that is enabled if it’s `number` field is odd.

## Functions to be implemented

Each feature flag requires 1 function to be implemented:

- `is_enabled.<FEATURE_FLAG>` - S3 method computing whether the feature flag is enabled

However, it is recommended to additionally include:

- `create_<YOUR_FEATURE_FLAG>_feature_flag` - function that creates an instance of your feature flag (note: this function can be called however you want, this is just a suggestion)

- `is_<YOUR_FEATURE_FLAG_NAME>` - function that returns TRUE if the passed object is an instance of the feature flag class (that we are currently defining)

## Create feature flag function

Let’s start with a construction method that will use create a base feature flag, add the `number` field to it and assign it to the proper class.

    library(featureflag)

    create_odd_feature_flag <- function(number) {
      flag <- create_feature_flag()

      flag$number <- number
      class(flag) <- c("odd_feature_flag", class(flag))

      flag
    }

## Class checking function

Next, we will create a function checking the class attribute of the object.

    is_odd_feature_flag <- function(feature_flag) {
      inherits(feature_flag, "odd_feature_flag")
    }

## is_enabled function

Finally, we define the `is_enabled` function which checks whether the `number` field of our flag is odd.

    is_enabled.odd_feature_flag <- function(feature_flag) {
      stopifnot(is_odd_feature_flag(feature_flag))
      (feature_flag$number %% 2) != 0
    }

Now, let’s test if everything is working

    my_odd_feature_flag <- create_odd_feature_flag(number = 3)

    if (is_enabled(my_odd_feature_flag)) {
      print("The feature flag is on!")
    }
    #> [1] "The feature flag is on!"