Finally introducing users with S3 classes

Our object-oriented system is almost finalized. We're only missing the User definition. In this case, we will use S3 to define the User class. The user_constructor() function takes an email and a Storage instance in storage to create a User instance. However, before it does, it checks that the email is valid with the valid_email() function defined below. After the user has been created, the get_wallets() method is called upon it to get the wallets associated to the user before it's sent back.

The valid_email() function simply receives a string which is supposed to be an email address, and checks whether at least one @ and one . symbol are contained within it. Of course, this is not a robust mechanism to check whether or not it's an email address, and it's put here just to illustrate how a checking mechanism could be implemented:

source("../assets/wallets/wallet.R", chdir = TRUE)

user_constructor <- function(email, storage) {
    if (!valid_email...