Build out a banking protocol, as seen in the example below:
> dfx canister call bank deposit '(500)'
()The bank canister behaves as you would expect a regular bank should, with some additional functionality. Account holders can deposit, withdraw, and transfer their funds to other account holders. The bank is also able to grant account holders an "allowance" which entitles them to spend from the balance of a different account holder, up to a certain amount. Naturally, the account holder whose balance is being spent needs to approve the allowance before any spending occurs.
To carry out its role, the bank canister leverages two additional modules: one for database management, and another for permissioning.
The Database module is responsible for the retrieval and storage of account holder information.
The Permissions module is responsible for maintaining user groups and granting privileges to account holders based on the user group they belong to.
Note that these modules may each serve as independent protocols for other applications to build on top of, they are not necessarily bank-specific.
In this exercise, you will implement the aforementioned libraries for the bank to use. You’ll need to use your understanding of the bank's operations to ensure that the libraries are leveraged successfully.
After navigating to the bank/, let’s take a look at the code in src/bank/Main.mo. You will find the expected methods getTotalSupply, getBalance, deposit, withdraw, and transfer are all fully implemented. You will also see that the Database and Permissions classes are initialized:
var db: Database.Database = Database.Database();
var permissions: Permissions.Permissions = Permissions.Permissions();Note how each is used, the arguments their methods take, and whether they are expected to return a value.
In src/bank/Database.mo, you'll find findAccount, findMultipleAccounts, updateAccount, and clear. Each of these makes calls to the underlying localDB HashMap. More specifically, findAccount and updateAccount are the getter and setter methods, respectively, while findMultipleAccounts is simply findAccount called on multiple inputs. Notice how none of these methods are permissioned, meaning anyone may be able to call a dangerous method like clear. It is up to the implementer—the bank canister in this case—to ensure permissions are set up appropriately.
In src/bank/Permissions.mo, you'll find hasPermission and addToGroup. The former method checks whether the given account holder falls within the specified group, whereas the latter method allows the caller to set an account holder's privileges.
Task: Complete the implementation of the Database and Permissions classes.
Database.mo: findAccount, findMultipleAccounts, updateAccount, clear
Permissions.mo: hasPermission, addToGroup
The following test should run to completion:
> dfx deploy
Deploying all canisters.
All canisters have already been created.
Building canisters...
Installing canisters...
Installing code for canister Bank, with canister_id rrkah-fqaaa-aaaaa-aaaaq-cai
...
> dfx canister call Test run
()