Look on my works, ye Mighty, and despair!

On hold

The next step for our bank account is to place a hold on an account.

Using state functions, this is as simple as:

open({call, From}, place_hold, Data) ->
    {next_state, held, Data, [{reply, From, hold_placed}]};

...

open({call, From}, {deposit, Amount}, Data) ->
    handle_deposit(Amount, Data, From);

...

held({call, From}, {deposit, Amount}, Data) ->
    handle_deposit(Amount, Data, From);

held({call, From}, remove_hold, Data) ->
    {next_state, open, Data, [{reply, From, hold_removed}]}.

handle_deposit(Amount, #{balance:=Balance} = Data, From) when is_number(Amount) andalso Amount > 0 ->
    NewBalance = Balance + Amount,
    {keep_state, Data#{balance:=NewBalance}, [{reply, From, deposit_made}]}.

Now any attempt to withdraw funds from a held account, will cause the gen_statem process to crash (hopefully having previously persisted any important data!).

And using handle event:

handle_event({call, From}, place_hold, open, Data) ->
    {next_state, held, Data, [{reply, From, hold_placed}]};

...

handle_event({call, From}, remove_hold, held, Data) ->
    {next_state, open, Data, [{reply, From, hold_removed}]};

...

handle_event({call, From}, {deposit, Amount}, open, Data) ->
    handle_deposit(Amount, Data, From);

handle_event({call, From}, {deposit, Amount}, held, Data) ->
    handle_deposit(Amount, Data, From);

...

handle_deposit(Amount, #{balance:=Balance} = Data, From) when is_number(Amount) andalso Amount > 0 ->
    NewBalance = Balance + Amount,
    {keep_state, Data#{balance:=NewBalance}, [{reply, From, deposit_made}]}.

Insufficient funds?

We can also add an availableToWithdraw method, with different behaviour for held accounts, without too much hassle:

open({call, From}, available_to_withdraw, #{balance:=Balance} = Data) ->
    {keep_state, Data, [{reply, From, Balance}]};

...

held({call, From}, available_to_withdraw, Data) ->
    {keep_state, Data, [{reply, From, 0}]};

Or:

handle_event({call, From}, available_to_withdraw, open, #{balance:=Balance} = Data) ->
    {keep_state, Data, [{reply, From, Balance}]};

...

handle_event({call, From}, available_to_withdraw, held, Data) ->
    {keep_state, Data, [{reply, From, 0}]};