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}]};
