From this, we get that accounts should certainly behave like state machines.
And from that, it’s reasonable that other pieces of code ought to be able to
dynamically inspect an account’s current state, as well as its complete graph
of states and transitions. That’s as much a part of its public interface as
any particular method.
I guess I'm fairly influenced by this book:
Conditional transitions, before- and after- actions, state entry conditions/validations, ... There are all sorts of places to go that have been proven fertile for developing robust software.
But you know... In one essay (two if you count its predecessor)... You have to draw the line somewhere. The goal is not to present a unified theory of modelling domain objects with state, nor is it to introduce some code that should be copied and pasted into a production pull request.
The simple goal is to provoke a conversation.
And if the outcome of that conversation for some people is, "We need conditional transitions, so the exact thing articulated in the essay is NoDamnGoodForUs," then I am actually quite satisfied that I've done my job as a blogger.
And if I can address your specific point--which I hope you understand that I agree with, even if it disagrees with that one paragraph--there are multiple approaches, and I think that if we really were modelling bank accounts as domain object with .deposit or .withdraw methods, we could do just as you suggest and bake a conditional transition to a Held state.
But we could also ask if the bank account is responsible for making that decision? In some architectures, we might say that this is burdening it with too much responsibility. I actually mulled this over with the following use case, which I decided to cut from the essay:
Consider a "suspicious deposit," or maybe a "suspicious transaction pattern," like a series of deposits and immediate withdrawals. PayPal is notorious for freezing accounts that violate some set of hidden rules.
Should the .deposit and .withdraw methods know about this and sometimes transition to "held?" One could argue this is not the account's responsibility, especially if some aspects of what makes a transaction suspicious involve things outside of the balance, like how long the customer has been with the institution.
Of course, there could be a line of code that calls a "Transaction Evaluator" thingummy from within the .deposit and .withdraw methods, or even a .validate method that always checks for this kind of thing after any action.
Or... Maybe this is a lot more like an MVC architecture, and the responsibility for checking the transaction falls to the controller, not the account model. So the controller calls .deposit or .withdraw, and the controller checks for suspicious transactions (or for negative balances), and the controller calls .placeHold on the account if necessary (and possibly on ALL accounts for the customer).
I'm not suggesting that this is superior to your suggestion, but it was something I thought about, and in some application architectures, I think what I just described is the way to do it. In others, what you described is the way to do it.
JM2C. Thanks for your comment.
Interestingly, that embedded device I made wound up being MVC without explicitly intending so -- the statecharts were the controllers. The only conditional transitions had to do with code reuse, where a single implementation lead to slightly different transitions depending on how it was configured.