问题
I am trying to model a vending machine program using alloy . I wish to create a model in which I could insert some money and provide the machine a selection option for an item and it would provide me the same and in case the money supplied is less then nothing would be provided . Here I am trying to input a coin along with a button as input and it should return the desired item from the vending machine provided the value ie. amount assigned to each item is provided as input. So here button a should require ten Rs, button b requires 5 rs, c requires 1 and d requires 2 . The op instance is the item returned once the money required is inserted. opc is the balance amount of coins to be returned. ip is input button and x is money input . How can I provide an instance such that it intakes multiple coins as input and also if the amount is greater than the item cost then it should return a no of coins back. If I could get some help it'll be greatly appreciated.
回答1:
If I were you, I'd proceed by asking myself what kinds of entities I care about; you've done that (signatures for coins and items -- do you also need some notion of a customer?).
Next, I'd ask myself what constitutes a legal state for the system -- sometimes it helps to think about it backwards by asking what would constitute an illegal or unacceptable state.
Then I'd try to define operations -- you've already mentioned insertion of money and selection of an item -- as transitions from one legal state of the system to the next.
At each stage I'd use the Analyzer to examine instances of the model and see whether what I'd done so far makes sense. One example of this pattern of defining entities, states, and state transitions in that order is given in the Whirlwind Tour chapter of Daniel Jackson's Software Abstractions -- if you have access to that book, you will find it helpful to review that chapter.
Good luck!
回答2:
module vending_machines
open util /ordering[Event]
fun fst:Event{ordering/first}
fun nxt:Event->Event{ordering/next}
fun upto[e:Event]:set Event{prevs[e]+e}
abstract sig Event{}
sig Coin extends Event{}
pred no_vendor_loss[product:set (Event-Coin)] {
all e:Event | let pfx=upto[e] | #(product&pfx)<=#(Coin&pfx)
来源：https://stackoverflow.com/questions/15872320/modelling-a-vending-box-using-alloy