合金的简单状态机

Question

i'm new to alloy and its function as well. Recently, i have an assignment about a simple state machine : begin_state->normal_state->end_state. There is only one begin_state, but some normal_state and some end_state. Then I can't make the instance view go right with these below alloy code:

abstract sig state
{
    prev : some state,
    next : some state
}

one sig begin extends state{}
some sig end extends state{}
sig mid extends state{}

//There is no state after end state, and there is no state before begin state

pred dosomething
{
    no s : state | s in begin.prev and s in end.next
}

run{dosomething}

So basically i just want no state before begin state, no state after the end state and the instance examples can something like this :

begin->normal->end

or

begin-> normal ->end
|
normal->normal->end
|
normal---normal
| |
end <--------

... something like that. Thank you

Answer 1

Consider the following propositions:

• Every state points to a preceding state.
• The begin-state is a state.
• The begin-state does not point to a preceding state.

If (as I hope) you believe that these three propositions contradict each other, then ask yourself (a) do these propositions resemble the rules given in your Alloy model? (b) how might you rephrase them to make sense and not contradict each other? (c) how might your rephrasing of them be translated into Alloy?

I hope this helps.

Answer 2

Watch your quantifiers! The formula

no s : state | s in begin.prev and s in end.next

says there is no state s that is BOTH a predecessor of begin AND a successor of end.