Finite State Machines are a classic tool from the 1950s, meant to allow a
system to be better defined. In formal and high safety systems they are a
critical tool. FSL, the Finite State Language, exists to make them easier to
write, debug, and maintain.
Most likely, you're already pretty familiar with a lot of state machines. On those grounds, we teach state machines by example.
An easy starting example is the idealized light switch: it's either turned On,
or turned Off. When the switch is On, it can be turned Off, but when it's
On, it can't be turned On again; the rules are similar for Off.
In FSL, we write states as just their names, and then connections as arrows
->; as such, we would write a light switch this way:
On -> Off -> On;
Or, to save time, we can use a double-sided arrow <->:
On <-> Off;
It might also be reasonable to say that to toggle is to switch from either
state to the other, without needing to know ahead of time. We call that an
action, and write it in single quotes ', inbetween the state and the
relevant arrow.
On 'toggle' -> Off 'toggle' -> On;
The placement of the action on double-sided arrows matches the arrow itself:
On 'toggle' <-> 'toggle' Off;
And were we to graph this, it might look like so:
