Richard Suchenwirth 2003-02-12 - The following little state machine was created, with help from dkf, in the Tcl chatroom, instigated by the quote (Alan Cox?): "A computer is a state machine. Threads are for people who can't program state machines."
So here is one, starting with the necessary state transition command - goto in Tcl. Note some unusal points: the proc changes a variable (next) in its caller, and lets that continue, so it can only be called in a loop. }
proc goto label {
upvar 1 next next
set next $label
return -code continue
}if 0 {The "machine" itself takes a paired list of alternating labels and state code (which could be implemented as dict, once that is available - 8.5?); if a state code does not end in a goto or break, the same state will be repeated as long as not left, with goto or break (implicit endless loop). Execution starts at the first of the states. }
proc statemachine states {
array set state $states
set next [lindex $states 0]
while 1 {eval $state($next)}
}RS 2006-08-26: Three years later, I like this version better, which keeps the goto command locally - it is not for general consumption after all, and hence disappears when the state machine returns:
proc statemachine states {
array set S $states
proc goto label {
uplevel 1 set this $label
return -code continue
}
set this [lindex $states 0]
while 1 {eval $S($this)}
rename goto {}
}# Testing: a tiny state machine that greets you as often as you wish, and ends if you only hit <Return> on the "how often?" question:
statemachine {
1 {
puts "hello - how often?"
gets stdin n
if {$n eq ""} {goto 3}
goto 2
} 2 {
if {$n==0} {goto 1}
puts Howdy!
incr n -1
} 3 {puts "Thank you!"; break}
}A more elaborate page: Finite state machines