Q. Why doesn't TCL have a 'goto' command?
A. (Donal Fellows) Because "goto"s are the bane of structured programming. Let's examine what you might use them for:
1) Implementing a standard control structure.
This simply doesn't fit with the Tcl way of doing things. Remember, [if], [for] and [while] are all ordinary commands with no special privileges. If you want, you can even create your own control structures which are just as much a part of the language as they are.
proc repeat {script untilKeyword expression} {
if {![string equal $untilKeyword "until"]} {
return -code error "malformed repeat: should be \"repeat\
script until expression\""
}
uplevel 1 $script
set test [list expr $expression]
while {![uplevel 1 $test]} {
uplevel 1 $script
}
}2) Resource cleanup on error exit.
Most Tcl resources are cleaned up automatically on exit from scope. The rest can be handled with the use of [catch] or something built on top of it; it is a better way to do it too, as it is writing code that is saying what you actually mean. (Alas, the code for try/finally is a bit too long for me to reproduce here.)
KBK (8 November 2000) -- Here's one possible implementation of try ... finally ...
3) Creating a state-machine.
It is equally possible to use a mechanism based on Tcl arrays and [eval] to do this, like this:
array set transition {
initialState state1
state1 {
set state state2
set x 1
}
state2 {
set state state3
incr x $x
}
state3 {
set state [expr {($x < 10) ? "state2" : "state4"}]
}
state4 {
break
}
}
set state $transition(initialState)
while {1} {eval $transition($state)}
puts $x ;# Can you guess what this does without running the code?This can even be extended fairly easily into working over events. I leave that as an exercise though.
4) Err. I can't think of a 4) at the moment. :^)
As you can see, you can do a lot with Tcl even without a goto. It often even ends up clearer to the person maintaining the code like that. Everyone's a winner!
David Cuthbert comments on 4):
If you're a code generator, you'll often use a goto for a combination of the above reasons. Of course, if you're a code generator, you're not human, and your code is not intended to be read by humans.
You're also probably spitting out C or assembly instead of Tcl. :-)
Also, see Goto in Tcl.