TCL upvar abuse

fib.tcl

#!/usr/bin/tclsh

# TCL has its unique `upvar` method to map a variable from
# a superiour scope to a local variable.
# This makes it possible to access the values of previous
# iterations in recursive functions

proc fib {stop {step 0}} {
	if {$step < 2} {
		set result [expr $step == 0 ? 0 : 1] ;# first two values
	} elseif {$step <= $stop} {
		upvar 1 result previous ;# make result of previous iteration available
		upvar 2 result preprevious ;# and one further iteration before
		set result [expr {$previous + $preprevious}]
	} elseif {$step > $stop} {
		upvar 1 result result ;# End of recursion
		return $result
	}
	return [fib $stop [expr "$step + 1"]]
}

puts [fib 20] ;# print 20Th Fibonacci number