prefix_calculator

set stack {}
 
while 1 {
	puts "{ $stack }"
	set input [gets stdin]
	if {$input in {+ - * /}} {
		# calculate new_element (trick with parsing input as part of the command)
		set new_element [eval [list tcl::mathop::$input [lindex $stack [expr {[llength $stack] - 1}]] [lindex $stack [expr {[llength $stack] - 2}]]]]
 
		# remove previous values
		# stack = stack[:-2]
		set stack [lreplace $stack [expr {[llength $stack] - 2}] [llength $stack]]
 
		# append
		lappend stack $new_element
	} else {
		lappend stack $input
	}
}