proc step {name {yesno 1}} {
    set mode [expr {$yesno? : "remove"}]
    trace $mode execution $name {enterstep leavestep} interact
 }

 proc interact args {
    if {[lindex $args end] eq "leavestep"} {
        puts ==>[lindex $args 2]
        return
    }
    puts -nonewline "$args --"
    while 1 {
        puts -nonewline "> "
        flush stdout
        gets stdin cmd
        if {$cmd eq "c" || $cmd eq ""} break
        catch {uplevel 1 $cmd} res
        if {[string length $res]} {puts $res}
    }
 }

#----------------------------Test case, a simple string reverter:
 proc sreverse str {
    set res ""
    for {set i [string length $str]} {$i > 0} {} {
        append res [string index $str [incr i -1]]
    }
    set res
 }
#-- Turn on stepping for [[sreverse]]:
 step sreverse

 sreverse hello

#-- Turn off stepping (you can also type this command from inside interact):
 step sreverse 0
 puts [sreverse Goodbye]

#-------------------------------------------- gives in a tclsh:

 {set res {}} enterstep -->
 ==>
 {for {set i [string length $str]} {$i > 0} {} {
        append res [string index $str [incr i -1]]
    }} enterstep -->
 {string length hello} enterstep -->
 ==>5
 {set i 5} enterstep -->
 ==>5
 {incr i -1} enterstep -->
 ==>4
 {string index hello 4} enterstep -->
 ==>o
 {append res o} enterstep -->
 ==>o
 {incr i -1} enterstep -->
 ==>3
 {string index hello 3} enterstep -->
 ==>l
 {append res l} enterstep -->
 ==>ol
 {incr i -1} enterstep -->
 ==>2
 {string index hello 2} enterstep -->
 ==>l
 {append res l} enterstep -->
 ==>oll
 {incr i -1} enterstep -->
 ==>1
 {string index hello 1} enterstep -->
 ==>e
 {append res e} enterstep -->
 ==>olle
 {incr i -1} enterstep -->
 ==>0
 {string index hello 0} enterstep -->
 ==>h
 {append res h} enterstep -->
 ==>olleh
 ==>
 {set res} enterstep -->
 ==>olleh
 eybdooG