I'm easily amused (especially when amusing myself). See [Tail call optimization] for other variations.

 proc continue: {args} {
    after idle $args
 }

 proc fac {n result {accum 1}} {
    if {$n > 1} {
	continue: fac [expr {$n-1}] $result [expr {$accum * $n}]
    } else {
	upvar $result res
	set res $accum
    }
 }

 fac 10 ::foo
 vwait ::foo
 puts $::foo

Hey, and there is room for more things to be done while calculating...
 proc do_ntimes {cnt code} {
    uplevel $code
    if {$cnt > 0} {
	continue: do_ntimes [incr cnt -1] $code
    }
 }

 fac 10 ::foo
 do_ntimes 10 {puts "hello world"}
 vwait ::foo
 puts $::foo
 
-- [Todd Coram]