Tcl 8.7/Tk8.7a5 Documentation > Tcl Commands > tailcall

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tailcall — Replace the current procedure with another command


tailcall command ?arg ...?


The tailcall command replaces the currently executing procedure, lambda application, or method with another command. The command, which will have arg ... passed as arguments if they are supplied, will be looked up in the current namespace context, not in the caller's. Apart from that difference in resolution, it is equivalent to:

return [uplevel 1 [list command ?arg ...?]]

This command may not be invoked from within an uplevel into a procedure or inside a catch inside a procedure or lambda.


Compute the factorial of a number.

proc factorial {n {accum 1}} {
    if {$n < 2} {
        return $accum
    tailcall factorial [expr {$n - 1}] [expr {$accum * $n}]

Print the elements of a list with alternating lines having different indentations.

proc printList {theList} {
    if {[llength $theList]} {
        puts "> [lindex $theList 0]"
        tailcall printList2 [lrange $theList 1 end]
proc printList2 {theList} {
    if {[llength $theList]} {
        puts "< [lindex $theList 0]"
        tailcall printList [lrange $theList 1 end]


apply, proc, uplevel


call, recursion, tail recursion
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