tailcall manual page - Tcl Built-In Commands

## Tcl/Tk Documentation > Tcl Commands, version 8.6.18 > tailcall

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### NAME

tailcall — Replace the current procedure with another command

### SYNOPSIS

tailcall command ?arg ...?

### DESCRIPTION

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.

### EXAMPLE

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]
        }
    }

### SEE ALSO

apply, proc, uplevel

### KEYWORDS

call, recursion, tail recursion
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