Purpose: example of code to apply aritmethic sum operation to
all elements of a list.

As of Tcl 8.5, one preferably uses the [tcl::mathop]::[+] command and [{*}]:

  proc ladd {l} {::tcl::mathop::+ {*}$l}

Before that, other techniques were necessary...

----

 # Pass ladd a list and receive back a single value which is a total of
 # all the elements.  WARNING: assumes all elements are integer and makes
 # no effort to prevent overflow.
 proc ladd {l} {
        set total 0
        foreach nxt $l {
                incr total $nxt
        }
        return $total
 }
----
See also: [Sample math programs] - [Arts and crafts of Tcl-Tk programming]
----
Do you think it would be harmful to students to see
    proc ladd l {
        if {![llength $l]} {return 0}
        return [expr [join $l +]]
    }
?
----

Harmful?  Probably not.  If it is faster, that would be fine.  Now you can add doubles as well, but the overflow
warning still applies ...  Use of [Mpexpr] or some other extended math
package is required to get around the innate limitations based on Tcl's
implicit use of C numeric types.
----
You can get even more functional and elegant if you append a dummy "+0", so an empty list causes no problems (and needs not to be tested):
    proc ladd L {expr [join $L +]+0} ;# RS

----
[Category Mathematics]