set data {1 2 3 4 5} join $data ", " → 1, 2, 3, 4, 5
Using join to flatten a list by a single level:
set data {1 {2 3} 4 {5 {6 7} 8}} join $data → 1 2 3 4 5 {6 7} 8
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Licensed under Tcl/Tk terms
https://www.tcl.tk/man/tcl/TclCmd/join.htm