Command Substitution is one of the fundamental features of Tcl, in which a script is evaluated and the result is substituted into a word in a command. A new stack frame is not created when the script is evaluated, so using return or break or the like will cause the caller to return, etc.
PYK 2019-02-12 2019-11-24: To perform each command subsitution, including nested substitutions, Tcl evaluates the script at the current level. No additional levels are created:
puts [
set n 10
set a {
puts $n
expr {[incr n -1]? [try $a] : {liftoff}}
}
try $a
]AMG: Command substitution can be used in combination with list and {*} to embed comments in lists. The syntax is awkward, but it does work:
switch -regexp $input [list {*}[
# Handle words starting with a capital letter
] {^[A-Z]} {
# Do the thing
theThing
} {*}[
# Handle everything else
] default {
# Don't do the thing
}]Though also consider using decomment for this same purpose.