Note that the list command quotes sequences of words in such a way that they are not further expanded by the eval command; for any values, $a, $b, and $c, these two lines are effectively equivalent:
eval [list $a $b $c] $a $b $c
set script {
puts "logging now"
lappend $myCurrentLogVar
}
set myCurrentLogVar log1
# Set up a switch of logging variable part way through!
after 20000 set myCurrentLogVar log2
for {set i 0} {$i<10} {incr i} {
# Introduce a random delay
after [expr {int(5000 * rand())}]
update ;# Check for the asynch log switch
eval $script $i [clock clicks]
}
Note that in the most common case (where the script fragment is actually just a list of words forming a command prefix), it is better to use {*}$script when doing this sort of invocation pattern. It is less general than the eval command, and hence easier to make robust in practice. The following procedure acts in a way that is analogous to the lappend command, except it inserts the argument values at the start of the list in the variable:
proc lprepend {varName args} {
upvar 1 $varName var
# Ensure that the variable exists and contains a list
lappend var
# Now we insert all the arguments in one go
set var [eval [list linsert $var 0] $args]
}
However, the last line would now normally be written without eval, like this:
set var [linsert $var 0 {*}$args]
or indeed like this:
set var [list {*}$args {*}$var]