09-25-2012
Quoting from
The New Kornshell Command and Programming Language by Bolsky and Korn (copyright 1995):
Quote:
It is often desirable to pass the name of a variable rather than the value of the variable, as an argument to a function. A reference variable provides a mechanism for providing a local name for this variable. A reference variable is created with typeset -n, or with the preset alias nameref. The name of a reference variable must be an identifier. The value of the variable at the time the typeset -n attribute is specified, defines the variable that will be referenced by the reference variable. For instance, if the variable foo is assigned the value bar, and foo is given the name reference attribute, then each assignment to foo will be an assignment to bar.
There is probably a way to get replace the use of name references using eval, but I have other things I need to do tonight and don't have time to prepare and test an example tonight.
Sorry,
Don
This User Gave Thanks to Don Cragun For This Post:
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eval(n) Tcl Built-In Commands eval(n)
__________________________________________________________________________________________________________________________________________________
NAME
eval - Evaluate a Tcl script
SYNOPSIS
eval arg ?arg ...?
_________________________________________________________________
DESCRIPTION
Eval takes one or more arguments, which together comprise a Tcl script containing one or more commands. Eval concatenates all its argu-
ments in the same fashion as the concat command, passes the concatenated string to the Tcl interpreter recursively, and returns the result
of that evaluation (or any error generated by it). Note that the list command quotes sequences of words in such a way that they are not
further expanded by the eval command.
EXAMPLES
Often, it is useful to store a fragment of a script in a variable and execute it later on with extra values appended. This technique is
used in a number of places throughout the Tcl core (e.g. in fcopy, lsort and trace command callbacks). This example shows how to do this
using core Tcl commands:
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] |
SEE ALSO
catch(n), concat(n), error(n), interp(n), list(n), namespace(n), subst(n), tclvars(n), uplevel(n)
KEYWORDS
concatenate, evaluate, script
Tcl eval(n)