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  4. eval(n)

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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
    arguments 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