eval

eval(n)                      Tcl Built-In Commands                     eval(n)



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NAME
       eval - Evaluate a Tcl script

SYNOPSIS
       eval arg ?arg ...?
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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), errorCode(n), errorInfo(n), interp(n),
       list(n), namespace(n), subst(n), uplevel(n)

KEYWORDS
       concatenate, evaluate, script



Tcl                                                                    eval(n)