eval(n) - phpMan

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 com-
       mand,  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  lap-
       pend  command, except it inserts the argument values at the start of the list in the vari-
       able:
              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)

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