eval(3tcl)

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(3tcl),  concat(3tcl),  error(3tcl),   interp(3tcl),   list(3tcl),
   namespace(3tcl), subst(3tcl), tclvars(3tcl), uplevel(3tcl)

KEYWORDS

   concatenate, evaluate, script

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