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 fur- ther 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 actu- ally 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 prac- tice. 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)