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| -rw-r--r-- | doc/eval.n | 33 |
1 files changed, 31 insertions, 2 deletions
@@ -5,7 +5,7 @@ '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" -'\" RCS: @(#) $Id: eval.n,v 1.4 2002/08/28 14:46:50 dkf Exp $ +'\" RCS: @(#) $Id: eval.n,v 1.5 2004/05/28 10:37:04 dkf Exp $ '\" .so man.macros .TH eval n "" Tcl "Tcl Built-In Commands" @@ -27,9 +27,38 @@ Tcl interpreter recursively, and returns the result of that evaluation (or any error generated by it). Note that the \fBlist\fR command quotes sequences of words in such a way that they are not further expanded by the \fBeval\fR command. +.SH EXAMPLE +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 \fBfcopy\fR, +\fBlsort\fR and \fBtrace\fR command callbacks). This example shows how +to do this using core Tcl commands: +.CS +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 $i [clock clicks] +} +.CE +.VS 8.5 +Note that in the most common case (where the script fragment is +actually just a list of words forming a command prefix), it is better +to use \fB{expand}$script\fR when doing this sort of invokation +pattern. It is less general than the \fBeval\fR command, and hence +easier to make robust in practice. +.VE 8.5 .SH KEYWORDS concatenate, evaluate, script .SH "SEE ALSO" -catch(n), concat(n), error(n), list(n), subst(n), tclvars(n) +catch(n), concat(n), error(n), interp(n), list(n), namespace(n), subst(n), tclvars(n), uplevel(n) |