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Diffstat (limited to 'doc/eval.n')
| -rw-r--r-- | doc/eval.n | 63 |
1 files changed, 56 insertions, 7 deletions
@@ -5,10 +5,8 @@ '\" 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 $ -'\" -.so man.macros .TH eval n "" Tcl "Tcl Built-In Commands" +.so man.macros .BS '\" Note: do not modify the .SH NAME line immediately below! .SH NAME @@ -16,7 +14,6 @@ eval \- Evaluate a Tcl script .SH SYNOPSIS \fBeval \fIarg \fR?\fIarg ...\fR? .BE - .SH DESCRIPTION .PP \fBEval\fR takes one or more arguments, which together comprise a Tcl @@ -27,9 +24,61 @@ 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 EXAMPLES +.PP +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: +.PP +.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 + \fBeval\fR $script $i [clock clicks] +} +.CE +.PP +Note that in the most common case (where the script fragment is +actually just a list of words forming a command prefix), it is better +to use \fB{*}$script\fR when doing this sort of invocation +pattern. It is less general than the \fBeval\fR command, and hence +easier to make robust in practice. +The following procedure acts in a way that is analogous to the +\fBlappend\fR command, except it inserts the argument values at the +start of the list in the variable: +.PP +.CS +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 [\fBeval\fR [list linsert $var 0] $args] +} +.CE +.PP +However, the last line would now normally be written without +\fBeval\fR, like this: +.PP +.CS +set var [linsert $var 0 {*}$args] +.CE +.SH "SEE ALSO" +catch(n), concat(n), error(n), errorCode(n), errorInfo(n), interp(n), list(n), +namespace(n), subst(n), uplevel(n) .SH KEYWORDS concatenate, evaluate, script - -.SH "SEE ALSO" -catch(n), concat(n), error(n), list(n), subst(n), tclvars(n) +'\" Local Variables: +'\" mode: nroff +'\" End: |