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Diffstat (limited to 'doc/eval.n')
| -rw-r--r-- | doc/eval.n | 46 |
1 files changed, 33 insertions, 13 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 1.5 2004/05/28 10:37:04 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 @@ -27,12 +24,14 @@ 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 +.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" @@ -46,19 +45,40 @@ 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] + \fBeval\fR $script $i [clock clicks] } .CE -.VS 8.5 +.PP Note that in the most common case (where the script fragment is actually just a list of words forming a command prefix), it is better -to use \fB{expand}$script\fR when doing this sort of invokation +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. -.VE 8.5 - +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), interp(n), list(n), namespace(n), subst(n), tclvars(n), uplevel(n) +'\" Local Variables: +'\" mode: nroff +'\" End: |