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Diffstat (limited to 'doc/eval.n')
| -rw-r--r-- | doc/eval.n | 62 |
1 files changed, 58 insertions, 4 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 1.2 1998/09/14 18:39:52 stanton 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 @@ -25,6 +22,63 @@ script containing one or more commands. fashion as the \fBconcat\fR 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 \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 +'\" Local Variables: +'\" mode: nroff +'\" End: |