85 lines
2.7 KiB
Plaintext
85 lines
2.7 KiB
Plaintext
'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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.TH eval n "" Tcl "Tcl Built-In Commands"
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.so man.macros
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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eval \- Evaluate a Tcl script
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.SH SYNOPSIS
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\fBeval \fIarg \fR?\fIarg ...\fR?
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.BE
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.SH DESCRIPTION
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.PP
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\fBEval\fR takes one or more arguments, which together comprise a Tcl
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script containing one or more commands.
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\fBEval\fR concatenates all its arguments in the same
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fashion as the \fBconcat\fR command, passes the concatenated string to the
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Tcl interpreter recursively, and returns the result of that
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evaluation (or any error generated by it).
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Note that the \fBlist\fR command quotes sequences of words in such a
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way that they are not further expanded by the \fBeval\fR command.
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.SH EXAMPLES
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.PP
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Often, it is useful to store a fragment of a script in a variable and
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execute it later on with extra values appended. This technique is used
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in a number of places throughout the Tcl core (e.g. in \fBfcopy\fR,
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\fBlsort\fR and \fBtrace\fR command callbacks). This example shows how
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to do this using core Tcl commands:
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.PP
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.CS
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set script {
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puts "logging now"
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lappend $myCurrentLogVar
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}
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set myCurrentLogVar log1
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# Set up a switch of logging variable part way through!
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after 20000 set myCurrentLogVar log2
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for {set i 0} {$i<10} {incr i} {
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# Introduce a random delay
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after [expr {int(5000 * rand())}]
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update ;# Check for the asynch log switch
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\fBeval\fR $script $i [clock clicks]
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}
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.CE
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.PP
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Note that in the most common case (where the script fragment is
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actually just a list of words forming a command prefix), it is better
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to use \fB{*}$script\fR when doing this sort of invocation
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pattern. It is less general than the \fBeval\fR command, and hence
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easier to make robust in practice.
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The following procedure acts in a way that is analogous to the
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\fBlappend\fR command, except it inserts the argument values at the
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start of the list in the variable:
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.PP
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.CS
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proc lprepend {varName args} {
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upvar 1 $varName var
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# Ensure that the variable exists and contains a list
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lappend var
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# Now we insert all the arguments in one go
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set var [\fBeval\fR [list linsert $var 0] $args]
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}
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.CE
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.PP
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However, the last line would now normally be written without
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\fBeval\fR, like this:
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.PP
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.CS
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set var [linsert $var 0 {*}$args]
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.CE
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.SH "SEE ALSO"
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catch(n), concat(n), error(n), errorCode(n), errorInfo(n), interp(n), list(n),
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namespace(n), subst(n), uplevel(n)
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.SH KEYWORDS
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concatenate, evaluate, script
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'\" Local Variables:
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'\" mode: nroff
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'\" End:
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