1 | '\" |
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2 | '\" Copyright (c) 1993 The Regents of the University of California. |
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3 | '\" Copyright (c) 1994-1997 Sun Microsystems, Inc. |
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4 | '\" |
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5 | '\" See the file "license.terms" for information on usage and redistribution |
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6 | '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. |
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7 | '\" |
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8 | '\" RCS: @(#) $Id: uplevel.n,v 1.10 2007/12/13 15:22:33 dgp Exp $ |
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9 | '\" |
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10 | .so man.macros |
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11 | .TH uplevel n "" Tcl "Tcl Built-In Commands" |
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12 | .BS |
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13 | '\" Note: do not modify the .SH NAME line immediately below! |
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14 | .SH NAME |
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15 | uplevel \- Execute a script in a different stack frame |
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16 | .SH SYNOPSIS |
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17 | \fBuplevel \fR?\fIlevel\fR?\fI arg \fR?\fIarg ...\fR? |
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18 | .BE |
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19 | .SH DESCRIPTION |
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20 | .PP |
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21 | All of the \fIarg\fR arguments are concatenated as if they had |
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22 | been passed to \fBconcat\fR; the result is then evaluated in the |
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23 | variable context indicated by \fIlevel\fR. \fBUplevel\fR returns |
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24 | the result of that evaluation. |
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25 | .PP |
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26 | If \fIlevel\fR is an integer then |
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27 | it gives a distance (up the procedure calling stack) to move before |
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28 | executing the command. If \fIlevel\fR consists of \fB#\fR followed by |
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29 | a number then the number gives an absolute level number. If \fIlevel\fR |
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30 | is omitted then it defaults to \fB1\fR. \fILevel\fR cannot be |
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31 | defaulted if the first \fIcommand\fR argument starts with a digit or \fB#\fR. |
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32 | .PP |
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33 | For example, suppose that procedure \fBa\fR was invoked |
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34 | from top-level, and that it called \fBb\fR, and that \fBb\fR called \fBc\fR. |
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35 | Suppose that \fBc\fR invokes the \fBuplevel\fR command. If \fIlevel\fR |
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36 | is \fB1\fR or \fB#2\fR or omitted, then the command will be executed |
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37 | in the variable context of \fBb\fR. If \fIlevel\fR is \fB2\fR or \fB#1\fR |
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38 | then the command will be executed in the variable context of \fBa\fR. |
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39 | If \fIlevel\fR is \fB3\fR or \fB#0\fR then the command will be executed |
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40 | at top-level (only global variables will be visible). |
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41 | .PP |
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42 | The \fBuplevel\fR command causes the invoking procedure to disappear |
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43 | from the procedure calling stack while the command is being executed. |
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44 | In the above example, suppose \fBc\fR invokes the command |
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45 | .CS |
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46 | \fBuplevel\fR 1 {set x 43; d} |
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47 | .CE |
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48 | where \fBd\fR is another Tcl procedure. The \fBset\fR command will |
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49 | modify the variable \fBx\fR in \fBb\fR's context, and \fBd\fR will execute |
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50 | at level 3, as if called from \fBb\fR. If it in turn executes |
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51 | the command |
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52 | .CS |
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53 | \fBuplevel\fR {set x 42} |
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54 | .CE |
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55 | then the \fBset\fR command will modify the same variable \fBx\fR in \fBb\fR's |
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56 | context: the procedure \fBc\fR does not appear to be on the call stack |
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57 | when \fBd\fR is executing. The \fBinfo level\fR command may |
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58 | be used to obtain the level of the current procedure. |
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59 | .PP |
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60 | \fBUplevel\fR makes it possible to implement new control |
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61 | constructs as Tcl procedures (for example, \fBuplevel\fR could |
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62 | be used to implement the \fBwhile\fR construct as a Tcl procedure). |
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63 | .PP |
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64 | The \fBnamespace eval\fR and \fBapply\fR commands offer other ways |
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65 | (besides procedure calls) that the Tcl naming context can change. |
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66 | They add a call frame to the stack to represent the namespace context. |
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67 | This means each \fBnamespace eval\fR command |
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68 | counts as another call level for \fBuplevel\fR and \fBupvar\fR commands. |
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69 | For example, \fBinfo level 1\fR will return a list |
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70 | describing a command that is either |
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71 | the outermost procedure call or the outermost \fBnamespace eval\fR command. |
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72 | Also, \fBuplevel #0\fR evaluates a script |
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73 | at top-level in the outermost namespace (the global namespace). |
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74 | .SH EXAMPLE |
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75 | As stated above, the \fBuplevel\fR command is useful for creating new |
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76 | control constructs. This example shows how (without error handling) |
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77 | it can be used to create a \fBdo\fR command that is the counterpart of |
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78 | \fBwhile\fR except for always performing the test after running the |
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79 | loop body: |
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80 | .CS |
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81 | proc do {body while condition} { |
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82 | if {$while ne "while"} { |
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83 | error "required word missing" |
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84 | } |
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85 | set conditionCmd [list expr $condition] |
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86 | while {1} { |
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87 | \fBuplevel\fR 1 $body |
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88 | if {![\fBuplevel\fR 1 $conditionCmd]} { |
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89 | break |
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90 | } |
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91 | } |
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92 | } |
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93 | .CE |
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94 | .SH "SEE ALSO" |
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95 | apply(n), namespace(n), upvar(n) |
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96 | .SH KEYWORDS |
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97 | context, level, namespace, stack frame, variables |
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