NAME

       watch - call Tcl procedures before and after each command

SYNOPSIS

       watch create watchName ?options?

       watch activate watchName

       watch deactivate watchName

       watch delete watchName

       watch configure watchName ?options

       watch info watchName

       watch names
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DESCRIPTION

       The  watch  command  arranges for Tcl procedures to be invoked before and after the execution of each Tcl
       command.

INTRODUCTION

       When an error occurs in Tcl, the global variable errorInfo will  contain  a  stack-trace  of  the  active
       procedures when the error occured.  Sometimes, however, the stack trace is insufficient.  You may need to
       know exactly where in the program's execution the error occured.  In cases  like  this,  a  more  general
       tracing facility would be useful.

       The  watch command lets you designate Tcl procedures to be invoked before and after the execution of each
       Tcl command.  This means you can display the command  line  and  its  results  for  each  command  as  it
       executes.   Another  use  is  to profile your Tcl commands.  You can profile any Tcl command (like if and
       set), not just Tcl procedures.

EXAMPLE

       The following example use watch to trace Tcl commands (printing to standard error) both before and  after
       they are executed.

              proc preCmd { level command argv } {
                  set name [lindex $argv 0]
                  puts stderr "$level $name => $command"
              }

              proc postCmd { level command argv retcode results } {
                  set name [lindex $argv 0]
                  puts stderr "$level $name => $argv0= ($retcode) $results"
              }
              watch create trace \
                -postcmd postCmd -precmd preCmd

OPERATIONS

       The following operations are available for the watch command:

       watch activate watchName
              Activates the watch, causing Tcl commands the be traced to the maximum depth selected.

       watch create watchName ?options?...
              Creates  a  new  watch  watchName.  It's an error if another watch watchName already exists and an
              error message will be returned.  Options may  have  any  of  the  values  accepted  by  the  watch
              configure command.  This command returns the empty string.

       watch configure watchName ?options...?
              Queries  or modifies the configuration options of the watch watchName.  WatchName is the name of a
              watch.  Options may have any of the following values:

              -active boolean
                     Specifies if the watch is active.  By default, watches are active when created.

              -postcmd string
                     Specifies a Tcl procedure to be called immediately after each Tcl command.  String is  name
                     of  a  Tcl procedure and any extra arguments to be passed to it.  Before string is invoked,
                     five more arguments are appended: 1) the current level 2) the current  command  line  3)  a
                     list  containing the command after substitutions and split into words 4) the return code of
                     the command, and 5) the results of the command.  The return status of the postcmd procedure
                     is always ignored.

              -precmd string
                     Specifies a Tcl procedure to be called immediately before each Tcl command.  String is name
                     of a Tcl procedure and any extra arguments to be passed to it.  Before string  is  invoked,
                     three  arguments  are  appended: 1) the current level 2) the current command line, and 3) a
                     list containing the command after substitutions and split into words.  The return status of
                     the -precmd procedure is always ignored.

              -maxlevel number
                     Specifies the maximum evaluation depth to watch Tcl commands.  The default maximum level is
                     10000.

       watch deactivate watchName
              Deactivates the watch.  The -precmd and -postcmd procedures will no longer be invoked.

       watch info watchName
              Returns the configuration information associated with the watch watchName.  WatchName is the  name
              of a watch.

       watch names ?state?
              Lists  the  names  of  the  watches for a given state.  State may be one of the following: active,
              idle, or ignore.  If a state argument isn't specified,
               all watches are listed.

KEYWORDS

       debug, profile