**Intro**

This is a quick little note regarding a common problem Tclers may see: Error strings which are blank apart from "    (command bound to event)". It's one of those nasty errors in that it doesn't provide the developer any context to explain what in the code is actually triggering it.

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''[Lars H]: Is this how it appears in [errorInfo], or is it the actual error result?''

[peterc]: Yep, that's all ::errorInfo gives you with this particular error.

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**Cause**

This error is typically seen in dialogs and is triggered when the dialog is closed by the user.

The cause is essentially the setting of values and such in -command or in event bindings:

 grid [button .b -command { 
  set a [someDialog]
  puts $a
  (... other code)
  }]

or:

 bind .b <Button-1> { 
  set a [someDialog]
  (... other code)
  }

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[Lars H]: This may be what triggers the error, but what is it that makes Tcl consider it an error? It's probably an error in programming — the code is evaluated in the global context rather than, as the programmer might have expected, a local context, which means all variables [set] are global — but this as such cannot be what Tcl balks at, can it?

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**Solution**

The best way to avoid this error is to simply ensure that all programming logic launched by -command and events are in the form of single procs:

 grid [button .b -command someProc]
 bind .b <Button-1> someProc

 proc someProc {} {
  set a [someDialog]
  (... other code)
  }]

A pleasant side-effect is your code becomes more readable too :-).

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[Lars H]: Indeed. If you're still having trouble figuring out what event caused your command to be called, you can even include a dummy argument where the caller can identify itself:
 proc someProc {{caller ""}} {
  set a [someDialog]
  (... other code)
  }]

 grid [button .b -command {someProc "Button .b -command"}]
 bind .b <Button-1> {someProc "<Button-1> event"}

These "Button .b -command" or "<Button-1> event" arguments will then be visible if you [trace] the execution of '''someProc'''.
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%| [Category Design] |%
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