http://purl.org/tcl/home/man/tcl8.4/TclCmd/bgerror.htm is the official man page. As of Tcl 8.5, interp bgerror provides a more flexible handling of these things.

[fill in useful tricks and examples for using bgerror]

Does the documentation describe the calling convention? I didn't notice it. My experience is that the error-handler invokes it as something like

      bgerror [firstLineOf $::errorInfo]

During development, one might rush a hack like

      proc bgerror message {
	  puts stderr "An event-based script faulted with '$message'."
      }

into use.

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Ken: I want to enquire what is the best method if i got a proc that runs every 20s like the follow code but i want to catch its errors if any of the proc that run afterwards run into errors? I heard using bgerror is not recommended, so what is the best method?

 proc a { } {

  [do something]

  after 1000 a
 }

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KPV 2008/06/13 : NB. in 8.4 the default bgerror code is not tile aware. Thus, the following idiomatic (but incorrect) code will fail (more precisely, the bgerror code configures a scrollbar's -relief which tile doesn't allow):

  package require -exact Tk 8.4
  package require tile
  namespace import -force ::ttk::*
  after 10 asdf

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Tcl syntax help Arts and crafts of Tcl-tk programming Category Command