# Procedure: trap
# Essentially works like 'if catch then body'.
# Trap stores the latest error in the 'traperror' auto variable
# in the caller's scope.
# Returns 1 on error 0 on success
# Example usages:
# trap {lassing} -> ignore the error and continue
# trap {lassign} {puts $traperror}
# trap {lassign} {puts "Recovering ..."; lassign {b} a}
# if {[trap {lassign}]} { puts $traperror; do this } else {do that}
proc trap {cmd {body ""}} {
  set level [expr {[info level] - 1}]
  set caller [lindex [info level $level] 0]
  set code [catch {uplevel 1 $cmd} err]

  if {$code} {
    if {$level == 0} {
      set err "Trap in level 0 in proc: ${caller} <- '$cmd': $err"
    } else {
      set err "Trap in proc: ${caller} <- '$cmd': $err"
    }

    uplevel 1 [list set traperror $err]
    if {$body ne ""} {
      uplevel 1 $body
    }
  }
  return $code
}