An example problem:
set a 1 set b {} ;# <---- this represents bad data from elsewhere
expr {$a+$b} can't use empty string as operand of "+"
Which doesn't tell us which part of the expression was bad
set _debugExpr_ 1
if {[info exists _debugExpr_]} {
rename expr _expr
proc expr {args} {
if {[catch {set result [uplevel _expr $args]}]} {
error [uplevel list "error in expression: '" {*}$args "'"]
}
set result
}
}
% expr {$a+$b}
error in expression: ' {$a+$b} '
Beware, this comes at the price of speed:
% time {expr $a/.5} 1000
25.975 microseconds per iteration
% time {_expr $a/.5} 1000
17.065 microseconds per iteration
% time {expr {$a/.5}} 1000
15.988 microseconds per iteration
% time {_expr {$a/.5}} 1000
0.709 microseconds per iteration