An error experiment

if 0 {Richard Suchenwirth 2003-07-29 - Errors happen, and in Tcl they're less depressing and more educational than in most other languages. Also, one can try things like resuming at the position where the error happened - almost. Consider this code, where error is renamed into the Tcl namespace}

 rename error tcl::error

if 0 {and redefined to a simple interactive prompt (a minimal debugger), where you can

check what went wrong ("p" to see the proc body where the error happened, and the arguments with which it was called),
maybe fix it, and decide to
"c"ontinue (i.e. resume after the place the error was raised) or
keep the error for the real handler ("x"):}

 proc error {code args} {
    puts $code/$args
    while 1 {
        puts -nonewline "% "
	flush stdout
	gets stdin line
	if {$line=="x"} {tcl::error $code}
	if {$line=="p"} {
	    set self [info level -1]
	    puts "$self {[info body [lindex $self 0]]}"
	    continue
	}
	if {$line=="c"} break
	catch {uplevel 1 $line} res
	puts $res
    }
 }

if 0 {The test code is heavily putsy so one sees what goes on, and does two deliberate errors in the "try" proc: one by just calling an error, one by referencing an undefined variable.

 puts "before try"
 proc try {args} {
    puts "in try"
    error intended
    puts x=$x
    puts "after error"
 }
 try a little bit
 puts "after try"

if 0 {Experimenting with this, I find the intended error triggers indeed, I can see where I am, and it lets me fix the missing x bug, and resume:

 suchenwi@jaguar% tclsh error2.tcl
 before try
 in try
 intended/
 % p
 try a little bit {
    puts "in try"
    error intended
    puts x=$x
    puts "after error"
 }
 % set x wow!
 wow!
 % c
 x=wow!
 after error
 after try

but if I just let the intended error resume, missing x appears to be hard-wired to the original error, not my overloaded version:

 suchenwi@jaguar% tclsh error2.tcl
 before try
 in try
 intended/
 % c ;# continue without fixing
 can't read "x": no such variable
    while executing
 "puts x=$x"
    (procedure "try" line 4)
    invoked from within
 "try"
    (file "error2.tcl" line 22)

So the usability of this is limited - you can only intercept errors that you explicitly raised yourself.

An interesting point is however the behavior of the bytecode compiler: it does not notice or complain about the fact that $x is used before defined - possibly because a preceding command (like my redefined "error") might have set it with upvar...

Arts and crafts of Tcl-Tk programming }