Re: equality and errors

From: Aric Bills (aricb_at_u.wwashingtonn.edu)
Date: 12/11/03 

Date: Thu, 11 Dec 2003 10:53:55 -0800

Hi Marvin,

This seems to me like a non-standard use of the catch mechanism. Normally,
I use catch to warn me if something is going to break so I can keep it from
breaking, like if I need to do division where the denominator might be zero,
for example.

If I understand right, you have a proc which validates the contents of two
entry widgets. If there is a problem, it throws an error, which the calling
code catches. That code then throws another error. That structure is
reasonable, although there's a problem with one of your catch statements.
If I were writing this program, though, I would use a different approach
besides catch and error.

> set fail [catch { $first < 1} range1]
> if {$fail} {
> error "range1 error"
> }

The catch above is not doing what you think it's doing... It's going to try
to execute the command [$first < 1]. Unless $first contains a Tcl command
that takes two arguments, catch is going to catch this error: "invalid
command name <whatever was in $first>". What you want here, instead of
[catch], is [if] :

if {$first < 1} {
    error "value ($first) is less than 1"
}

If I were writing the program, I would have the errorProc return a value
telling 1) whether there was an error and 2) what the error was. Then I'd
have the calling code respond intelligently to that value, either with an
error message, a popup window, or some code to read the user's mind and fix
his/her mistakes automagically. Here's a quick implementation that might
give you some ideas:

proc errorProc {val1 val2} {
    if {$val1 < 0 && $val2 < 0} {
        return "val1 and val2 are too small"
    } elseif {$val1 < 0} {
        return "val1 too small"
    } elseif {$val2 < 0} {
        return "val2 too small"
    } else {
        return ""
    }
}

proc button_code {} {
    set a [Toplevel1.Entry1 get]
    set b [Toplevel1.Entry2 get]

    set errorMsg [errorProc $a $b]
    if {$errorMsg == ""} {
        # do whatever the button does
    } else {
        tk_messageBox -message $errorMsg
    }
}

To use this code with your button, you would configure the button to call
button_code when pressed (using the -command option).

Regards,
Aric