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Errors and Trapping Them
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else { die "Woah: Couldn't open the file $!"; }
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This procedure is most useful when you want to be able to account for two possible outcomes if the statement works, then continue and execute these statements; if it doesn t succeed, then do these statements instead Alternatively, we can reduce the statement to one line in situations where it makes sense to do so; for example:
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die "Woah: Something went wrong\n" if (error());
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See the upcoming section on Error Checking Guidelines for more information on when, and indeed whether, to use this format
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Using unless
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The unless function is the logical opposite to if: statements can completely bypass the success status and only be executed if the expression returns false For example:
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unless(chdir("/etc")) { die "Can't change directory!: $!"; }
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The unless statement is best used when you want to raise an error or alternative only if the expression fails The statement also makes sense when used in a single-line statement:
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die "Can't change directory!: $!" unless(chdir("/etc"));
Here we die only if the chdir operation fails, and it reads nicely
Using the Conditional Operator
For very short tests, you can use the conditional operator:
print(exists($hash{value}) 'There' : 'Missing',"\n");
It s not quite so clear here what we re trying to achieve, but the effect is the same as using an if or unless statement The conditional operator is best used when you want to quickly return one of two values within an expression or statement
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It s not really an error trapping statement, since there s not enough scope to do anything, but it can be used to help communicate status information back to the user Consider the following example:
chdir("/tmp") print "Using /tmp\n" : warn "Can't use /tmp: $!";
Here it s a useful way of highlighting a potential problem without actually doing anything about it The same basic principles can be used from within functions when returning values:
return (@results) @results : undef;
Using Short-Circuit Logic
For many situations, especially when you want to immediately exit the script without actually handling the error, the short-circuit capabilities of the or operator work best:
mkdir("/tmp",0755) or die "Can't make directory!: $!";
See Symbolic Logical Or in 3 for more details on why this works and the related dangers The || symbolic logical or can also be used as a way to provide alternatives when the first-choice option doesn t work For example, the line
$host = param('host') || $user->{prihost} || 'azus';
will use the browser-supplied value, then the user-configured value, and finally a default value if the other options fail
Error Checking Guidelines
There are some general guidelines for testing for errors in this way The first guideline is to make it obvious what you are testing and what you are trying to do For example, the statement
if (!open(DATA,$file))
will work fine, except that it would make more sense to use the unless statement, as in
unless(open(DATA,$file))
9:
Errors and Trapping Them
The difference is that the if statement reads If I didn t open, and the unless statement reads Unless I can open It s a minor difference but will make the code easier to read and, therefore, easier to debug Here s another example that s difficult to read:
die "Couldn't change directory: $!" unless(chdir("/etc"));
FUNDAMENTALS
This should be changed to
chdir("/etc") or die "Couldn't change directory: $!";
The second guideline is that you should make it obvious what the actual problem was; simply reporting that there was an error isn t enough, either for you to debug the program, or for your user to rectify it Where relevant, also include information on the system error message, as provided by $! Also remember the $^E variable, which contains the extended OS error on non-Unix platforms For example, the line
open(DATA,$file) or die "Can't open";
is useless compared to
open(DATA,$file) or die "Can't open $file: $!, stopped";
Coupled with this, you should always report an error to STDERR by using either warn or die The exception to this rule is when you working with a GUI or web-based application, for which there is no logical STDERR file handle See the end of this chapter for information on reporting errors when no terminal interface is available
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