Re: Question about eval

[tiny_tim]

Thank you for the replies. It is beginning to make sense, but why does perl allow me to hand a print statement a junk token if I do not use strict ? Also, can I please get a few more uses of eval ? I am still not 100% on eval. Thank you all :)

[shmem]

Because of TIMTOWDI (there's more than one way to do it). Attempting to print something somewhere is perfectly fine, even on a closed filehandle. But then, you should check the return value of print as it tells you wether the print actually succeeded. If you "use warnings" perl will warn you:

use warnings;
my $junkprint   = print JUNK "foo";
my $stdoutprint = print STDOUT "junkprint = '$junkprint'\n";
print "stdoutprint = '$stdoutprint'\n";
__END__
Name "main::JUNK" used only once: possible typo at - line 2.
print() on unopened filehandle JUNK at - line 2.
Use of uninitialized value in concatenation (.) or string at - line 3.
junkprint = ''
stdoutprint = '1'

Read the eval, do, require and use entries in perlfunc, as well as perlfaq8 which contains the difference between do, require and use. Then there is Super Search.

--shmem

[ikegami]

why does perl allow me to hand a print statement a junk token if I do not use strict ?

It's a filehandle, not a junk token. There's no way at compile time to know whether the filehandle has been opened or not, so STDOUT, FH, petrol, $fh are all the same and all perfectly valid.

I am still not 100% on eval.

require either returns true, or throws an exception. eval catches exceptions. You can check if an exception has been caught or not by checking $@.

eval returns undef if an exception has been caught, so you can also do something like

if (eval { require "module/that/does/not/exist"; 1 } ) {
   print "loaded module ok\n";
} else {
   print "could not load module: $@\n";
}

or since require always returns true,

if (eval { require "module/that/does/not/exist" } ) {
   print "loaded module ok\n";
} else {
   print "could not load module: $@\n";
}

[ikegami]

It's a filehandle, not a junk token. There's no way at compile time to know whether the filehandle has been opened or not, so STDOUT, FH, petrol, $fh are all the same and all perfectly valid.

[lbjay]

This was hinted at in a couple of the above replies, but combined with die (or croak) perl's eval provides a handy way to implement simple exception handling similar to the try/catch of other languages like Java and JavaScript.

# some code that might fail
eval {
  open MYFILE, "> some_file.txt" or die $!;
  ...
  close MYFILE;
};

# catch the "exception"
if ($@) {
  print "Errors during file operation: $@\n";
}

For more, search for "exception" on CPAN; there are several modules that provide more elaborate exception handling mechanisms.