Question about eval

tiny_tim has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Question about eval by imp (Priest) on Jan 16, 2007 at 07:16 UTC
require actually requires the file/module, so an error is raised when the file does not exist. You should wrap the require in an eval block (or expression). When using eval it is usually better to have the last statement within the eval always return true, and use that instead of $@ when testing for success. e.g. `if (eval "something; 1;") {}` eval and require: `if(eval { require "foo.pm"; 1; } ) { print "loaded module ok\n"; }else{ print "could not load module $@\n"; }` [download] eval and use: `if(eval " use foo; 1" ) { print "loaded module ok\n"; }else{ print "could not load module $@\n"; }` [download] As for this line: `print petrol "hello there !";` [download] The reason why that does not generate an error is that the first argument to print can be a bareword, which is interpreted as a filehandle name. e.g.: `print STDOUT "hello\n";` [download] If you use strict and warnings (which you always should) then you will receive this error: `Unquoted string "petrol" may clash with future reserved word at p line + 3. Name "main::petrol" used only once: possible typo at p line 3. print() on unopened filehandle petrol at p line 3.` [download]	[reply] [d/l] [select]
Re: Question about eval by sgifford (Prior) on Jan 16, 2007 at 07:18 UTC
At what point of processing the script does an eval function get parsed ? Is it not compile time ? Or is it run time ? There are two types of `eval` which behave quite differently: `eval STRING` and `eval BLOCK`. If `eval` is passed a string parameter, it is parsed and compiled at runtime; if it is passed an unquoted block of code, it is parsed and compiled at compile-time. Also, why does `if(require "module/that/does/not/exist") { print "loaded module ok\n"; }else{ print "could not load module $@\n"; }` [download] not work ? `require` `die`s if it can't find the file it's looking for, rather than simply returning an error. `require` is a sort of assertion, and if it fails it's considered too severe to keep on running. if I do `eval { require "module/that/does/not/exist"; }` [download] nothing happens ? Do I have to check $@ after eval ? Do i have to check if $@ is defined after every eval call ? Yes, you have to check `$@` after every `eval` to see if anything in the `eval` failed. Also, why does this small print statement not generate error ? `print petrol "hello there !";` [download] If you run perl without warnings and strict, undeclared variables simply spring into existence. In this case, `petrol` is created the first time you use it. It doesn't go anywhere, so output to it is simply discarded. If you add: `use warnings; use strict;` [download] to the top of your Perl code, you'll get an error if you try to do this. I recommend starting off all of your scripts that way. -- sgifford's Web page	[reply] [d/l] [select]
Re: Question about eval by shigetsu (Hermit) on Jan 16, 2007 at 06:59 UTC
You might want to read about eval. eval is sometimes used to probe loading of modules and act accordingly when loading fails by examining `$@`. `eval { require Foo::Bar }; die $@ if $@;` [download] is "functionally" almost equal to `use Foo::Bar;` [download] If you read carefully through the documentation, you'll see there are many other interesting use cases for eval. You should also give Super Search a try since others have already provided useful insight.	[reply] [d/l] [select]
Re: Question about eval by DrHyde (Prior) on Jan 16, 2007 at 10:24 UTC
Lots of people have told you lots of useful things. But they're all details. I recommend that you go and buy a copy of "Learning Perl" (aka the Llama book). Once you've understood it, go and buy a copy of "Programming Perl" (the camel book). All will become clear.	[reply]
Re: Question about eval by tiny_tim (Sexton) on Jan 16, 2007 at 07:39 UTC
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 :)	[reply]
Re^2: Question about eval by shmem (Chancellor) on Jan 16, 2007 at 08:53 UTC
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 = '$stdoutprint'\n"; __END__ Name "main::JUNK" used only once: possible typo at - line 2. print() on unopened filehandle JUNK at - line 2. Use of uninitialized value in concatenation (.) or string at - line 3. junkprint = '' stdoutprint = '1'` [download] Read the eval, do, require and use entries in perlfunc, as well as perlfaq8 which contains the difference between do, require and use. Then there is Super Search. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l]
Re^2: Question about eval by ikegami (Patriarch) on Jan 16, 2007 at 15:58 UTC
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"; }` [download] 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"; }` [download]	[reply] [d/l] [select]
Re^2: Question about eval by ikegami (Patriarch) on Jan 16, 2007 at 15:55 UTC
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.	[reply]
Re^2: Question about eval by lbjay (Novice) on Jan 17, 2007 at 20:59 UTC
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: $@\n"; }` [download] For more, search for "exception" on CPAN; there are several modules that provide more elaborate exception handling mechanisms.	[reply] [d/l]
Re: Question about eval by jesuashok (Curate) on Jan 16, 2007 at 07:08 UTC
novice 'die' help requested and eval problem Sounds very interesting	[reply]
Re^2: Question about eval by tiny_tim (Sexton) on Jan 16, 2007 at 07:15 UTC
Care to explain the `print petrol "hi\n";` no error question ? I don't understand. Show me some examples please jesuashok.	[reply] [d/l]
Re^3: Question about eval by logie17 (Friar) on Jan 16, 2007 at 18:46 UTC
If you enable warnings and strict the following is displayed: `Unquoted string "petrol" may clash with future reserved word at ./blah +.pl line 5. Name "main::petrol" used only once: possible typo at ./blah.pl line 5. print() on unopened filehandle petrol at ./blah.pl line 5.` [download] Cheers, s;;5776?12321=10609$d=9409:12100$xx;;s;(\d*);push @_,$1;eg;map{print chr(sqrt($_))."\n"} @_;	[reply] [d/l]


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