In Perl, you call modules using :: as path separator. So, if you have a module on site/lib/GD/Image.pm, you call use GD::Image.
However, long time ago I found out that you can also call use GD'Image and things like my $img = new GD'Image;, and there are also modules on CPAN using that syntax on ther names/documentation.
What is the purpose or logic behind that? Is it maybe, as many things in Perl, just a feature intended to humanize sentences and allow you to create and use modules like Acme::Don't?
Does it have any other intention different to ::?
See perlmod for explanation:
The old package delimiter was a single quote, but double colon is now the preferred delimiter
So, the reason is history.
The single quote is an old ADA separator. However, it didn't play well with Emacs, so the double colon became used.
Good God! ADA? Emacs? I am old.
Related
I was writing some Perl code in vim and accidentally typed a single quote character in a variable name and noticed that it highlighted it in a different color than normal single quoted strings.
I thought that was odd, so I wrote a small test program (shown above) and tried to run it to see how Perl would handle it and I got this error:
"my" variable $var::with::apostrophes can't be in a package
What exactly is going on here? Are there situations where single quotes in variable names are actually valid? If so, what meaning do single quotes have when used in this context?
The single quote is the namespace separator used in Perl 4, replaced by the double colon :: in Perl 5. Because Perl is mostly backwards compatible, this still works. It's great for golfing, but not much else.
Here's an article about it on perl.com that doesn't explain it.
I know all about the history of different OSes having different path formats, but at this point in time there seems to be a general agreement (with one sorta irrelevant holdout*) about how paths work. I find the whole File::Spec route of path management to be clunky and a useless pain.
Is it really worth having this baroque set of functions to manipulate paths? Please convince me I am being shortsighted.
* Irrelevant because even MS Windows allows forward slashes in paths, which means the only funky thing is the volume at the start and that has never really been a problem for me.
Two major systems have volumes. What's the parent of C:? In unix, it's C:/... In Windows, it's C:... (Unfortunately, most people misuse File::Spec to the point of breaking this.)
There are three different set of path separators in the major systems. The fact that Windows supports "/" could simplify building paths, but it doesn't help in parsing them or to canonising them.
File::Spec also provides useful functions that make it useful even if every system did use the same style of paths, such as the one that turns a path into a relative path.
That said, I never use File::Spec. I use Path::Class instead. Without sacrificing any usability or usefulness, Path::Class provides a much better interface. And it doesn't let users mishandle volumes.
For usual file management inside Perl, No, File::Spec is not necessary and using forward slahes everywhere makes much less pain and works on Win32 anyways.
cpanminus is a good example used by lots of people and have been proved work great on win32 platform. it doesn't use File::Spec for most file path manipulation and just uses forward slashes - that was even suggested so by the experienced Perl-Win32 developers.
The only place I had to use File::Spec's catfile in cpanm, though, is where I extract file paths from a perl error message (Can't locate File\Path.pm blah blah) and create a file path to pass to the command line (i.e. cmd.exe).
Meanwhile File::Spec provides useful functions such as canonical and rel2abs - that's not "necessary" per se but really useful.
Yes absolutely.
Golden rule of programming, never hard code string literals.
Edit: One of the best ways to avoid porting issues is to avoid OS specific constants especially in the form of inline literals.
i.e e.g drive + ":/" + path + "/" + filename
It is bad practice yet We all commit these attrocities in the haste of the moment or because it doesn't matter for that piece of code. File::Spec is there for when a programmer is adhering to gospel programming.
In addition it provides the values of special and often used system directories e.g tmp or devnull which can vary from one distribution/OS to another.
If anything it could probably do with some other members added to it like user to point to the users home directory
makepp (makepp.sourceforge.net) has a makefile variable $/ which is either / or \ (on non-Cygwin Win). The reason is that Win accepts / in filenames, but not in command names (where it starts an option).
From http://perldoc.perl.org/File/Spec.html:
catdir
Concatenate two or more directory names to form a complete path ending with a directory. But remove the trailing slash from the resulting string, because it doesn't look good, isn't necessary and confuses OS/2. Of course, if this is the root directory, don't cut off the trailing slash :-)
So for example in this example I wouldn't need the regex to remove the trailing slash if I would use catdir.
I am screwing around with a tiny script I am making and one thing I am trying to figure out is how to make a perl variable reflect an executable, for example.
$putty = C:\putty.exe;
When ever I run it like this it tells me "C:\ is not recognizable command, what am I doing wrong? I have also tried surrounding it in quotes and no help by that.
You should be quoting literal strings, for example like
my $putty = 'C:\putty.exe';
If this is news to you, you might have been missing out on the strict pragma before. I highly recommend having a look at that and using it in all of your code.
Something I keep doing is removing comments from a file as I process it. I was was wondering if there a module to do this.
Sort of code I keep writing time and again is
while(<>) {
s/#.*// ;
next if /^ \s+ $/x ;
**** do something useful here ****
}
Edit Just to clarify, the input is not Perl. It is a text file of my own making that might have data I want to process in some way or other. I want to beable to place comments that are ignored by my programs
Unless this is a learning experience I suggest you use Regexp::Common::comment instead of writing your own regular expressions.
It supports quite a few languages.
The question does not make clear what type of file it is. Are we dealing with perl source files? If so, your approach is not entirely correct - see gbacon's comment. Perl source files are notoriously difficult (impossible?) to parse with regex. In that case, or if you need to deal with several types of files, use Regexp::Common::comment as suggested by Niffle. Otherwise, if you think your regex logic is correct for your scenario, then I personally prefer to write it explicitly, it's just a pair of strighforward lines, there is little to be gained by using a module (and you introduce a dependency).
I'm trying to understand someone else's Perl code without knowing much Perl myself. I would appreciate your help.
I've encountered a Perl function along these lines:
MyFunction($arg1,$arg2__size,$arg3)
Is there a meaning to the double-underscore syntax in $arg2, or is it just part of the name of the second argument?
There is no specific meaning to the use of a __ inside of a perl variable name. It's likely programmer preference, especially in the case that you've cited in your question. You can see more information about perl variable naming here.
As in most languages underscore is just part of an identifier; no special meaning.
But are you sure it's Perl? There aren't any sigils on the variables. Can you post more context?
As far as the interpreter is concerned, an underscore is just another character allowed in identifiers. It can be used as an alternative to concatenation or camel case to form multi-word identifiers.
A leading underscore is often used to mean an identifier is for local use only, e.g. for non-exported parts of a module. It's merely a convention; the interpreter doesn't care.
In the context of your question, the double underscore doesn't have any programmatic meaning. Double underscores does mean something special for a limited number of values in Perl, most notably __FILE__ & __LINE__. These are special literals that aren't prefixed with a sigil ($, % or #) and are only interpolated outside of quotes. They contain the full path & name of the currently executing file and the line that is being executed. See the section on 'Special Literals' in perldata or this post on Perl Monks
I'm fairly certain arg2__size is just the name of a variable.
Mark's answer is of course correct, it has no special meaning.
But I want to note that your example doesn't look like Perl at all. Perl variables aren't barewords. They have the sigils, as you will see from the links above. And Perl doesn't have "functions", it has subroutines.
So there may be some confusion about which language we're talking about.
You will need to tell the interpreter that "$arg2" is the name of a variable. and not "$arg2__size". For this you will need to use the parenthesis. (This usage is similar to that seen in shell).
This should work
MyFunction($arg1,${arg2}__size,$arg3)
--Binu