<?xml version="1.0" encoding="utf-8"?>
<document xmlns="http://www.shlomifish.org/open-source/projects/XML-Grammar/Vered/" xmlns:xlink="http://www.w3.org/1999/xlink" version="0.2.0" xml:lang="en-GB" xml:id="index">
<info>
<title>Perl Elements to Avoid</title>
</info>
<body>
<preface xml:id="intro">
<info>
<title>Introduction</title>
</info>
<p>
Often when people ask for help with Perl code, they show Perl code that
suffers from many bad or outdated elements. This is expected, as there
are many bad Perl tutorials out there, and lots of bad code that people
have learned from, but it is still not desirable. In order to not get
"yelled at" for using these, here is the document of the bad elements that
people tend to use and some better practices that should be used instead.
</p>
<p>
A book I read said, that as opposed to most previous idea systems, they
were trying to <b>liquidate negatives</b> instead of instilling positives
in people. So in the spirit of liquidating negatives, this tutorial-in-reverse
aims to show you what <b>not to do</b>.
</p>
<p>
<strong>Note:</strong> Please don't think this advice is meant as gospel.
There are some instances where one can expect to deviate from it, and a lot
of it can be considered only my own opinion. I tried to filter the various
advice I found in the <a xlink:href="#sources_of_advice">sources for this advice</a>
and get rid of things that are a matter of taste or not so critical, or have
arguments here or there (so-called <a xlink:href="http://bikeshed.com/">colour of
the bike shed arguments</a>), but some of the advice here may still be
controversial.
</p>
</preface>
<section xml:id="bad-elements">
<info>
<title>The List of Bad Elements</title>
</info>
<item xml:id="no-indentation">
<info>
<title>No Indentation</title>
</info>
<p>
<a xlink:href="http://en.wikipedia.org/wiki/Indent_style">Indentation</a> means
that the contents of every block are promoted from their containing environment
by using a shift of some space. This makes the code easier to read and follow.
</p>
<p>
Code without indentation is harder to read and so should be avoided.
<a xlink:href="http://en.wikipedia.org/wiki/Indent_style">The Wikipedia article</a>
lists several styles - pick one and follow it.
</p>
</item>
<item xml:id="no-strict-and-warnings">
<info>
<title>No "use strict;" and "use warnings;"</title>
</info>
<p>
All modern Perl code should have the "use strict;" and "use warnings;" pragmas
that prevent or warn against misspelling variable names, using undefined
values, and other bad practices. So start your code (in every file) with this:
</p>
<code_blk syntax="perl">
use strict;
use warnings;
</code_blk>
<p>
Or:
</p>
<code_blk syntax="perl">
package MyModule;
use strict;
use warnings;
</code_blk>
</item>
<item xml:id="open-function-style">
<info>
<title>Correct style for using the open function</title>
</info>
<p>
<a xlink:href="http://perldoc.perl.org/functions/open.html">The open function</a>
is used to open files, sub-processes, etc. The correct style for it is:
</p>
<code_blk syntax="perl">
open my $input_fh, "<", $input_filename
or die "Could not open '$input_filename' - $!";
</code_blk>
<p>
some <b>wrong</b>, insecure and/or outdated styles are:
</p>
<bad_code syntax="perl">
# Bareword filehandle (type glob), two arguments open (insecure) and no
# error handling
open INPUT, "<$filename";
# Also opens from $INPUT.
open INPUT;
# Bareword filehandle with three args open and no exception thrown.
open INPUT, "<", $filename;
# Bareword filehandle with two-args open and exception (rare, but possible):
open INPUT, "<$filename"
or die "Cannot open $filename - $!";
# Lexical file handle with two-args open (instead of three-args open)
# and no exception
open my $input_fh, "<$filename";
</bad_code>
</item>
<item xml:id="calling-variables-file">
<info>
<title>Calling variables "file"</title>
</info>
<p>
Some people call their variables "file". However, file can mean either
<a xlink:href="http://en.wikipedia.org/wiki/File_descriptor">file handles</a>,
file names, or the contents of the file. As a result, this should be avoided
and one can use the abbreviations "fh" for file handle, or "fn" for filenames
instead.
</p>
</item>
<item xml:id="identifiers-without-underscores">
<info>
<title>Identifiers without underscores</title>
</info>
<p>
Some people name their identifiers as several words all in lowercase and
not separated by underscores ("_"). As a result, this makes the code harder
to read. So instead of:
</p>
<code_blk syntax="perl">
my @namesofpresidents;
</code_blk>
<p>
Say:
</p>
<code_blk syntax="perl">
my @names_of_presidents;
</code_blk>
<p>
Or maybe:
</p>
<code_blk syntax="perl">
my @presidents_names;
</code_blk>
</item>
<item xml:id="prototypes">
<info>
<title>Don't use prototypes for subroutines</title>
</info>
<p>
Some people are tempted to declare their subroutines using
<code>sub my_function ($$@)</code>, with the signature of the accepted parameter
types, which is called a prototype. However, this tends to break code more
often than not, and should be avoided.
</p>
<p>
If you're looking for parameter lists to functions and methods, take a look
at <cpan_self_dist d="Devel-Declare" /> from
CPAN. But don't use prototypes.
</p>
<p>
For more information, see:
</p>
<ol>
<li>
<p>
<a xlink:href="https://www.socialtext.net/perl5/prototype">Discussion on the Perl 5 Wiki</a>
</p>
</li>
<li>
<p>
<a xlink:href="http://stackoverflow.com/questions/297034/why-are-perl-5s-function-prototypes-bad">“Why
are Perl 5’s function prototypes bad?”</a> on Stack Overflow.
</p>
</li>
</ol>
</item>
<item xml:id="ampersand-in-subroutine-calls">
<info>
<title>Ampersand in Subroutine Calls</title>
</info>
<p>
One should not call a subroutine using <code>&myfunc(@args)</code> unless
you're sure that is what you want to do (like overriding prototypes). Normally
saying <code>myfunc(@args)</code> is better.
</p>
<p>
For more information see
<a xlink:href="https://www.socialtext.net/perl5/subroutines_called_with_the_ampersand">the
relevant page</a> on the Perl 5 Wiki.
</p>
</item>
<item xml:id="assigning-from-dollar-underscore">
<info>
<title>Assigning from $_</title>
</info>
<p>
Some people write code like the following:
</p>
<code_blk syntax="perl">
while (<$my_fh>)
{
my $line = $_;
# Do something with $line…
}
</code_blk>
<p>
Or:
</p>
<code_blk syntax="perl">
foreach (@users)
{
my $user = $_;
# Process $user…
}
</code_blk>
<p>
However, you can easily assign the explicit and lexical variables in the
loop's opening line like so:
</p>
<code_blk syntax="perl">
while (my $line = <$my_fh>)
{
# Do something with $line…
}
</code_blk>
<p>
and:
</p>
<code_blk syntax="perl">
foreach my $user (@users)
{
# Process $user…
}
</code_blk>
</item>
<item xml:id="foreach-lines">
<info>
<title>Using "foreach" on lines</title>
</info>
<p>
Some people may be tempted to write this code:
</p>
<code_blk syntax="perl">
foreach my $line (<$my_file_handle>)
{
# Do something with $line.
}
</code_blk>
<p>
This code appears to work but what it does is read the entire contents of the
file pointed by $my_file_handle into a (potentially long) list of lines, and
then iterate over them. This is inefficient. In order to read one line
at a time, use this instead:
</p>
<code_blk syntax="perl">
while (my $line = <$my_file_handle>)
{
# Do something with $line.
}
</code_blk>
</item>
<item xml:id="string-notation">
<info>
<title>String Notation</title>
</info>
<p>
Perl has a flexible way to write strings and other delimiters, and you should
utilize it for clarity. If you find yourself writing long strings, write them
as <a xlink:href="http://en.wikipedia.org/wiki/Here_document">here-documents</a>:
</p>
<code_blk syntax="perl">
my $long_string_without_interpolation = <<'EOF';
Hello there. I am a long string.
I am part of the string.
And so am I.
EOF
</code_blk>
<p>
There are also <code><<"EOF"</code> for strings with interpolation
and <code><<`EOF`</code> for trapping command output. Make sure you never
use bareword here documents <code><<EOF</code> which are valid syntax,
but many people are never sure whether they are <code><<"EOF"</code> or
<code><<'EOF'</code>.
</p>
<p>
If your strings are not too long but contain the special characters that
correspond with the default delimiters (e.g: <code>'</code>, <code>"</code>,
<code>`</code>, <code>/</code> etc.), then you can use the initial letter followed
by any arbitrary delimiter notation: <code>m{\A/home/sophie/perl}</code>,
<code>q/My name is 'Jack' and I called my dog "Diego"./</code>.
</p>
</item>
<item xml:id="slurp">
<info>
<title>Slurping a file (i.e: Reading it all into memory)</title>
</info>
<p>
One can see several bad ways to read a file into memory in Perl. Among them
are:
</p>
<code_blk syntax="perl">
# Not portable and suffers from possible
# shell code injection.
my $contents = `cat $filename`;
# Wasteful of CPU and memory:
my $contents = join("", <$fh>);
# Even more so:
my $contents = '';
while (my $line = <$fh>)
{
$contents .= $line;
}
</code_blk>
<p>
You should avoid them all. Instead the proper way to read an entire file
into a long string is to either use CPAN distributions for that such as
<cpan_self_dist d="File-Slurp" /> or
<cpan_self_dist d="IO-All" />, or alternatively
write down the following function and use it:
</p>
<code_blk syntax="perl">
sub _slurp
{
my $filename = shift;
open my $in, '<', $filename
or die "Cannot open '$filename' for slurping - $!";
local $/;
my $contents = <$in>;
close($in);
return $contents;
}
</code_blk>
</item>
<item xml:id="paragraphs">
<info>
<title>Write code in Paragraphs using Empty Lines</title>
</info>
<p>
If one of your blocks is long, split it into "code paragraphs", with empty
lines between them and with each paragraph doing one thing. Then, it may be a
good idea to precede each paragraph with a comment explaining what it does, or
to extract it into its own function or method.
</p>
</item>
<item xml:id="io-socket">
<info>
<title>Use IO::Socket and friends instead of lower-level calls</title>
</info>
<p>
One should use <cpan_mod m="IO::Socket">the IO::Socket</cpan_mod> family of
modules for networking Input/Output instead of the
lower-level socket()/connect()/bind()/etc. calls. As of this writing,
<pdoc d="perlipc"></pdoc> contains outdated information demonstrating how
to use the lower-level API which is not recommended.
</p>
</item>
<item xml:id="subroutine-arguments">
<info>
<title>Subroutine Arguments Handling</title>
</info>
<p>
The first thing to know about handling arguments for subroutines is to avoid
referring to them directly by index. Imagine you have the following code:
</p>
<code_blk syntax="perl">
sub my_function
{
my $first_name = $_[0];
my $street = $_[1];
my $city = $_[2];
my $country = $_[3];
.
.
.
}
</code_blk>
<p>
Now, what if you want to add <code>$last_name</code> between <code>$first_name</code>
and <code>$street</code>?
You'll have to promote all the indexes after it! Moreover, this scheme
is error-prone and you may reuse the same index more than once, or
miss some indexes.
</p>
<p>
Instead do either:
</p>
<code_blk syntax="perl">
sub my_function
{
my $first_name = shift;
my $street = shift;
my $city = shift;
my $country = shift;
.
.
.
}
</code_blk>
<p>
Or:
</p>
<code_blk syntax="perl">
sub my_function
{
my ($first_name, $street, $city, $country) = @_;
.
.
.
}
</code_blk>
<p>
The same thing holds for unpacking <code>@ARGV</code>, the array containing the
command-line arguments for a Perl program, or any other array. Don't use
<code>$ARGV[0]</code>, <code>$ARGV[1]</code> etc. directly, but instead unpack
<code>@ARGV</code> using the methods given above. For processing
command line arguments, you should also consider using
<cpan_self_mod m="Getopt::Long" />.
</p>
<item xml:id="clobbering-arrays-or-hashes">
<info>
<title>Don’t clobber arrays or hashes</title>
</info>
<p>
Often people ask how to pass arrays or hashes to subroutines. The answer is
that the right way to do it is to pass them as a reference as an argument
to the subroutine:
</p>
<code_blk syntax="perl">
sub calc_polynomial
{
my ($x, $coefficients) = @_;
my $x_power = 1;
my $result = 0;
foreach my $coeff (@{$coefficients})
{
$result += $coeff * $x_power;
}
continue
{
$x_power *= $x;
}
return $result;
}
print "(4*x^2 + 2x + 1)(x = 5) = ", calc_polynomial(5, [1, 2, 4]);
</code_blk>
<p>
You shouldn't clobber the subroutine's arguments list with entire arrays
or hashes (e.g: <code>my_func(@array1, @array2);</code> or
<code>my_func(%myhash, $scalar)</code> ), as this will make it difficult to
extract from <code>@_</code>.
</p>
</item>
<item xml:id="named-parameters">
<info>
<title>Named Parameters</title>
</info>
<p>
If the number of parameters that your subroutine accepts gets too long, or
if you have too many optional parameters, make sure you convert it to use
named arguments. The standard way to do it is to pass a hash reference or
a hash of arguments to the subroutine:
</p>
<code_blk syntax="perl">
sub send_email
{
my $args = shift;
my $from_address = $args->{from};
my $to_addresses = $args->{to};
my $subject = $args->{subject};
my $body = $args->{body};
.
.
.
}
send_email(
{
from => 'shlomif@perl-begin.org',
to => ['shlomif@perl-begin.org', 'sophie@perl-begin.org'],
subject => 'Perl-Begin.org Additions',
.
.
.
}
);
</code_blk>
</item>
</item>
<item xml:id="chop">
<info>
<title>Avoid using chop() to trim newlines characters from lines</title>
</info>
<p>
Don't use <a xlink:href="http://perldoc.perl.org/functions/chop.html">the built-in
function chop()</a> in order to remove newline characters from the end
of lines read using the diamond operator (<code><></code>), because this
may cause the last character in a line without a line feed character to be
removed. Instead, use <a xlink:href="http://perldoc.perl.org/functions/chomp.html">chomp()</a>.
</p>
<p>
If you expect to process DOS/Windows-like text files whose lines end with the
dual Carriage Return-Line Feed character on Unix systems then use the
following in order to trim them: <code>$line =~ s/\x0d?\x0a\z//;</code>.
</p>
<p>
For more information see:
</p>
<ul>
<li>
<p>
<a xlink:href="http://onlamp.com/pub/a/onlamp/2006/08/17/understanding-newlines.html">"Understanding Newlines"</a> - by Xavier Noria on OnLAMP.com.
</p>
</li>
<li>
<p>
<a xlink:href="http://en.wikipedia.org/wiki/Newline">Wikipedia article about newlines</a>
</p>
</li>
</ul>
</item>
<item xml:id="lowercase_modules_and_pkgs">
<info>
<title>Don't start Modules and Packages with a Lowercase Letter</title>
</info>
<p>
Both modules and packages (the latter also known as namespaces) and all
intermediate components thereof should always start with an uppercase letter,
because modules and packages that start with a lowercase letter are
reserved for pragmas. So this is bad:
</p>
<bad_code syntax="perl">
# This is file person.pm
package person;
use strict;
use warnings;
1;
</bad_code>
<p>
And this would be better:
</p>
<code_blk syntax="perl">
# Better code!
# This is file MyProject/Person.pm
package MyProject::Person;
use strict;
use warnings;
.
.
.
1;
</code_blk>
</item>
<item xml:id="indirect-object-notation">
<info>
<title>Avoid Indirect Object Notation</title>
</info>
<p>
Don't use the so-called “Indirect-object notation” which can be seen in a lot
of old code and tutorials and is more prone to errors:
</p>
<bad_code syntax="perl">
my $new_object = new MyClass @params;
</bad_code>
<p>
Instead use the <code>MyClass->new(…)</code> notation:
</p>
<code_blk syntax="perl">
my $new_object = MyClass->new(@params);
</code_blk>
<p>
For more information and the motivation for this advice, see chromatic’s article
<a xlink:href="http://modernperlbooks.com/mt/2009/08/the-problems-with-indirect-object-notation.html">“The
Problems with Indirect Object Notation”</a>.
</p>
</item>
<item xml:id="dollar-dollar">
<info>
<title>$$myarray_ref[$idx] or $$myhash_ref{$key}</title>
</info>
<p>
Don't write <code>$$myarray_ref[$idx]</code>, which is cluttered and can be easily
confused with <code>(${$myarray_ref})->[$idx]</code>. Instead, use the
arrow operator - <code>$myarray_ref->[$idx]</code>. This also applies for
hash references - <code>$myhash_ref->{$key}</code>.
</p>
</item>
<item xml:id="c-style-for-loops">
<info>
<title>C-style for loops</title>
</info>
<p>
Some beginners to Perl tend to use C-style-for-loops to loop over an array's
elements:
</p>
<code_blk syntax="perl">
for (my $i=0 ; $i < @array ; $i++)
{
# Do something with $array[$i]
}
</code_blk>
<p>
However, iterating over the array itself would normally be preferable:
</p>
<code_blk syntax="perl">
foreach my $elem (@array)
{
# Do something with $elem.
}
</code_blk>
<p>
If you still need the index, do:
</p>
<code_blk syntax="perl">
foreach my $idx (0 .. $#array)
{
my $elem = $array[$idx];
# Do something with $idx and $elem.
}
# perl-5.12.0 and above:
foreach my $idx (keys(@array))
{
my $elem = $array[$idx];
# Do something with $idx and $elem.
}
# Also perl-5.12.0 and above.
while (my ($idx, $elem) = each(@array))
{
# Do something with $idx and $elem.
}
</code_blk>
<p>
An arbitrary C-style for loop can be replaced with a while loop with
a “continue” block.
</p>
</item>
<item xml:id="non-intrusive-commenting">
<info>
<title>Avoid Intrusive Commenting</title>
</info>
<p>
Some commenting is too intrusive and interrupts the flow of reading the code.
Examples for that are the <code>########################</code> hard-rules that
some people put in their code, the comments using multiple
<a xlink:href="http://en.wikipedia.org/wiki/Number_sign">number signs ("#")</a>,
like <code>####</code>, or excessively long comment block. Please avoid all those.
</p>
<p>
Some schools of software engineering argue that if the code's author feels
that a comment is needed, it usually indicates that the code is not clear
and should be factored better (like extracting a method or a subroutine with
a meaningful name.). It probably does not mean that you should avoid writing
comments altogether, but excessive commenting could prove as a red flag.
</p>
<p>
If you're interested in documenting the public interface of your modules and
command-line programs, refer to <pdoc d="perlpod">, Perl's Plain Old
Documentation (POD)</pdoc>, which allows one to quickly and easily document
one's code. POD has
<a xlink:href="http://search.cpan.org/search?query=pod&mode=all">many extensions
available on CPAN</a>, which may prove of use.
</p>
</item>
<item xml:id="accessing_object_slots_directly">
<info>
<title>Accessing Object Slots Directly</title>
</info>
<p>
Since <a xlink:href="../../topics/object-oriented/">Perl objects</a> are simple
references some programmers are tempted to access them directly:
</p>
<bad_code syntax="perl">
$self->{'name'} = "John";
print "I am ", $self->{'age'}, " years old\n";
# Or even: (Really bad code)
$self->[NAME()] = "John";
</bad_code>
<p>
However, this is sub-optimal as explained in
<a xlink:href="http://www.shlomifish.org/lecture/Perl/Newbies/lecture5/accessors/">the
Perl
for Newbies section about "Accessors"</a>, and one should use accessors
using code like that:
</p>
<code_blk syntax="perl">
# Good code.
$self->_name("John");
print "I am ", $self->_age(), " years old\n";
</code_blk>
<p>
As noted in the link, you can use one of CPAN's many accessor generators to
generate accessors for you.
</p>
</item>
<item xml:id="caret_and_dollar_sign_in_regexes">
<info>
<title>'^' and '$' in Regular Expressions</title>
</info>
<p>
Some people use "^" and "$" in regular expressions to mean
beginning-of-the-string or end-of-the-string. However, they can mean
beginning-of-a-line and end-of-a-line respectively using the <code>/m</code> flag
which is confusing. It's a good idea to use <code>\A</code> for start-of-string
and <code>\z</code> for end-of-string always, and to specify the <code>/m</code> flag
if one needs to use "^" and "$" for start/end of a line.
</p>
</item>
<item xml:id="magic_numbers">
<info>
<title>Magic Numbers</title>
</info>
<p>
Your code should not include <a xlink:href="http://en.wikipedia.org/wiki/Magic_number_%28programming%29#Unnamed_numerical_constants">unnamed
numerical constants also known as "magic numbers" or "magic constants"</a>.
For example, there is one in this code to shuffle a deck of cards:
</p>
<bad_code syntax="perl">
for my $i (0 .. 51)
{
my $j = $i + int(rand(52-$i));
@cards[$i,$j] = @cards[$j,$i];
}
</bad_code>
<p>
This code is bad because the meaning of 52 and 51 is not explained and they
are arbitrary. A better code would be:
</p>
<code_blk syntax="perl">
# Good code.
# One of:
my $deck_size = 52;
Readonly my $deck_size => 52;
for my $i (0 .. $deck_size-1)
{
my $j = $i + int(rand($deck_size-$i));
@cards[$i,$j] = @cards[$j,$i];
}
</code_blk>
<p>
(Of course in this case, you may opt to use a shuffle function from CPAN,
but this is just for the sake of demonstration.).
</p>
</item>
<item xml:id="vars_in_quotes">
<info>
<title>String Variables Enclosed in Double Quotes</title>
</info>
<p>
One can sometimes see people write code like that:
</p>
<bad_code syntax="perl">
my $name = shift(@ARGV);
print "$name", "\n";
if ("$name" =~ m{\At}i)
{
print "Your name begins with the letter 't'";
}
</bad_code>
<p>
However, it's not necessary to enclose $name in double quotes (i.e:
<code>"$name"</code>) because it's already a string. Using it by itself as
<code>$name</code> will do just fine:
</p>
<code_blk syntax="perl">
# Better code.
my $name = shift(@ARGV);
print $name, "\n";
if ($name =~ m{\At}i)
{
print "Your name begins with the letter 't'";
}
</code_blk>
<p>
Also see <a xlink:href="../../uses/text-generation/">our page about text
generation</a> for other ways to delimit text.
</p>
<p>
Note that sometimes enclosing scalar variables in double-quotes makes sense -
for example if they are objects with overloaded stringification. But this is
the exception rather than the rule.
</p>
</item>
<item xml:id="at-array-for-subscripting">
<info>
<title>@array[$idx] for array subscripting</title>
</info>
<p>
Some newcomers to Perl 5 would be tempted to write <code>@array[$index]</code>
to subscript a single element out of the array <code>@array</code>. However,
<code>@array[$index]</code> is a single-element array <b>slice</b>. To get
a single subscript out of <code>@array</code> use <code>$array[$idx]</code> (with
a dollar sign). Note that if you want to extract several elements, you can
use an array slice such as <code>@array[@indexes]</code> or
<code>@array[$x,$y] = @array[$y,$x]</code>. However, then it's a list which should
be used in list context.
</p>
</item>
<item xml:id="vars-a-and-b">
<info>
<title>Variables called $a and $b</title>
</info>
<p>
One should not create lexical variables called <code>$a</code> and <code>$b</code>
because there are built-in-variables called that used for
<pdoc_f f="sort">sorting</pdoc_f> and other uses (such as reduce in
<cpan_self_mod m="List::Util" />), which the lexical variables will interfere
with:
</p>
<bad_code syntax="perl">
my ($a, $b) = @ARGV;
.
.
.
# Won't work now.
my @array = sort { length($a) <=> length($b) } @other_array;
</bad_code>
<p>
Instead, use other single-letter variable names such as
<code>$x</code> and <code>$y</code>, or better yet give more descriptive names.
</p>
</item>
<item xml:id="flow-stmts-without-labels">
<info>
<title>Flow Control Statements Without an Explicit Label</title>
</info>
<p>
One can sometimes see flow-control statements such as
<pdoc_f f="next">next</pdoc_f>, <pdoc_f f="last">last</pdoc_f> or
<pdoc_f f="redo">redo</pdoc_f> used without an explicit label following
them, in which case they default to re-iterating or breaking out of the
innermost loop. However, this is inadvisable, because later on, one may modify
the code to insert a loop in between the innermost loop and the flow control
statement, which will break the code. So always append a label to "next",
"last" and "redo" and label your loops accordingly:
</p>
<code_blk syntax="perl">
LINES:
while (my $line = <>)
{
if ($line =~ m{\A#})
{
next LINES;
}
}
</code_blk>
</item>
<item xml:id="abuse_of_array_last_index">
<info>
<title>($#array + 1) and Other Abuses of $#.</title>
</info>
<p>
The <code>$#array</code> notation gives the last index in <code>@array</code> and
is always equal to the array length minus one. Some people use it to signify
the length of the array:
</p>
<bad_code syntax="perl">
my @flags = ((0) x ($#names +1))
</bad_code>
<p>
However this is unnecessary because one can better do it by evaluating
<code>@names</code> in scalar context, possibly by saying <code>scalar(@names)</code>.
</p>
<code_blk syntax="perl">
# Better code.
my @flags = ((0) x @names);
</code_blk>
</item>
<item xml:id="last_elems_of_array">
<info>
<title>$array[$#array], $array[$#array-1], etc.</title>
</info>
<p>
One can sometimes see people references the last elements of arrays using
notation such as <code>$array[$#array]</code>, <code>$array[$#array-1]</code>
or even <code>$array[scalar(@array)-1]</code>. This duplicates the identifier
and is error prone and there's a better way to do it in Perl using
negative indexes. <code>$array[-1]</code> is the last element of the array,
<code>$array[-2]</code> is the second-to-last, etc.
</p>
</item>
<item xml:id="re_string_interpolate">
<info>
<title>Interpolating Strings into Regular Expressions</title>
</info>
<p>
One can often see people interpolate strings directly into regular expressions:
</p>
<bad_code syntax="perl">
my $username = shift(@ARGV);
open my $pass_fh, '<', '/etc/passwd'
or die "Cannot open /etc/passwd - $!";
PASSWD:
while (my $line = <$pass_fh>)
{
if ($line =~ m{\A$username}) \# Bad code here.
{
print "Your username is in /etc/passwd\n";
last PASSWD;
}
}
close($pass_fh);
</bad_code>
<p>
The problem is that when a string is interpolated into a regular expression
it is interpolated as a mini-regex, and special characters there behave like
they do in a regular expression. So if I input <code>'.*'</code> into the command
line in the program above, it will match all lines. This is a special case
of <a xlink:href="http://community.livejournal.com/shlomif_tech/35301.html">code
or markup injection</a>.
</p>
<p>
The solution to this is to use \Q and \E to signify a
<pdoc_f f="quotemeta">quotemeta()</pdoc_f> portion that will treat the
interpolated strings as plaintext with all the special characters escaped.
So the line becomes: <code>if ($line =~ m{\A\Q$username\E})</code>.
</p>
<p>
Alternatively, if you do intend to interpolate a sub-regex, signify this
fact with a comment. And be careful with regular expressions that are accepted
from user input.
</p>
</item>
<item xml:id="overuse_dollar_underscore">
<info>
<title>Overusing $_</title>
</info>
<p>
It's a good idea not to overuse <code>$_</code> because using it, especially in
large scopes, is prone to errors, including many subtle ones. Most Perl
operations support operating on other variables and you should use lexical
variables with meaningful names instead of $_ whenever possible.
</p>
<p>
Some places where you have to use <code>$_</code> are <pdoc_f f="map">map</pdoc_f>,
<pdoc_f f="grep">grep</pdoc_f> and other functions like that, but even in
that case it might be desirable to set a lexical variable to the value of
<code>$_</code> right away: <code>map { my $line = $_; … } @lines</code>.
</p>
</item>
<item xml:id="mixing_tabs_and_spaces">
<info>
<title>Mixing Tabs and Spaces</title>
</info>
<p>
Some improperly configured text editors may be used to write code that, while
indented well at a certain tab size looks terrible on other tab sizes, due
to a mixture of tabs and spaces. So either use tabs for indentation or make
sure your tab key expands to a constant number of spaces. You may also wish
to make use of <cpan_self_dist d="Perl-Tidy" /> to properly format your
code.
</p>
</item>
<item xml:id="qx_for_command_execution">
<info>
<title>`…` or qx// for Executing Commands</title>
</info>
<p>
Some people are tempted to use backticks (<code>`…`</code>) or <code>qx/…/</code>
for executing commands for their side-effects. E.g:
</p>
<bad_code syntax="perl">
use strict;
use warnings;
my $temp_file = "tempfile.txt";
`rm -f $temp_file`;
</bad_code>
<p>
However, this is not idiomatic because <code>`…`</code> and <code>qx/…/</code> are
used to trap a command's output and to return it as a big string or as a list
of lines. It would be a better idea to use
<pdoc_f f="system">system()</pdoc_f> or to seek more idiomatic Perl-based
solutions on CPAN or in the Perl core (such as using
<pdoc_f f="unlink">unlink()</pdoc_f> to delete a file in our case.).
</p>
<p>
Some people even go and ask how to make the <code>qx/…/</code> output go to
the screen, which is a clear indication that they want to use system().
</p>
</item>
<item xml:id="explicit_return">
<info>
<title>No Explicit Returns</title>
</info>
<p>
As noted in "Perl Best Practices", all functions must have an explicit
“return” statement, as otherwise they implicitly return the last
expression, which would be subject to change upon changing the code. If you
don't want the subroutine to return anything (i.e: it's a so-called
"procedure"), then write <code>return;</code> to always return a false value,
which the caller won't be able to do anything meaningful with.
</p>
<p>
Another mistake is to write "return 0;" or "return undef;" to return
false, because in list context, they will return a one-element list which
is considered true. So always type <code>return;</code> to return false.
</p>
</item>
<item xml:id="varvarname">
<info>
<title>"Varvarname" - Using a variable as another variable's name.</title>
</info>
<p>
Mark Jason Dominus has written about
<a xlink:href="http://perl.plover.com/varvarname.html">varvarname -
"Why it's stupid to `use a variable as a variable name'"</a>, namely if
<code>$myvar</code> is <code>'foobar'</code> they want to operate on the value of
<code>$foobar</code>. While there are ways to achieve similar things in Perl,
the best way is to use <a xlink:href="../../topics/hashes/">hashes</a> (possibly
pointing to complex records with more information) and lookup them by
the string you want to use. Read the link by Mark Jason Dominus for more
information.
</p>
</item>
<item xml:id="leading_underscores">
<info>
<title>Use Leading Underscores ('_') for Internal Methods and Functions</title>
</info>
<p>
When writing a module use leading underscores in identifiers of methods and
functions to signify those that are: 1. Subject to change. 2. Are used
internally by the module. 3. Should not be used from outside. By using
<cpan_self_dist d="Pod-Coverage" /> one can make sure that the external API
of the module is documented and it will skip the identifiers with leading
underscores, that can be thought of as "private" ones.
</p>
<p>
Here's an example:
</p>
<code_blk syntax="perl">
package Math::SumOfSquares;
use strict;
use warnings;
use List::Utils qw(sum);
sub _square
{
my $n = shift;
return $n * $n;
}
sub sum_of_squares
{
my ($numbers) = @_;
return sum(map { _square($_) } @$numbers);
}
1;
</code_blk>
</item>
<item xml:id="print_to_fh">
<info>
<title>print $fh @args</title>
</info>
<p>
It is preferable to write <code>print {$write_fh} @args</code>
over <code>print $write_fh @args</code> because the latter can more easily be
mistaken for <code>print $write_fh, @args</code> (which does something different)
and does not provide enough visual hints that you are writing to the
<code>$write_fh</code> filehandle. Therefore, always wrap the file-handle in
curly braces (so-called "dative block"). (Inspired by "Perl Best Practices").
</p>
</item>
<item xml:id="STDIN_instead_of_ARGV">
<info>
<title>Using STDIN instead of ARGV</title>
</info>
<p>
One can write code while reading from STDIN:
</p>
<bad_code syntax="perl">
use strict;
use warnings;
# Strip comments.
LINES:
while (my $line = <STDIN>)
{
if ($line =~ m{\A *#})
{
next LINES;
}
print $line;
}
</bad_code>
<p>
However, it is usually better to use <code>ARGV</code> instead of <code>STDIN</code>
because it also allows processing the filenames from the command line. This
can also be achieved by simply saying <code><></code>. So the code becomes:
</p>
<code_blk syntax="perl">
# Better code:
use strict;
use warnings;
# Strip comments.
LINES:
while (my $line = <>)
{
if ($line =~ m{\A *#})
{
next LINES;
}
print $line;
}
</code_blk>
</item>
<item xml:id="modifying_iterated_array">
<info>
<title>Modifying arrays or hashes while iterating through them.</title>
</info>
<p>
Some people ask about how to add or remove elements to an existing array or
hash when iterating over them using “foreach” and other loops. The
answer to that is that Perl will likely not handle it too well, and it expects
that during loops the keys of a data structure will remain constant.
</p>
<p>
The best way to achieve something similar is to populate a new array or hash
during the loop by using <pdoc_f f="push">push()</pdoc_f> or a hash lookup
and assignment. So do that instead.
</p>
</item>
<item xml:id="code_in_foreign_lang">
<info>
<title>Comments and Identifiers in a Foreign Language</title>
</info>
<p>
Apparently, many non-native English speakers write code with comments and
even identifiers in their native language. The problem with this is that
programmers who do not speak that language will have a hard time understanding
what is going on here, especially after the writers of the foreign language
code post it in to an Internet forum in order to get help with it.
</p>
<p>
Consider what Eric Raymond wrote in
<a xlink:href="http://www.catb.org/~esr/faqs/hacker-howto.html#skills4">his
"How to Become a Hacker" document</a> (where hacker is a software enthusiast
and not a computer intruder):
</p>
<blockquote>
<p>
4. If you don't have functional English, learn it.
</p>
<p>
As an American and native English-speaker myself, I have previously been
reluctant to suggest this, lest it be taken as a sort of cultural imperialism.
But several native speakers of other languages have urged me to point out that
English is the working language of the hacker culture and the Internet, and
that you will need to know it to function in the hacker community.
</p>
<p>
Back around 1991 I learned that many hackers who have English as a second
language use it in technical discussions even when they share a birth tongue;
it was reported to me at the time that English has a richer technical
vocabulary than any other language and is therefore simply a better tool for
the job. For similar reasons, translations of technical books written in
English are often unsatisfactory (when they get done at all).
</p>
<p>
Linus Torvalds, a Finn, comments his code in English (it apparently never
occurred to him to do otherwise). His fluency in English has been an important
factor in his ability to recruit a worldwide community of developers for Linux.
It's an example worth following.
</p>
<p>
Being a native English-speaker does not guarantee that you have language skills
good enough to function as a hacker. If your writing is semi-literate,
ungrammatical, and riddled with misspellings, many hackers (including myself)
will tend to ignore you. While sloppy writing does not invariably mean sloppy
thinking, we've generally found the correlation to be strong — and we have no
use for sloppy thinkers. If you can't yet write competently, learn to.
</p>
</blockquote>
<p>
So if you're posting code for public scrutiny, make sure it is written with
English identifiers and comments.
</p>
</item>
<item xml:id="perlform">
<info>
<title>Using perlform for formatting text.</title>
</info>
<p>
One should not use “perlform” for formatting text, because it makes
use of global identifiers, and should use the
<cpan_self_dist d="Perl6-Form" /> CPAN distribution instead. Also see
our <a xlink:href="../../uses/text-generation/">text generation page</a> for
more information. (Inspired by "Perl Best Practices").
</p>
</item>
<item xml:id="obj_new">
<info>
<title>Using $obj->new for object construction.</title>
</info>
<p>
Sometimes you can see class constructors such as:
</p>
<bad_code syntax="perl">
sub new
{
my $proto = shift;
my $class = ref($proto) || $proto;
my $self = {};
…
}
</bad_code>
<p>
The problem here is that this allows one to do
<code>$my_object_instance->new</code> to create a new instance of the object,
but many people will expect it to be invalid or to clone the object. So don't
do that and instead write your constructors as:
</p>
<code_blk syntax="perl">
# Better code:
sub new
{
my $class = shift;
my $self = {};
bless $self, $class;
…
}
</code_blk>
<p>
Which will disable it and will just allow
<code>ref($my_object_instance)->new(…)</code>. If you need a clone method,
then code one called "clone()" and don't use "new" for that.
</p>
<p>
(Thanks to
<a xlink:href="http://www.stonehenge.com/merlyn/UnixReview/col52.html">Randal L.
Schwartz's post "Constructing Objects"</a> for providing the insight to this).
</p>
</item>
<item xml:id="law_of_demeter">
<info>
<title>Law of Demeter</title>
</info>
<p>
See the <a xlink:href="http://en.wikipedia.org/wiki/Law_of_Demeter">Wikipedia article
about "Law of Demeter" for more information</a>. Namely, doing many nested
method calls like
<code>$self->get_employee('sophie')->get_address()->get_street()</code>
is not advisable, and should be avoided.
</p>
<p>
A better option would be to provide methods in the containing objects to
access those methods of their contained objects. And an even better way would
be to structure the code so that each object handles its own domain.
</p>
</item>
<item xml:id="delegating_parameter_passing">
<info>
<title>Passing parameters in delegation</title>
</info>
<p>
Sometimes we encounter a case where subroutines each pass the same parameter
to one another in delegation, just because the innermost subroutines in the
callstack need it.
</p>
<p>
To avoid it, create a class, and declare methods that operate on the
fields of the class, where you can assign the delegated arguments.
</p>
</item>
<item xml:id="duplicate_code">
<info>
<title>Duplicate Code</title>
</info>
<p>
As noted in
<a xlink:href="http://www.shlomifish.org/philosophy/books-recommends/#refactoring">Martin
Fowler's "Refactoring"</a> book (but held as a fact for a long time
beforehand),
<a xlink:href="http://en.wikipedia.org/wiki/Duplicate_code">duplicate code</a> is a
code smell, and should be avoided. The solution is to extract duplicate
functionality into subroutines, methods and classes.
</p>
</item>
<item xml:id="long_functions">
<info>
<title>Long Functions and Methods</title>
</info>
<p>
Another common code smell is
<a xlink:href="http://c2.com/cgi/wiki?LongMethodSmell">long
subroutines and methods</a>. The solution to these is to extract several
shorter methods out, with meaningful names.
</p>
</item>
<item xml:id="map_instead_of_foreach">
<info>
<title>Using map instead of foreach for side-effects</title>
</info>
<p>
You shouldn't be using <pdoc_f f="map">map</pdoc_f> to iterate on a list
instead of foreach if you're not interested in constructing a new list and
all you are interested in are the side-effects. For example:
</p>
<bad_code syntax="perl">
use strict;
use warnings;
map { print "Hello $_!\n"; } @ARGV;
</bad_code>
<p>
Would be better written as:
</p>
<code_blk syntax="perl">
use strict;
use warnings;
foreach my $name (@ARGV)
{
print "Hello $name!\n";
}
</code_blk>
<p>
Which better conveys one's intention and may be a bit more efficient.
</p>
</item>
<item xml:id="ternary_operator_instead_of_if_else">
<info>
<title>Using the ternary operator for side-effects instead of if/else</title>
</info>
<p>
A similar symptom to the above is people who wish to use the ternary
inline- conditional operator (<code>? :</code>) for choosing to execute between
two different statements with side-effects
instead of using <code>if</code> and <code>else</code>. For example:
</p>
<bad_code syntax="perl">
$cond_var ? ($hash{'if_true'} .= "Cond var is true")
: ($hash{'if_false'} .= "Cond var is false")
</bad_code>
<p>
(This is assuming the ternary operator was indeed written correctly, which
is not always the case).
</p>
<p>
However, the ternary operator is meant to be an expression that is a choice
between two values and should not be used for its side-effects. To do the
latter, just use <code>if</code> and <code>else</code>:
</p>
<code_blk syntax="perl">
if ($cond_var)
{
$hash{'if_true'} .= "Cond var is true";
}
else
{
$hash{'if_false'} .= "Cond var is false";
}
</code_blk>
<p>
This is safer, and better conveys one’s intentions.
</p>
<p>
For more information, refer to
<a xlink:href="http://www.nntp.perl.org/group/perl.beginners/2012/04/msg120480.html">a
relevant thread on the Perl beginners mailing list</a> (just make sure you read
it in its entirety).
</p>
</item>
<item xml:id="nested_top_level_subroutines">
<info>
<title>Nested top-level subroutines</title>
</info>
<p>
One should not nest an inner top-level subroutine declared using
<code>sub inner</code> inside of an outer one, like so:
</p>
<bad_code syntax="perl">
sub outer
{
sub inner
{
.
.
.
}
# Use inner here
}
</bad_code>
<p>
This code will compile and run, but may break in subtle ways.
</p>
<p>
The first problem with this approach is that <code>inner()</code> will still be
visible outside <code>outer()</code>, but the more serious problem is that the
inner subroutine will only get one copy of the lexical variables inside
<code>outer()</code>.
</p>
<p>
The proper and safer way to declare an inner subroutine is to declare
a lexical variable and set it to an anonymous subroutine, which is
also known as a closure:
</p>
<code_blk syntax="perl">
sub outer
{
my ($foo, $bar) = @_;
my $print_foo = sub {
print "Foo is '$foo'\n";
return;
};
$print_foo->();
$foo++;
$print_foo->();
return;
}
</code_blk>
</item>
<item xml:id="grep_instead_of_any">
<info>
<title>Using grep instead of any and friends</title>
</info>
<p>
Sometimes one can see people using <pdoc_f f="grep">grep</pdoc_f> to find
the first matching element in an array, or whether such an element exists at
all. However, grep is intended to extract <b>all</b> matching elements out
of a list, not just the first one, and as a result will not stop until it
finds them all. To remedy this look at either <code>first()</code> from
<cpan_self_mod m="List::Util" /> (to find the first match) or
"any/all/notall/none" from <cpan_self_mod m="List::MoreUtils" /> (to find
whether a single element exists). These better convey one's intention
and may be more efficient because they stop on the first match.
</p>
<p>
One should note that if one does such lookups often, then they should try
to use a <a xlink:href="../../topics/hashes/">hash</a> instead.
</p>
</item>
<item xml:id="FileHandle_module">
<info>
<title>Using the FileHandle Module</title>
</info>
<p>
The FileHandle module is old and bad, and should not be used. One should
use the <a xlink:href="http://perldoc.perl.org/IO/Handle.html">IO::Handle</a>
family of modules instead.
</p>
</item>
<item xml:id="file_includes">
<info>
<title>"Including" files instead of using Modules</title>
</info>
<p>
We are often asked how one can "include" a file in a Perl program (similar
to <a xlink:href="http://php.net/manual/en/function.include.php">PHP's include</a>
or <a xlink:href="http://ss64.com/bash/period.html">the shell's
"source" or "." operators</a>. The answer is that the better way is to extract
the common functionality from all the programs into
<a xlink:href="../../topics/modules-and-packages/">modules</a> and load them by
using "use" or "require".
</p>
<p>
Note that <pdoc_f f="do">do</pdoc_f> can be used to evaluate a file (but in
a different scope), but it's almost always not needed.
</p>
<p>
Some people are looking to supply a common configuration to their programs
as global variables in the included files, and those people should look at
CPAN configuration modules such as <cpan_self_dist d="Config-IniFiles" />
or <a xlink:href="http://search.cpan.org/search?query=json&mode=all">the
various JSON modules</a> for the ability to read configuration files
in a safer and better way.
</p>
</item>
<item xml:id="global_vars_iface">
<info>
<title>Using Global Variables as an Interface to the Module</title>
</info>
<p>
While it is possible to a large extent, one should generally not use global
variables as an interface to a module, and should prefer having a procedural
or an object oriented interface instead. For information about this see our
<a xlink:href="../../topics/modules-and-packages/">page about modules and
packages</a> and our <a xlink:href="../../topics/object-oriented/">our page
about object oriented programming in Perl</a>.
</p>
</item>
<item xml:id="declaring_all_vars_at_top">
<info>
<title>Declaring all variables at the top</title>
</info>
<p>
Some inexperienced Perl programmers, possibly by influence from languages
such as C, like to declare all variables used by the program at the top of
the program or the relevant subroutines. Like so:
</p>
<bad_code syntax="perl">
my $first_name;
my $last_name;
my $address;
my @people;
my %cities;
.
.
.
</bad_code>
<p>
However, this is bad form in Perl, and the preferable way is to declare all
the variables when they are first used, and at the innermost scope where they
should retain their value. This will allow to keep track of them better.
</p>
</item>
<item xml:id="switch_pm">
<info>
<title>Using Switch.pm</title>
</info>
<p>
One should not use Switch.pm to implement a
<a xlink:href="http://en.wikipedia.org/wiki/Switch_statement">switch statement</a>
because it's a source filter, tends to break a lot of code, and causes
unexpected problems. Instead one should either use <code>given/when</code>, which
are only available in perl-5.10 and above, or dispatch tables, or alternatively
plain <code>if/elsif/else</code> structures.
</p>
</item>
<item xml:id="threads">
<info>
<title>Using threads in Perl</title>
</info>
<p>
Some beginners, when thinking they need to multitask their programs start
thinking they should use perl threads. However, as mentioned in
<pdoc d="perlthrtut"></pdoc>, perl threads are very much unlike
the traditional thread modules, share nothing by default and are in fact
heavyweight processes (instead of the usual lightweight ones). See also
<a xlink:href="http://www.perlmonks.org/index.pl?node_id=288022">Elizabeth
Mattijsen’s write-up about perl's ithreads on perlmonks</a>.
</p>
<p>
To sum up, usually threads are the wrong answer and you should be using
forking processes or something like POE (see our
<a xlink:href="../../uses/multitasking/">page about multitasking</a>) instead.
</p>
</item>
<item xml:id="calling-the-shell-too-much">
<info>
<title>Calling Shell Commands Too Much</title>
</info>
<p>
Some people are tempted to use shell commands for performing
various tasks using <code>`…`</code>, <code>qx/…/</code>, <code>system()</code>,
piped-open, etc. However, usually Perl has built-in routines or alternatively
CPAN modules, which are more portable, and often would be faster than
calling the shell for help, and they should be used instead.
</p>
<p>
As an extreme example, the site <i>The Daily WTF</i>
had <a xlink:href="http://thedailywtf.com/Articles/The_UNIX_Philosophy.aspx">a
feature</a> which featured the following code to determine the file size
in Perl:
</p>
<bad_code syntax="perl">
my $filesize = `wc -c $file | cut -c0-8 | sed 's/ //g'`;
</bad_code>
<p>
Reportedly, replacing this line with <code>my $filesize = -s $file</code> (which
as noted earlier should have been called <code>$filename</code> instead), resulted
in the program being 75 minutes faster on average (!).
</p>
<p>
Normally, if you find yourself using UNIX text processing commands such as
“sed”, “awk”, “grep”, and “cut”, you should
implement it in pure-Perl code.
</p>
</item>
<item xml:id="missing-semicolons-at-the-end-of-blocks">
<info>
<title>Missing Semicolons at the end of blocks</title>
</info>
<p>
The perl interpreter allows one to omit the last trailing semicolon (";") in
the containing block. Like so:
</p>
<bad_code syntax="perl">
if ( COND() )
{
print "Success!\n";
call_routine() \# No semicolon here.
}
</bad_code>
<p>
However, this isn't a good idea, because it is inconsistent, and may cause
errors (or obscure failures) if one-or-more statements are added afterwards.
</p>
<p>
As a result, you should end every statement with a semicolon (";") even i
it’s the last one. A possible exception to this may be single-line and/or
single-statement blocks like in <pdoc_f f="map">map</pdoc_f>.
</p>
</item>
<item xml:id="list-form-of-open-with-one-arg">
<info>
<title>List form of open with one argument.</title>
</info>
<p>
Recent versions of of perl introduced the list-forms of piping to and from a
command, such as <code>open my $fh, '-|', 'fortune', $collection</code> or
<code>open my $printer, '|-', 'lpr', '-Plp1'</code>. However, not only they are
not implemented on Windows and other UNIX-like systems yet, but when one passes
only one argument to them, they pass it to the shell verbatim.
</p>
<p>
As a result, if one passes an array variable to them, as in:
</p>
<bad_code syntax="perl">
open my $fh, '-|', @foo
or die "Could not open program! - $!"
</bad_code>
<p>
One can pass only a single argument to <code>@foo</code>, which would be dangerous.
To mitigate that, one should use the <cpan_self_dist d="IPC-Run" />
or the <cpan_self_dist d="IPC-System-Simple" /> CPAN distributions.
</p>
</item>
<item xml:id="trailing-whitespace">
<info>
<title>Trailing Whitespace</title>
</info>
<p>
With many editors, it can be common to write new code or modify existing
one, so that some lines will contain trailing whitespace, such as
spaces (ASCII 32 or 0x20) or tabs characters. These trailing spaces normally
do not cause much harm, but they are not needed and can be distracted.
</p>
<p>
While you should not feel bad about having trailing space, it is a good idea
to sometimes search for them using a command such as <code>ack '[ \t]+$'</code>
(or <code>ack -a '[ \t]+$'</code> for completeness - see
<a xlink:href="http://betterthangrep.com/">ack</a>, and get rid of them.
</p>
<p>
Some editors also allow you to highlight trailing whitespace when present. See
for example:
</p>
<ul>
<li>
<p>
<a xlink:href="http://vim.wikia.com/wiki/Highlight_unwanted_spaces">Highlight
unwanted spaces in Vim</a>. Also see <a xlink:href="http://vim.wikia.com/wiki/Highlight_unwanted_spaces">this post</a>.
</p>
</li>
<li>
<p>
<a xlink:href="http://emacswiki.org/emacs/ShowWhiteSpace">EmacsWiki:
Show White Space</a>.
</p>
</li>
</ul>
</item>
<item xml:id="string-eval">
<info>
<title>Misusing String Eval</title>
</info>
<p>
String <pdoc_f f="eval">eval</pdoc_f> allows one to compile and execute
(possibly generated) strings as Perl expressions. While it is a powerful
feature, there are usually better and safer ways to achieve what you want
using string <code>eval ""</code>. So you should only use it, if you are an expert
and really know what you are doing.
</p>
<p>
Related to string eval, is using two or more <code>/e</code> flags in the
<code>s///</code> substitution. While one /e flag is often useful (for example
when substituting counters like in <code>s/#\./($i++)."."/ge</code>) the second
/e flags just evaluates the generated expression again. This can easily be done
with using string eval inside the right-hand-side, assuming it is needed which
is normally not the case.
</p>
</item>
<item xml:id="dash-starting-named-params">
<info>
<title>Named Parameters That Start With Dash</title>
</info>
<p>
If you're defining interfaces that accept a flattened hash or a hash reference
of named parameters, there is no need to call the parameters with keys starting
with a dash, like so:
</p>
<bad_code syntax="perl">
my $obj = MyClass->new(
{
-name => "George",
-occupation => "carpenter",
-city => "Inverness",
}
);
</bad_code>
<p>
The dashes are not needed because Perl can safely escape and deal with plain
names that only contain alphanumeric characters and underscores, and they
just add clutter to the code. Named arguments starting with dashes were
prevalent in some early modules such as <cpan_self_dist d="Tk" /> or
<cpan_self_dist d="Config-IniFiles" />, but they should not be used in
more modern modules.
</p>
<p>
Instead, design your interfaces with calling conventions like so:
</p>
<code_blk syntax="perl">
my $obj = MyClass->new(
{
name => "George",
occupation => "carpenter",
city => "Inverness",
}
);
</code_blk>
</item>
<item xml:id="code_and_markup_injection">
<info>
<title>Code and Markup Injection</title>
</info>
<p>
Care must be taken when constructing statements that are passed to an
interpreter, when putting arbitrary strings inside (using string interpolation
or other methods). This is because if the strings are subject to input from
the outside world (including the users), then one can use specially crafted
strings for executing arbitrary commands and exploiting the system.
</p>
<p>
An example of this is outputting HTML using
<code>print "<p>" . $paragraph_text . "</p>\n";</code> which may allow
inserting arbitrary, malicious, markup inside <code>$paragraph_text</code>,
which may include malicious JavaScript, that can steal passwords or alter
the page’s contents.
</p>
<p>
For more information, see:
</p>
<ol>
<li>
<p>
<a xlink:href="../../topics/security/code-markup-injection/">“Code/Markup Injection
and Its Prevention”</a> resource on this site.
</p>
</li>
<li>
<p>
Wikipedia articles about
<a xlink:href="http://en.wikipedia.org/wiki/SQL_injection">SQL injection</a>
and
<a xlink:href="http://en.wikipedia.org/wiki/Cross-site_scripting">Cross-site
scripting</a>.
</p>
</li>
<li>
<p>
The site <a xlink:href="http://bobby-tables.com/">Bobby Tables</a> about SQL
injections.
</p>
</li>
</ol>
</item>
<item xml:id="init_arrays_from_arrayrefs">
<info>
<title>Initializing Arrays and Hashes from Anonymous References</title>
</info>
<p>
Some beginners to Perl are tempted to use the anonymous array reference
constructor (<code>[ … ]</code>) to initialise array variables, or alternatively
anonymous hash references (<code>{ … }</code>) to initialise hash variables, like
so:
</p>
<bad_code syntax="perl">
my @arr = [1 .. 10];
my %uk_info = { continent => "Europe", capital => "London", };
</bad_code>
<p>
However, these reference constructors actually create a single scalar that
contains a reference and, as a result, in the case of the array, one will have
a single element array, and in case of the hash, one will have an error
with a hash that was initialised only with a single key (that was converted
to a nonsensical string).
</p>
<p>
Array and hash variables should be initialized using lists enclosed in
parentheses:
</p>
<code_blk syntax="perl">
my @arr = (1 .. 100);
my %uk_info = ( continent => "Europe", capital => "London", );
</code_blk>
<p>
For more information about the difference between references and aggregate
variables, refer to our <a xlink:href="../../topics/references/">references</a>
page.
</p>
</item>
<item xml:id="long_lines">
<info>
<title>Overly Long Lines in the Source Code</title>
</info>
<p>
It is a good idea to avoid overly long lines in the source code, because
they need to be scrolled to read, and may not fit within the margins of your
co-developers’ text editors. If the lines are too long, you should break
them or reformat them (for example, by adding a newline before or after an
operator), and by breaking long string constants into several lines using
the string concatenation operator - <code>.</code>.
</p>
<p>
Many coding standards require lines to fit within 80 characters or
78 characters or so, and you should standardise on a similar limit for your
own code.
</p>
</item>
<item xml:id="no_upwards_for_dirs">
<info>
<title>Getting rid of special entries in directory contents</title>
</info>
<p>
Calling <pdoc_f f="readdir">readdir()</pdoc_f> repetitively, or calling it
in list context will normally return the two special entries of <filepath>.</filepath>
(the same directory) and <filepath>..</filepath> (the parent directory) which should not
be checked, and should normally be ignored. One can often find that people
are trying to skip them in various sub-optimal ways:
</p>
<bad_code syntax="perl">
if ($dir_entry =~ m/\A\./) # Will skip all directories that start with dot.
if ($dir_entry =~ m/^\./) # Same but \A is preferable for start-of-string.
if ($dir_entry =~ m/\A\.\.?\z/) # Obfuscated.
if ($dir_entry =~ m/\A\.{1,2}\z/) # Not much better.
if ($dir_entry eq "." or $dir_entry eq "..") # May not be portable.
</bad_code>
<p>
The best way to do that is to use <cpan_self_mod m="File::Spec" />’s
<code>no_upwards()</code> function:
</p>
<code_blk syntax="perl">
foreach my $entry (File::Spec->no_upwards(readdir($dir_handle))
{
}
</code_blk>
<p>
Note that <cpan_self_dist d="File-Slurp" /> wraps that for you in its
<code>read_dir()</code> function and other file system abstraction modules provide
similar functionality.
</p>
</item>
<item xml:id="assigning_list_to_scalar">
<info>
<title>Assigning a List to a Scalar Variable</title>
</info>
<p>
Normally, assigning from a function or an expression that returns a list
to a scalar variable, will not yield what you want:
</p>
<bad_code syntax="perl">
my $characters = split(//, $string);
</bad_code>
<p>
This will cause the list as returned by split to be evaluated in scalar
context, and to return a single (and not very meaningful) scalar item.
You normally want one of those:
</p>
<code_blk syntax="perl">
my @characters = split(//, $string);
my $chars_aref = [ split(//, $string) ];
my $num_chars = () = split(//, $string); \# Use length instead in this case.
</code_blk>
<p>
A lot of the confusion stems from the fact that people expect arrays in Perl
to be contained directly in scalars. For more information about that,
consult <a xlink:href="../../topics/references/">our page about references</a>.
</p>
</item>
<item xml:id="dot_asterisk">
<info>
<title>Regular Expressions starting or ending with “.*”</title>
</info>
<p>
It is not necessary to put <code>.*</code> or <code>.*?</code> into the beginning or
end of regular
expressions to match something anywhere inside the string. So for example
<code>if ($hay_stack =~ /.*ab+c.*/)</code> can be replaced with the simpler:
<code>if ($hay_stack =~ /ab+c/)</code>. If you wish to match and extract the
prefix, you should say <code>(.*?)</code> or <code>(.*)</code>.
</p>
</item>
<item xml:id="recursive_directory_traversal">
<info>
<title>Recursive Directory Traversal Without Using File::Find and Friends</title>
</info>
<p>
Some beginners to Perl are tempted to write a recursive directory traversal
(i.e: finding all files in a directory, its sub-directories, its
sub-sub-directories, etc.) by using procedural recursion or other sub-optimal
means. However, the idiomatic way is to use the core module File::Find or
its CPAN friends. For more information, see
<a xlink:href="../../uses/sys-admin/#directory_traversal">our resources about
directory traversal</a>.
</p>
</item>
<item xml:id="non_recursive_file_find">
<info>
<title>Using File::Find for listing the contents of a directory non-recursively</title>
</info>
<p>
Alternatively, sometimes people are tempted to use File::Find or similar
modules to non-recursively list the contents of a single directory. However,
in this case, it is a better idea to simply use
<pdoc_f f="opendir">opendir()</pdoc_f>,
<pdoc_f f="readdir">readdir()</pdoc_f> and
<pdoc_f f="closedir">closedir()</pdoc_f>, in conjunction with
<a xlink:href="#no_upwards_for_dirs">no_upwards</a>, or an abstraction of them.
</p>
<p>
File::Find and friends should be reserved for a recursive traversal.
</p>
</item>
<item xml:id="populating_array_with_same_reference">
<info>
<title>Populating an Array with Multiple Copies of the Same Reference</title>
</info>
<p>
You can sometimes see code like that:
</p>
<bad_code syntax="perl">
my @array_of_arrays = ([]) x $num_rows;
</bad_code>
<p>
Or:
</p>
<bad_code syntax="perl">
my @row;
my @array_of_rows;
foreach my $elem (@existing_array)
{
@row = generate_row($elem);
push @array_of_rows, \@row;
}
</bad_code>
<p>
The problem with code like this is that the same referent (see
<a xlink:href="../../topics/references/">our resources about references in
Perl</a>) is being used in all places in the array, and so they will
always be synchronised to the same contents.
</p>
<p>
As a result, the two code excerpts should be written as such instead:
</p>
<code_blk syntax="perl">
my @array_of_arrays = map { [] } (1 .. $num_rows);
</code_blk>
<p>And:</p>
<code_blk syntax="perl">
my @array_of_rows;
foreach my $elem (@existing_array)
{
my @row = generate_row($elem);
push @array_of_rows, \@row;
}
</code_blk>
<p>
Or alternatively:
</p>
<code_blk syntax="perl">
my @array_of_rows;
foreach my $elem (@existing_array)
{
push @array_of_rows, [generate_row($elem)];
}
</code_blk>
</item>
<item xml:id="conditional_my_decls">
<info>
<title>Conditional my declarations.</title>
</info>
<p>
It is not a good idea to append a trailing if statement modifier to a
declaration of a lexical variable using <code>my</code>:
</p>
<bad_code syntax="perl">
my $var = VALUE() if (COND());
my ($var1, @array2) if (COND());
</bad_code>
<p>
This code might compile and appear to run but you probably want to declare
a lexical variable for the rest of its scope. If you need to assign to it
conditionally, then do it in a separate statement:
</p>
<code_blk syntax="perl">
my $var;
if (COND())
{
$var = VALUE();
}
</code_blk>
</item>
<item xml:id="one_var_for_two_purposes">
<info>
<title>Using One Variable for Two (or More) Different Purposes</title>
</info>
<p>
Within the scope of its declaration, a variable should serve one purpose, and
serve it well. One should not re-use a variable for a completely different
purpose later on in the scope. Creating new variables is cheap in Perl and
should not be a concern to avoid clarity.
</p>
</item>
<item xml:id="backslash_n_on_rhs">
<info>
<title>Using \1 instead of $1 on the Right Hand Side of a Substitution</title>
</info>
<p>
There is no good reason to use <code>\\1</code>, <code>\\2</code>, etc. in the
right-hand-side of a substitution instead of <code>$1</code> <code>$2</code>
etc. While this may work, the backslash-digits variables are aimed at
back-references, such as matching the exact string of a capture again within
the left hand side of a regular expression:
</p>
<bad_code syntax="perl">
$s =~ s/(H\w+)\s+(W\w+)/\1 [=] \2/;
</bad_code>
<p>
Better code:
</p>
<code_blk syntax="perl">
$s =~ s/(H\w+)\s+(W\w+)/$1 [=] $2/;
</code_blk>
</item>
<item xml:id="premature_optimization">
<info>
<title>Premature Optimisation</title>
</info>
<p>
On various online Perl forums, we are often getting asked questions like:
“What is the speediest way to do task X?” or “Which of these pieces of code
will run faster?”. The answer is that in this day and age of extremely fast
computers, you should optimise for clarity and modularity first, and worry
about speed when and if you find it becomes a problem. Remember Professor
Don Knuth’s words that “Premature Optimisation is the root of all evil.”
(attributing it to C.A.R. Hoare).
</p>
<p>
If you do find that your program runs too slowly, refer to our
<a xlink:href="../../topics/optimising-and-profiling/">page about Optimising and
Profiling Perl code</a>, but don't optimise prematurely.
</p>
</item>
<item xml:id="version_control">
<info>
<title>Not Using Version Control</title>
</info>
<p>
For everything except for short throwaway scripts, or otherwise incredibly
short programs, there is no good excuse, not to use a version control system
(a.k.a: "revision control systems", "source control systems", or more in
general as part of "software configuration management"). This is especially
true nowadays given the availability of several powerful, easy to use,
open-source (and as a result free-of-charge), and cross-platform, version
control systems, that you should have not a lot of problems to deploy, learn
and use.
</p>
<p>
For more information and the motivation behind using version control systems,
see
<a xlink:href="../../tutorials/perl-for-newbies/part5/#page--version-control--DIR">the
relevant section out of the fifth part of “Perl for Perl Newbies”</a>
for more discussion about the motivation behind that, some links and a
demonstration.
</p>
<p>
Some links for further discussion:
</p>
<ul>
<li>
<p>
<a xlink:href="http://better-scm.shlomifish.org/">The Better SCM Site</a>
</p>
</li>
<li>
<p>
<a xlink:href="http://producingoss.com/en/vc-systems.html">The Free Version Control
Systems Appendix of <i>Producing Open Source Software</i></a>.
</p>
</li>
<li>
<p>
The Wikipedia
<a xlink:href="http://en.wikipedia.org/wiki/List_of_revision_control_software">List
of revision control software</a>.
</p>
</li>
<li>
<p>
<a xlink:href="http://perlhacks.com/2012/03/you-must-hate-version-control-systems/">“You
Must Hate Version Control Systems”</a> - a discussion on Dave Cross’ blog
about best practices in the software development industry.
</p>
</li>
</ul>
</item>
<item xml:id="automated_tests">
<info>
<title>Writing Automated Tests</title>
</info>
<p>
Automated tests help verify that the code is working correctly, that bugs
are not introduced due to refactoring or the addition of new feature, and also
provide specifications and interface documentation to the code. As a result,
automated tests have been considered a good practise for a long time.
</p>
<p>
For more information about how to write automated tests, see
<a xlink:href="../../uses/qa/">our page about quality assurance in
Perl</a>.
</p>
</item>
</section>
<section xml:id="sources_of_advice">
<info>
<title>Sources of This Advice</title>
</info>
<p>
This is a short list of the sources from which this advice was taken which
also contains material for further reading:
</p>
<ol>
<li>
<p>
<a xlink:href="$(ROOT)/books/advanced/#pbp">The
Book "Perl Best Practices"</a> by Damian Conway - contains a lot of good
advice and food for thought, but sometimes should be deviated from.
Also see the
<a xlink:href="https://www.socialtext.net/perl5/index.cgi?pbp_module_recommendation_commentary">"PBP
Module Recommendation Commentary"</a> on the Perl 5 Wiki.
</p>
</li>
<li>
<p>
<a xlink:href="https://www.socialtext.net/perl5/index.cgi?ancient_perl">"Ancient
Perl"</a> on the Perl 5 Wiki.
</p>
</li>
<li>
<p>
<a xlink:href="http://modernperlbooks.com/">chromatic's "Modern Perl" Book and
Blog</a>
</p>
</li>
<li>
<p>
The book <a xlink:href="http://www.refactoring.com/"><i>Refactoring</i> by Martin
Fowler</a> - not particularly about Perl, but still useful.
</p>
</li>
<li>
<p>
The book
<a xlink:href="http://pragprog.com/book/tpp/the-pragmatic-programmer"><i>The Pragmatic
Programmer: From Journeyman to Master</i></a> - also not particularly about
Perl, and I found it somewhat disappointing, but it is an informative book.
</p>
</li>
<li>
<p>
Advice given by people on <a xlink:href="$(ROOT)/irc/#freenode">Freenode's #perl
channel</a>, on the Perl Beginners mailing list, and on other Perl forums.
</p>
</li>
</ol>
</section>
</body>
</document>