Introduction to writing readable and maintainable Perl

Introduction to writing readable
and maintainable Perl
Or

Perl Best Practices: The Best Bits

Or
Perl is more readable than Java!

Or
Your code is bad and you should feel bad

Who Am I?

Alex Balhatchet Working for a small company

Super Nerd since 1985 ~5 years of code
Perl Hacker since 2002 582 modules
Londoner since 2004 Lots of legacy code
Paid Perl Hacker since All Perl
2006

Who are you guys?

Perl Oldies?
Perl Newbies?
Curious non-Perl types?

I'm here to convince you that Perl can be readable!

my @files = @ARGV;

foreach my $file (@files) {
open(my $fh, '<', $file);

while (my $line = readline($fh)) {
print $line;
}

close($fh);
}

Summary
Pragmas
CPAN
Best Perl Best Practices
Legacy Code
Perl::Critic & Perl::Tidy
Questions

Always use strict

use strict makes your code safer

requires that all variables are declared with "my", "our", etc.
- stops you from making typos in variable names
stops you from using symbolic (string) references
- stops you from writing terrible terrible code
does not allow non-subroutine barewords
- stops you making typos in subroutine calls

...and use warnings

use warnings ensures that odd things do not silently try to "do
what you mean."

print(undef) - uninitialized value in print()
1 + "bananas" - non-numeric value in addition
%hash = (1, 2, 3); - odd number of elements in hash

Other Useful Pragmas

# make open() and others die on error
use autodie;

# enable say(), given(), state, etc.
use feature ':5.10'; # enable all of them
use feature 'say'; # enable one at a time

# make warnings throw exceptions
use warnings FATAL => 'all';

The CPAN

The CPAN is by far the best thing about Perl.

http://search.cpan.org

90,000 modules!

Using CPAN modules means your code gets maintained, bug-
fixed and optimized for you!

Picking CPAN Modules

With 90,000 modules it can be difficult to pick the right one...

Which is the right one for the job?


Use the CPAN Testers reports, rt bug tracker, and Reviews.

Every Distribution will have these!
http://search.cpan.org/dist/Data-Dumper/

CPAN Testers: PASS (561) FAIL (8) UNKNOWN (4)

Rating: (9 Reviews)


The Task::Kensho CPAN module is a documentation-and-
dependencies-only distribution which can be used as a
recommended modules list.
Some highlights are...
App::cpanminus, Test::Most, Try::Tiny,
Devel::NYTProf, Perl::Critic, DateTime,
Config::General, and App::Ack

It's a great starting point!

Code in paragraphs

Code which is written in paragraphs is much more readable.

# get ready...
read_commandline_arguments();
init();

# actual work here...
do_stuff();

# output...
format_output();
output_stuff();

Throw Exceptions

Modern programming wisdom gives us many reasons
Exceptions win out over error codes.

Impossible to ignore
Functions don't have to try to return two values
Separate exceptional from non-exceptional cases

Throw Exceptions

Perl implements Exceptions with strings and die().

die "Exception!";

You can use eval() to catch them, but the Try::Tiny module
gives us Java-style try/catch keywords which are much nicer.

try {
stuff();
}
catch {
# exception is in a lexically scoped $_ variable
}

Use builtins

Builtins in Perl are sensible and readable, especially when your
editor has decent syntax highlighting.

Perl is excellent at string manipulation and dealing with lists.
Use it to its full potential.

Perl's builtins have well defined names and behaviours, learn to
love them.

Use builtins

while (my $line = readline while (my $line = <$fh>) {
($fh)) { # ...
# ... }
}

warn "warning! something's print STDERR "is this a warning? who
weird"; knows?!";

Use builtins

if (defined $value){ if ($value){
# stuff... # stuff...
} }

my @files = glob("*. my @files = <*.txt>;
txt");

Use honorary builtins

There are a number of "honorary builtins" which are exported
by core modules.

use Scalar::Util qw(looks_like_number openhandle);

use List::Util qw(first max min shuffle sum);

use List::MoreUtils qw(any all none uniq apply);

Avoid overly confusing idioms and cleverness

Perl lets you write code however you want.

TIMTOWTDI - There is more than one way to do it.

A big part of writing readable Perl is about admitting
that some of the ways to do it suck!


What does this do?

my $result =
[ $result1 => $result2 ]
->[ $result2 <= $result1 ];


Maybe it's more obvious like this...

use List::Util qw(min);
my $result = min($result1, $result2);

Be consistent with existing code

This is an important rule, because it often contradicts all the
others I've mentioned:

When dealing with an existing code base, be consistent.

Be consistent with existing code

If you change the existing code...

Make sure there are tests
Make sure there are good tests
Change the whole file so that consistency is maintained
Commit your changes to your VCS as a whole, without any
other changes!

Perl::Critic

Perl::Critic, and its binary friend perlcritic, is a tool which will tell
you what is wrong with your Perl code.

% perlcritic my_script.pl

Perl::Critic

#!/usr/bin/perl

use feature 'say';

open(IN, $0);
while (<IN>) {
chomp; foreach (sort keys %letters) {
for (split //, $_) { say "$_t$letters{$_}";
$letters{$_}++; }
}
} close(IN);

How many mistakes can you spot?

Perl::Critic

% perlcritic --verbose 11 bad_perl.pl

Bareword file handle opened at line 3, near 'open(IN, $0);'.
InputOutput::ProhibitBarewordFileHandles (Severity: 5)
Using bareword symbols to refer to file handles is particularly
evil because they are global, and you have no idea if that symbol
already points to some other file handle. You can mitigate some of that risk
by......

Contains the full Perl Best Practices text!!

Perl::Tidy

Perl::Tidy, and perltidy, is a tool for automatically tidying up Perl code
to make it more readable.

It can...

convert tabs into spaces
restrict lines to 80 characters
automatically line up "=>" characters in hashes and ","'s in lists
add semi-colons where they belong
un-cuddle elses

The perltidyrc file listed in Perl Best Practices can be found here:

http://www.perlmonks.org/?node_id=485885

Contact Me

http://kaoru.slackwise.net/

@kaokun on Twitter

My Code

http://search.cpan.org/~kaoru/

https://github.com/kaoru/

Slides

http://www.slideshare.net/kaokun

Introduction to writing readable and maintainable Perl

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