Introduction to Perl Best Practices

Introduction to Perl Best Practices José Castro <cog@cpan.org> Tokyo, September 2009

Who am I? • Perl Hacker from Portugal • COG@CPAN • YAPC::EU::20{04,05,06,07,08,09}, YAPC::NA::20{05,06,07,08}, YAPC::Asia 20{08,09}, LPW 20{04,05}, PPW 2008, OSDC::AU 2006 • Technology Evangelist @ SAPO SAPO/Portugal Telecom

What is SAPO?

What is SAPO? • Older than Google, kind of like Yahoo!

What is SAPO? • Older than Google, kind of like Yahoo! • 100+ techs

What is SAPO? • Older than Google, kind of like Yahoo! • 100+ techs • 100+ non-techs

What is SAPO? • Older than Google, kind of like Yahoo! • 100+ techs • 100+ non-techs • Mail, Blogs, Photos,Videos, Links, Cars, Real Estate, Jobs, Maps, Encyclopedia online, Search, Messaging, Social Network, etc., etc., etc.

What do I do at SAPO?

What do I do at SAPO? • Evangelism (motivation, productivity, quality)

What do I do at SAPO? • Evangelism (motivation, productivity, quality) • Organize the training program

What do I do at SAPO? • Evangelism (motivation, productivity, quality) • Organize the training program • Recruitment (lots of interviews every week)

What do I do at SAPO? • Evangelism (motivation, productivity, quality) • Organize the training program • Recruitment (lots of interviews every week) • Organize events

What do I do at SAPO? • Evangelism (motivation, productivity, quality) • Organize the training program • Recruitment (lots of interviews every week) • Organize events • Etc.

Introduction to Perl Best Practices José Castro <cog@cpan.org> Tokyo, September 2009

“What?” • a set of suggestions/guidelines to improve your code’s readability and reliability

Disclaimer • what works for a particular situation may not be adequate for another • YMMV

“Why?” • tomorrow, you may need to ﬁx a glitch in your code • next week, you may need to conﬁrm something • 6 months from now, you may need to develop a new feature

“How?” • using standards • deﬁning your best practices and following them

Code Sample #1

use strict; my @list=('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i=int ($rand+rand()); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

Block separation • A program is a list of instructions • You’re writing it as a letter • Separate your paragraphs, for clarity

Separate instructions • Each instruction should have its own line

Indentation • 2 spaces? • 4 spaces? • 6, 8? • Tabs?

Spaces around operators • Don’t clutter things up • Spaces around operators help you ﬁnd them more easily

use strict; my @list = ('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i = int ($rand + rand()); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

Naming variables • Don’t name them “var”, “list”, “hash” or anything else that does not have a meaning • Make sure the name helps you identify what it is

use strict; my @list = ('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i = int ($rand + rand()); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

use strict; my @editors = ('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i = int ($rand + rand()); print " $editors[$i] users are from Venus" . ", "$editors[1-$i] users are from Mars" . "n"; }

Vertical indentation • Vertical indentation helps your eyes ﬁnd things more easily

use strict; my @editors = ('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i = int ($rand + rand()); print " $editors[$i] users are from Venus" . ", "$editors[1-$i] users are from Mars" . "n"; }

Code Sample #2

my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47 my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377, for my $number (@list) { if (grep{$number==$_}@list2) {print "$number is }

2 is both a prime number and a fibonacci number 3 is both a prime number and a fibonacci number 5 is both a prime number and a fibonacci number 13 is both a prime number and a fibonacci number 89 is both a prime number and a fibonacci number

Strict and warnings • both strict and warnings will help you avoid lots of pesky mistakes

my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47 my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377, for my $number (@list) { if (grep{$number==$_}@list2) {print "$number is }

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47 my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377, for my $number (@list) { if (grep{$number==$_}@list2) {print "$number is }

Avoid lines over 80 charac • code is easier to read if it ﬁts on the screen • even if using line wrapping, having to ﬁgure out if two lines are different instructions or the same one is harder than not having to do it

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47 my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377, for my $number (@list) { if (grep{$number==$_}@list2) {print "$number is }

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ; my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765) ; for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number and a }

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ; my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765) ; for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " a }

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ; my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765) ; for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

When breaking lines, align vertically • a broken line is a visual accident waiting to happen

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ; my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765) ; for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,9 my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

Add spaces after a comma • spaces after a comma help you visually separate items

use strict; use warnings; my @list = (2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73,79,83,89,9 my @list2 = (1,1,2,3,5,8,13,21,34,55,89, 144,233,377,610,987,1597,2584,4181,6765 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @list = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 41 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

Organize long lists as tables • the brain identiﬁes patterns such as tables much more easily

use strict; use warnings; my @list = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 41 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 41 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, my @list2 = ( 1, 1, 2, 3, 5, 8, 13, 2 144, 233, 377, 610, 987, 1597, 2584, 41 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

Again, name variables appropriately • the brain identiﬁes patterns such as tables much more easily

use strict; use warnings; my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, my @list2 = ( 1, 1, 2, 3, 5, 8, 13, 2 144, 233, 377, 610, 987, 1597, 2584, 41 for my $number (@list) { if (grep{$number==$_}@list2) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if (grep{$number==$_}@fibonnaci) { print "$number is both a prime number " . " and a fibonacci numbern"} }

Spaces for clarity • around operators • around blocks • around anything else that may cause confusion

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if (grep{$number==$_}@fibonnaci) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"} }

Block closing • the closing curly brace should be in a line by itself • this helps better identify where the block ends

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"} }

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern" } }

Always end a statement with a semicolon • even if it’s the last statement in the block • even if the closing curly brace is on the same line • this will allow you to add another statement without having to check the line above

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern" } }

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

Inside a loop, use empty lines around the body • this is also going to help you identify the beginning and the end of the loop

use strict; use warnings; my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19, 37, 41, 43, 47, 53, 59, 61, 67, my @fibonnaci = ( 1, 1, 2, 3, 5, 8, 144, 233, 377, 610, 987, 1597, for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

Code Sample #3

for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

if ( grep { $number == $_ } @fibonnaci ) {

Using CPAN • Many modules on CPAN already implement what we need in a far better way than we could possibly do in a couple of minutes

grep { 2 == $_ } ( 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946) if ( grep { $number == $_ } @fibonnaci ) {

grep { 2 == $_ } ( 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946) n iterations, always if ( grep { $number == $_ } @fibonnaci ) {

if ( grep { $number == $_ } @fibonnaci ) {

use List::MoreUtils qw(any); if ( grep { $number == $_ } @fibonnaci ) {

use List::MoreUtils qw(any); if ( any { $number == $_ } @fibonnaci ) {

Structure • separate your code in subroutines • subroutines should have meaningful names

use List::MoreUtils qw(any); if ( any { $number == $_ } @fibonnaci ) {

use List::MoreUtils qw(any); if ( is_fibonnaci($number) ) { sub is_fibonnaci { my $number = shift; return any { $number == $_ } @fibonnaci }

for my $number (@prime_numbers) { if ( grep { $number == $_ } @fibonnaci ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

for my $number (@prime_numbers) { if ( is_fibonnaci($number) ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

for my $number (@prime_numbers) { if ( is_fibonnaci($number) ) { print "$number is both a prime number " . " and a fibonacci numbern"; } } sub is_fibonnaci { my $number = shift; return any { $number == $_ } @fibonnaci;

for my $number (@prime_numbers) { if ( is_fibonnaci($number) ) { print "$number is both a prime number " . " and a fibonacci numbern"; } }

for (@prime_numbers) { if ( is_fibonnaci($_) ) { print "$_ is both a prime number " . " and a fibonacci numbern"; } }

Code Sample #4

for ( 1 .. 1000000000 ) { my $flag = 1; $flag = 0 if is_prime( $_ ); $flag = 0 if is_fibonnaci( $_ ); if ( $flag ) { print "$_ seems interestingn"; } }

Underscores for large numbers • Large numbers are hard to read • Underscores every three digits can help

for ( 1 .. 1000000000 ) { my $flag = 1; $flag = 0 if is_prime( $_ ); $flag = 0 if is_fibonnaci( $_ ); if ( $flag ) { print "$_ seems interestingn"; } }

for ( 1 .. 1_000_000_000 ) { my $flag = 1; $flag = 0 if is_prime( $_ ); $flag = 0 if is_fibonnaci( $_ ); if ( $flag ) { print "$_ seems interestingn"; } }

Booleans • ﬂags shouldn’t be named “ﬂag” • they should have intuitive names

for ( 1 .. 1_000_000_000 ) { my $flag = 1; $flag = 0 if is_prime( $_ ); $flag = 0 if is_fibonnaci( $_ ); if ( $flag ) { print "$_ seems interestingn"; } }

for ( 1 .. 1_000_000_000 ) { my $is_interesting = 1; $is_interesting = 0 if is_prime( $_ ); $is_interesting = 0 if is_fibonnaci( $_ ); if ( $is_interesting ) { print "$_ seems interestingn"; } }

Tools and helpers

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• Perl::Tidy - “Parses and beautiﬁes perl source” • Perl::Critic - “Critique Perl source code for best-practices”

Perl::Tidy use strict; my @list=('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i=int ($rand+rand()); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

Perl::Tidy use strict; my @list = ( 'Emacs', 'Vi ' ); my $rand = rand(); print "10 random users said:n"; foreach ( 0 .. 10 ) { my $i = int( $rand + rand() ); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

Perl::Tidy • conﬁgurable (parameters or .perltidyrc) • integrates with most text editors • some people use it as an svn hook

Perl::Critic use strict; my @list=('Emacs', 'Vi '); my $rand = rand(); print "10 random users said:n"; foreach(0..10) { my $i=int ($rand+rand()); print " $list[$i] users are from Venus" . ", " . "$list[1-$i] users are from Mars" . "n"; }

Perl::Critic Loop iterator is not lexical at line 5, column 1. See page 108 of PBP. (Severity: 5)

Perl::Critic • conﬁgurable (several severity levels, extendable, .perlcriticrc) • integrates with most text editors • some people also use it as an svn hook • if you don’t own PBP, there’s also user- contributed documentation on each subject

Methods • Pair programming • Code review

Further documentation • http://127h.sl.pt - An Introduction to Perl Critic, by Josh McAdams • http://127i.sl.pt - Perl Tidy Perl Critic, by olegmmiller • http://127w.sl.pt - Perl Best Practices Reference Guide

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Introduction to Perl Best Practices

by José Castro on Sep 11, 2009

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