Writing Maintainable Perl

Writing Maintainable Perl
David M. Bradford

Who am I?
David Bradford
@tinypig
tinypig.com

Who am I?
● Web Engineer at OmniTI
– Full stack tech consulting
– Remote database management and consulting
– Large Scale / Mission Critical
– We're Hiring!
– omniti.com

Why This Talk?
● Is Perl a "write-only" language?

Why This Talk?
● Is Perl a "write-only" language? No!

Why This Talk?
● ...but it can be.

Why This Talk?
● It's our responsibility to keep code:

Why This Talk?
– Easy to read

Why This Talk?
– Easy to read, so it will be:

Why This Talk?
– Easy to read, so it will be:
– Easy to maintain

My Function, “listify”
sub listify {
my ($aref,$cc) = @_;
if( ref $aref eq 'ARRAY' && $cc > 0 ) {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}

Purpose of listify
Take a list, for example:
List # 1, with 10 items
one two three four five six seven eight nine ten
Give me several lists back, each containing no more than N items,
for example, 4:
List #1 with 4 items: one two three four
List #2 with 4 items: five six seven eight
List #3 with 2 items: nine ten

Prepare to call listify
listify( @list, 4 );
Result:
@array = (
[ 'one', 'two', 'three', 'four' ],
[ 'five', 'six', 'seven', 'eight' ],
[ 'nine', 'ten' ],
);
my @list = qw( one two three four five six seven eight nine ten );

Call to listify
Result:
@array = (
[ 'nine', 'ten' ],
);

Result of listify
Result:
@array = (
[ 'nine', 'ten' ],
);

Declare function listify
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
sub listify {
}

Get Parameters
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}

Validate Parameters
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
}
return;

Declare Return Variable
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
my $j;

Loop Through Input Array
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
for(my $i=0; $i<=$#$aref; $i+=$cc) {
}

Add List to Output Variable
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
push @$j, [@$aref[$i..$i+$cc-1]];

???
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
$#{$j->[$#{$j}]}=$#$aref%$cc;

Update Input Array In Place
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
@$aref = @$j;

Return 1 to Indicate Success
sub listify {
my $j;
for(my $i=0; $i<=$#$aref; $i+=$cc) {
push @$j, [@$aref[$i..$i+$cc-1]];
}
$#{$j->[$#{$j}]}=$#$aref%$cc;
@$aref = @$j;
return 1;
}
return;
}
return 1;

Clever (but not easy to read) Line
$#{$j->[$#{$j}]}=$#$aref%$cc;

Purpose of Clever Line
The purpose of the line is to truncate the final array to the number of remaining
elements so we don't end up with this:
@array = (
[ 'nine', 'ten', undef, undef ],
);

Improvement #2: Refactor
$#{ $j->[ -1 ] } = $#$aref % $cc;
$#{ $j->[ $#{$j} ] } = $#$aref % $cc;

Improvement #3: Naming
$#{ $result_array[ -1 ] } = $#$in_aref % $elements_per_array;

Improvement #4: Multiple Lines
$final_aref = $result_array[ -1 ];
$#$final_aref = $#$in_aref % $elements_per_array;

Improvement #5: Split Another Line
my $final_aref = $result_array[ -1 ];
my $elements_in_final = $#$in_aref % $elements_per_array;
$#$final_aref = $elements_in_final;

Improvement #6: More Whitespace

Improvement #7: A comment!
# Truncate final array

Easier to Read Now?
Original:
$#{$j->[$#{$j}]}=$#$aref%$cc;
Revised:

Apply Principles to Entire Function
sub listify {
my ( $in_aref, $elements_per_array ) = @_;
return if (
ref $in_aref ne 'ARRAY' or
$elements_per_array <= 0
);
my @result_array;
for( my $i = 0; $i <= $#$in_aref; $i += $elements_per_array ) {
push @result_array, [
@$in_aref[ $i..$i + $elements_per_array - 1 ]
];
}
# Continued on next slide

Apply Principles to Entire Function
# Continued from previous slide
@$in_aref = @result_array;
}

Helpful Modules
Perl::Critic
based on the book “Perl Best Practices” by Damian
Conway
Perl::Tidy
increase readability of code

perlcritic Result
$ perlcritic -1 listify.pl
Code not contained in explicit package at line 1, column 1. Violates
encapsulation. (Severity: 4)
Code before strictures are enabled at line 1, column 1. See page
429 of PBP. (Severity: 5)
Subroutine "listify" does not end with "return" at line 1, column 1.
See page 197 of PBP. (Severity: 4)
Code before warnings are enabled at line 1, column 1. See page 431 of
PBP. (Severity: 4)
No package-scoped "$VERSION" variable found at line 1, column 1. See
page 404 of PBP. (Severity: 2)
C-style "for" loop used at line 10, column 7. See page 100 of PBP.
(Severity: 2)
Double-sigil dereference at line 10, column 26. See page 228 of PBP.
(Severity: 2)
(more double-sigil warnings follow)

Critical Fixes
Code not contained in explicit package at line 1, column 1. Violates
encapsulation. (Severity: 4)
Code before strictures are enabled at line 1, column 1. See page 429
of PBP. (Severity: 5)
Code before warnings are enabled at line 1, column 1. See page 431
sub listify {
# --- cut ---
#!/usr/bin/perl
use strict;
use warnings;

Explicit Return
Subroutine "listify" does not end with "return" at line 2,
column 1. See page 197 of PBP. (Severity: 4)
@$in_aref = @result_array;
}
return;

$VERSION
No package-scoped "$VERSION" variable found at line 1,
column 1. See page 404 of PBP. (Severity: 2)
#!/usr/bin/perl
use strict;
use warnings;
sub listify {
# --- cut ---
our $VERSION = '1.19';

C-style "for" Loop
C-style "for" loop used at line 11, column 7. See page 100

C-style "for" Loop
● “What?” No C-style “for” loops

C-style "for" Loop
● “Why?” More difficult to read and maintain

C-style "for" Loop
● “I disagree.” That's OK. Modify Perl::Critic settings!

C-style "for" Loop
● “I disagree.” That's OK. Modify Perl::Critic settings!
But today, I'm going to fix all the defaults

Change to “while” loop
C-style "for" loop used at line 11, column 7. See page 100 of
PBP. (Severity: 2)
Old:
for( my $i = 0; $i <= $#$in_aref; $i += $elements_per_array ) {
# (loop contents)
}
New:
my $i = 0;
while($i <= $#$in_aref) {
# (loop contents)
$i += $elements_per_array;
}

Double-sigil dereference
● Caused by
– @$my_array_reference
● Fixed with
– @{ $my_array_reference }

JAPH: The Do-Not Example
sub q{ord($_[0])}sub qq{chr($_[0])}(@q=(j,q,q,,q,s,,u,o,[$;=@q,$"=sub{for(@q){
s/(.)/&qq(&q($1)-1)/e}},$_=q,*534`!./4(%2`0%2{`(!#+%2,,tr;{;,;,s/(.)/&q($1)>95?
&qq(&q($1)-64):&qq(&q($ 1)+64)/eg,% q=(q,qq,,sub{eval$ _[0]},q,q,,1)])),&{$ "},
$q{qq}->(qq.$;->[$q=$q{q}]${$;}[++$q]$;->[$[]${$;}[-1+$)-$ )+$#q]$;->[$#q-$q].)

s/(.)/&qq(&q($1)-1)/e}},$_=q,*534`!./4(%2`0%2{`(!#+%2,,tr;{;,;,s/(.)/&q($1)>95?
$q{qq}->(qq.$;->[$q=$q{q}]${$;}[++$q]$;->[$[]${$;}[-1+$)-$ )+$#q]$;->[$#q-$q].)
● Ugly variables $" $; $ " and even $_, @_

s/(.)/&qq(&q($1)-1)/e}},$_=q,*534`!./4(%2`0%2{`(!#+%2,,tr;{;,;,s/(.)/&q($1)>95?
$q{qq}->(qq.$;->[$q=$q{q}]${$;}[++$q]$;->[$[]${$;}[-1+$)-$ )+$#q]$;->[$#q-$q].)
– use English;

s/(.)/&qq(&q($1)-1)/e}},$_=q,*534`!./4(%2`0%2{`(!#+%2,,tr;{;,;,s/(.)/&q($1)>95?
$q{qq}->(qq.$;->[$q=$q{q}]${$;}[++$q]$;->[$[]${$;}[-1+$)-$ )+$#q]$;->[$#q-$q].)
– use English;
● 74% non-alpha/space

s/(.)/&qq(&q($1)-1)/e}},$_=q,*534`!./4(%2`0%2{`(!#+%2,,tr;{;,;,s/(.)/&q($1)>95?
$q{qq}->(qq.$;->[$q=$q{q}]${$;}[++$q]$;->[$[]${$;}[-1+$)-$ )+$#q]$;->[$#q-$q].)
– use English;
● 74% non-alpha/space
– More non-alpha characters, more difficult to read

Other Considerations
● Conformity can be good

● How to implement good change?

● The “next guy” might be YOU.

● The “next guy” might be YOU.
● Comment to preempt disputes or misunderstandings

Best Book on this Topic
Perl Best Practices by Damian Conway

Thank You!
David Bradford
@tinypig
tinypig.com
Web Engineer at OmniTI
omniti.com/presents

Writing Maintainable Perl

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