This month I started a new job at Alert Logic, a cybersecurity provider with Perl (among many other things) at its beating heart. I’ve been learning a lot, and part of the process has been understanding the APIs in the code base. To that end, I’ve been writing small test scripts to tease apart data structures, using Perl array-processing, list-processing, and hash- (i.e., associative array)-processing functions.
I’ve covered map, grep, and friends a coupletimes before. Most recently, I described using List::Util’s any function to check if a condition is true for any item in a list. In the simplest case, you can use it to check to see if a given value is in the list at all:
use feature 'say';
use List::Util 'any';
my @colors =
qw(red orange yellow green blue indigo violet);
say 'matched' if any { /^red$/ } @colors;
However, if you’re going to be doing this a lot with arbitrary strings, Perl FAQ section 4 advises turning the array into the keys of a hash and then checking for membership there. For example, here’s a simple script to check if the colors input (either from the keyboard or from files passed as arguments) are in the rainbow:
#!/usr/bin/env perl
use v5.22; # introduced <<>> for safe opening of arguments
use warnings;
my %in_colors = map {$_ => 1}
qw(red orange yellow green blue indigo violet);
while (<<>>) {
chomp;
say "$_ is in the rainbow" if $in_colors{$_};
}
List::Util has a bunch of functions for processing lists of pairs that I’ve found useful when pawing through hashes. pairgrep, for example, acts just like grep but instead assigns $a and $b to each key and value passed in and returns the resulting pairs that match. I’ve used it as a quick way to search for hash entries matching certain value conditions:
use List::Util 'pairgrep';
my %numbers = (zero => 0, one => 1, two => 2, three => 3);
my %odds = pairgrep {$b % 2} %numbers;
Sure, you could do this by invoking a mix of plain grep, keys, and a hash slice, but it’s noisier and more repetitive:
use v5.20; # for key/value hash slice
my %odds = %numbers{grep {$numbers{$_} % 2} keys %numbers};
pairgrep’s compiled C‑based XS code can also be faster, as evidenced by this Benchmark script that works through a hash made of the Unix words file (479,828 entries on my machine):
#!/usr/bin/env perl
use v5.20;
use warnings;
use List::Util 'pairgrep';
use Benchmark 'cmpthese';
my (%words, $count);
open my $fh, '<', '/usr/share/dict/words'
or die "can't open words: $!";
while (<$fh>) {
chomp;
$words{$_} = $count++;
}
close $fh;
cmpthese(100, {
grep => sub {
my %odds = %words{grep {$words{$_} % 2} keys %words};
},
pairgrep => sub {
my %odds = pairgrep {$b % 2} %words;
},
} );
I mentioned at the Ephemeral Miniconf last month that as soon as I write about one Perl module (or five), someone inevitably brings up another (or seven) I’ve missed. And of course, it happened again last week: no sooner had I written in passing that I was using Exception::Class than the denizens of the Libera Chat IRC #perl channel insisted I should use Throwable instead for defining my exceptions. (I’ve already blogged about various ways of catching exceptions.)
Why Throwable? Aside from Exception::Class’s author recommending it over his own work due to a “nicer, more modern interface,” Throwable is a Moo role, so it’s composable into classes along with other roles instead of mucking about with multiple inheritance. This means that if your exceptions need to do something reusable in your application like logging, you can also consume a role that does that and not have so much duplicate code. (No, I’m not going to pick a favorite logging module; I’ll probably get that wrong too.)
However, since Throwable is a role instead of a class, I would have to define several additional packages in my tiny modulino script from last week, one for each exception class I want. The beauty of Exception::Class is its simple declarative nature: just use it and pass a list of desired class names along with options for attributes and whatnot. What’s needed for simple use cases like mine is a declarative syntax for defining several exception classes without the noise of multiple packages.
Enter Throwable::SugarFactory, a module that enables you to do just that by adding an exception function for declaring exception classes. (There’s also the similarly-named Throwable::Factory; see the above discussion about never being able to cover everybody’s favorites.) The exception function takes three arguments: the name of the desired exception class as a string, a description, and an optional list of instructions Moo uses to build the class. It might look something like this:
package Local::My::Exceptions;
use Throwable::SugarFactory;
exception GenericError => 'something bad happened';
exception DetailedError => 'something specific happened' =>
( has => [ message => ( is => 'ro' ) ] );
1;
Throwable::SugarFactory takes care of creating constructor functions in Perl-style snake_case as well as functions for detecting what kind of exception is being caught, so you can use your new exception library like this:
#!/usr/bin/env perl
use experimental qw(isa);
use Feature::Compat::Try;
use JSON::MaybeXS;
use Local::My::Exceptions;
try {
die generic_error();
}
catch ($e) {
warn 'whoops!';
}
try {
die detailed_error( message => 'you got me' );
}
catch ($e) {
die encode_json( $e->to_hash )
if $e isa DetailedError and defined $e->message;
$e->throw if $e->does('Throwable');
die $e;
}
The above also demonstrates a couple of other Throwable::SugarFactory features. First, you get a to_hash method that returns a hash reference of all exception data, suitable for serializing to JSON. Second, you get all of Throwable’s methods, including throw for re-throwing exceptions.
So where does this leave last week’s FOAAS.com modulino client demonstration of object mocking tests? With a little bit of rewriting to define and then use our sweeter exception library, it looks like this. You can review for a description of the rest of its workings.
#!/usr/bin/env perl
package Local::CallFOAAS::Exceptions;
use Throwable::SugarFactory;
BEGIN {
exception NoMethodError =>
'no matching WebService::FOAAS method' =>
( has => [ method => ( is => 'ro' ) ] );
exception ServiceError =>
'error from WebService::FOAAS' =>
( has => [ message => ( is => 'ro' ) ] );
}
package Local::CallFOAAS; # this is a modulino
use Test2::V0; # enables strict, warnings, utf8
# declare all the new stuff we're using
use feature qw(say state);
use experimental qw(isa postderef signatures);
use Feature::Compat::Try;
use Syntax::Construct qw(non-destructive-substitution);
use WebService::FOAAS ();
use Package::Stash;
BEGIN { Local::CallFOAAS::Exceptions->import() }
my $foaas = Package::Stash->new('WebService::FOAAS');
my $run_as =
!!$ENV{CPANTEST} ? 'test'
: !defined scalar caller ? 'run'
: undef;
__PACKAGE__->$run_as(@ARGV) if defined $run_as;
sub run ( $class, @args ) {
try { say $class->call_method(@args) }
catch ($e) {
die 'No method ', $e->method, "\n"
if $e isa NoMethodError;
die 'Service error: ', $e->message, "\n"
if $e isa ServiceError;
die "$e\n";
}
return;
}
# Utilities
sub methods ($) {
state @methods = sort map s/^foaas_(.+)/$1/r,
grep /^foaas_/, $foaas->list_all_symbols('CODE');
return @methods;
}
sub call_method ( $class, $method = '', @args ) {
state %methods = map { $_ => 1 } $class->methods();
die no_method_error( method => $method )
unless $methods{$method};
return do {
try { $foaas->get_symbol("&$method")->(@args) }
catch ($e) { die service_error( message => $e ) }
};
}
# Testing
sub test ( $class, @ ) {
state $stash = Package::Stash->new($class);
state @tests = sort grep /^_test_/,
$stash->list_all_symbols('CODE');
for my $test (@tests) {
subtest $test => sub {
try { $class->$test() }
catch ($e) { diag $e }
};
}
done_testing();
return;
}
sub _test_can ($class) {
state @subs = qw(run call_method methods test);
can_ok $class, \@subs, "can do: @subs";
return;
}
sub _test_methods ($class) {
my $mock = mock 'WebService::FOAAS' => ( track => 1 );
for my $method ( $class->methods() ) {
$mock->override( $method => 1 );
ok lives { $class->call_method($method) },
"$method lives";
ok scalar $mock->sub_tracking->{$method}->@*,
"$method called";
}
return;
}
sub _test_service_failure ($class) {
my $mock = mock 'WebService::FOAAS';
for my $method ( $class->methods() ) {
$mock->override( $method => sub { die 'mocked' } );
my $exception =
dies { $class->call_method($method) };
isa_ok $exception, [ServiceError],
"$method throws ServiceError on failure";
like $exception->message, qr/^mocked/,
"correct error in $method exception";
}
return;
}
1;
[Updated, thanks to Dan Book, Karen Etheridge, and Bob Kleemann] The only goofy bit above is the need to put the exception calls in a BEGIN block and then explicitly call BEGIN { Local::CallFOAAS::Exceptions->import() }. Since the two packages are in the same file, I can’t do a use statement since the implied require would look for a corresponding file or entry in %INC. (You can get around this by messing with %INC directly or through a module like me::inlined that does that messing for you, but for a single-purpose modulino like this it’s fine.)
Over the past two years, I’ve gotten back into playing Dungeons & Dragons, the famous tabletop fantasy role-playing game. As a software developer and musician, one of my favorite character classes to play is the bard, a magical and inspiring performer or wordsmith. The list of basic bardic spells includes Vicious Mockery, enchanting verbal barbs that have the power to psychically damage and disadvantage an opponent even if they don’t understand the words. (Can you see why this is so appealing to a coder?)
Mocking has a role to play in software testing as well, in the form of mock objects that simulate parts of a system that are too brittle, too slow, too complicated, or otherwise too finicky to use in reality. They enable discrete unit testing without relying on dependencies external to the code being tested. Mocks are great for databases, web services, or other network resources where the goal is to test what you wrote, not what’s out in “the cloud” somewhere.
Speaking of web services and mocking, one of my favorites is the long-running FOAAS (link has language not safe for work), a surprisingly expansive RESTful insult service. There’s a corresponding Perl client API, of course, but what I was missing was a handy Perl script to call that API from the terminal command line. So I wrote the following over Thanksgiving break, trying to keep it simple while also showing the basics of mocking such an API. It also demonstrates some newer Perl syntax and testing techniques as well as brian d foy’s modulino concept from Mastering Perl (second edition, 2014) that marries script and module into a self-contained executable library.
#!/usr/bin/env perl
package Local::CallFOAAS; # this is a modulino
use Test2::V0; # enables strict, warnings, utf8
# declare all the new stuff we're using
use feature qw(say state);
use experimental qw(isa postderef signatures);
use Feature::Compat::Try;
use Syntax::Construct qw(non-destructive-substitution);
use WebService::FOAAS ();
use Package::Stash;
use Exception::Class (
NoMethodException => {
alias => 'throw_no_method',
fields => 'method',
},
ServiceException => { alias => 'throw_service' },
);
my $foaas = Package::Stash->new('WebService::FOAAS');
my $run_as =
!!$ENV{CPANTEST} ? 'test'
: !defined scalar caller ? 'run'
: undef;
__PACKAGE__->$run_as(@ARGV) if defined $run_as;
sub run ( $class, @args ) {
try { say $class->call_method(@args) }
catch ($e) {
die 'No method ', $e->method, "\n"
if $e isa NoMethodException;
die 'Service error: ', $e->error, "\n"
if $e isa ServiceException;
die "$e\n";
}
return;
}
# Utilities
sub methods ($) {
state @methods = sort map s/^foaas_(.+)/$1/r,
grep /^foaas_/, $foaas->list_all_symbols('CODE');
return @methods;
}
sub call_method ( $class, $method = '', @args ) {
state %methods = map { $_ => 1 } $class->methods();
throw_no_method( method => $method )
unless $methods{$method};
return do {
try { $foaas->get_symbol("&$method")->(@args) }
catch ($e) { throw_service( error => $e ) }
};
}
# Testing
sub test ( $class, @ ) {
state $stash = Package::Stash->new($class);
state @tests = sort grep /^_test_/,
$stash->list_all_symbols('CODE');
for my $test (@tests) {
subtest $test => sub {
try { $class->$test() }
catch ($e) { diag $e }
};
}
done_testing();
return;
}
sub _test_can ($class) {
state @subs = qw(run call_method methods test);
can_ok( $class, \@subs, "can do: @subs" );
return;
}
sub _test_methods ($class) {
my $mock = mock 'WebService::FOAAS' => ( track => 1 );
for my $method ( $class->methods() ) {
$mock->override( $method => 1 );
ok lives { $class->call_method($method) },
"$method lives";
ok scalar $mock->sub_tracking->{$method}->@*,
"$method called";
}
return;
}
sub _test_service_failure ($class) {
my $mock = mock 'WebService::FOAAS';
for my $method ( $class->methods() ) {
$mock->override( $method => sub { die 'mocked' } );
my $exception =
dies { $class->call_method($method) };
isa_ok $exception, ['ServiceException'],
"$method throws ServiceException on failure";
like $exception->error, qr/^mocked/,
"correct error in $method exception";
}
return;
}
1;
Let’s walk through the code above.
Preliminaries
First, there’s a generic shebang line to indicate that Unix and Linux systems should use the perl executable found in the user’s PATH via the env command. I declare a package name (in the Local:: namespace) so as not to pollute the default main package of other scripts that might want to require this as a module. Then I use the Test2::V0 bundle from Test2::Suite since the embedded testing code uses many of its functions. This also has the side effect of enabling the strict, warnings, and utf8 pragmas, so there’s no need to explicitly use them here.
(Why Test2 instead of Test::More and its derivatives and add-ons? Both are maintained by the same author, who recommends the former. I’m seeing more and more modules using it, so I thought this would be a great opportunity to learn.)
Next, I bring in the aforementioned FOAAS Perl API without importing any of its functions, Package::Stash to make metaprogramming easier, and a couple of exception classes so that the command line function and other consumers might better tell what caused a failure. In preparation for the methods below dynamically discovering what functions are provided by WebService::FOAAS, I gather up its symbol table (or stash) into the $foaas variable.
The next block determines how, if at all, I’m going to run the code as a script. If the CPANTEST environment variable is set, I’ll call the test class method sub, but if there’s no subroutine calling me I’ll execute the run class method. Either will receive the command line arguments from @ARGV. If neither of these conditions is true, do nothing; the rest of the code is method declarations.
Modulino methods, metaprogramming, and exceptions
The first of these is the run method. It’s a thin wrapper around the call_method class method detailed below, either outputting its result or dieing with an appropriate error depending on the class of exception thrown. Although I chose not to write tests for this output, future tests might call this method and catch these rethrown exceptions to match against them. The messages end with a \n newline character so die knows not to append the current script line number.
Next is a utility method called methods that uses Package::Stash’s list_all_symbols to retrieve the names of all named CODE blocks (i.e., subs) from WebService::FOAAS’s symbol table. Reading from right to left, these are then filtered with grep to only find those beginning in foaas_ and then transformed with map to remove that prefix. The list is then sorted and stored in a state variable and returned so it need not be initialized again.
(As an aside, although perlcritic sternly warns against it I’ve chosen the expression forms of grep and map here over their blockforms for simplicity’s sake. It’s OK to bend the rules if you have a good reason.)
sub call_method is where the real action takes place. Its parameters are the class that called it, the name of a FOAAS $method (defaulted to the empty string), and an array of optional arguments in @args. I build a hash or associative array from the earlier methods method which I then use to see if the passed method name is one I know about. If not, I throw a NoMethodException using the throw_no_method alias function created when I used Exception::Class at the beginning. Using a function instead of NoMethodException->throw() means that it’s checked at compile time rather than runtime, catching typos.
I get the subroutine (denoted by a & sigil) named by $method from the $foaas stash and pass it any further received arguments from @args. If that WebService::FOAAS subroutine throws an exception it’ll be caught and re-thrown as a ServiceException; otherwise call_method returns the result. It’s up to the caller to determine what, if anything, to do with that result or any thrown exceptions.
Testing the modulino with mocks
This is where I start using those Test2::Suite tools I mentioned at the beginning. The test class method starts by building a filtered list of all subs beginning with _test_ in the current class, much like methods did above with WebService::FOAAS. I then loop through that list of subs, running each as a subtest containing a class method with any exceptions reported as diagnostics.
The rest of the modulino is subtest methods, starting with a simple _test_can sanity check for the public methods in the class. Following that is _test_methods, which starts by mocking the WebService::FOAAS package and telling Test2::Mock I want to track any added, overridden, or set subs. I then loop through all the method names returned by the methods class method, overrideing each one to return a simple true value. I then test passing those names to call_method and use the hash reference returned by sub_tracking to check that the overridden sub was called. This seems a lot simpler than the Test::Builder-based mocking libraries I’ve tried like Test::MockModule and Test::MockObject.
_test_service_failure acts in much the same way, checking that call_method correctly throws ServiceExceptions if the wrapped WebService::FOAAS function dies. The main difference is that the mocked WebService::FOAAS subs are now overridden with a code reference (sub { die 'mocked' }), which call_method uses to populate the rethrown ServiceException’s error field.
Wrapping up
With luck, this article has given you some ideas, whether it’s in making scripts (perhaps legacy code) testable to improve them, or writing better unit tests that mock dependencies, or delving a little into metaprogramming so you can dynamically support and test new features of said dependencies. I hope you haven’t come away too offended, at least. Let me know in the comments what you think.
Six months ago I gave an overview of Perl’s list processing fundamentals, briefly describing what lists are and then introducing the built-in map and grep functions for transforming and filtering them. Later on, I compiled a list (how appropriate) of list processing modules available via CPAN, noting there’s some confusing duplication of effort. But you’re a busy developer, and you just want to know the Right Thing To Do™ when faced with a list processing challenge.
First, some credit is due: these are all restatements of several Perl::Critic policies which in turn codify standards described in Damian Conway’s Perl Best Practices (2005). I’ve repeatedly recommended the latter as a starting point for higher-quality Perl development. Over the years these practices continue to be re-evaluated (including by the author himself) and various authors release new policy modules, but perlcritic remains a great tool for ensuring you (and your team or other contributors) maintain a consistent high standard in your code.
With that said, on to the recommendations!
Don’t use grep to check if any list elements match
It might sound weird to lead off by recommending not to use grep, but sometimes it’s not the right tool for the job. If you’ve got a list and want to determine if a condition matches any item in it, you might try:
if (grep { some_condition($_) } @my_list) {
... # don't do this!
}
Yes, this works because (in scalar context) grep returns the number of matches found, but it’s wasteful, checking every element of @my_list (which could be lengthy) before finally providing a result. Use the standard List::Util module’s any function, which immediately returns (“short-circuits”) on the first match:
use List::Util 1.33 qw(any);
if (any { some_condition($_) } @my_list) { ... # do something }
As a side note for web developers, the Perl Dancer framework also includes an any keyword for declaring multiple HTTP routes, so if you’re mixing List::Util in there don’t import it. Instead, call it explicitly like this or you’ll get an error about a redefined function:
use List::Util 1.33;
if (List::Util::any { some_condition($_) } @my_list) { ... # do something }
I mentioned this back in March, but it bears repeating: map and grep are intended as pure functions, not mutators with side effects. This means that the original list should remain unchanged. Yes, each element aliases in turn to the $_ special variable, but that’s for speed and can have surprising results if changed even if it’s technically allowed. If you need to modify an array in-place use something like:
for (@my_array) { $_ = ...; # make your changes here }
If you want something that looks like map but won’t change the original list (and don’t mind a few CPAN dependencies), consider List::SomeUtils’ apply function:
use List::SomeUtils qw(apply);
my @doubled_array = apply {$_ *= 2} @old_array;
Lastly, side effects also include things like manipulating other variables or doing input and output. Don’t use map or grep in a void context (i.e., without a resulting array or list); do something with the results or use a for or foreach loop:
map { print foo($_) } @my_array; # don't do this print map { foo($_) } @my_array; # do this instead
map { push @new_array, foo($_) } @my_array; # don't do this @new_array = map { foo($_) } @my_array; # do this instead
my @new_array = map foo($_), @old_array; # don't do this my @new_array2 = grep !/^#/, @old_array; # don't do this
Or like this:
my @new_array = map { foo($_) } @old_array; my @new_array2 = grep {!/^#/} @old_array;
Do it the second way. It’s easier to read, especially if you’re passing in a literal list or multiple arrays, and the expression forms can conceal bugs. This recommendation is codified by the BuiltinFunctions::RequireBlockGrep and BuiltinFunctions::RequireBlockMap Perl::Critic policies and comes from Perl Best Practices.
Refactor multi-statement maps, greps, and other list functions
map, grep, and friends should follow the Unix philosophy of “Do One Thing and Do It Well.” Your readability and maintainability drop with every statement you place inside one of their blocks. Consider junior developers and future maintainers (this includes you!) and refactor anything with more than one statement into a separate subroutine or at least a for loop. This goes for list processing functions (like the aforementioned any) imported from other modules, too.
As previously written, I like list processing. Many computing problems can be broken down into transforming and filtering lists, and Perl has got the fundamentals covered with functions like map, grep, and sort. There is so much more you might want to do, though, and CPAN has a plethora of list and array processing modules.
However, due to the vicissitudes of Perl module maintenance, we have a situation where it’s not clear at a glance where to turn when you’ve got a list that needs processing. So here’s another list: the list modules of CPAN. Click through to discover what functions they provide.
We’ve got List::Util which has been released as part of Perl since version 5.7.3.
We’ve got List::MoreUtils which has some functions which are named the same as Util but behave differently.
We’ve got List::SomeUtils which duplicates MoreUtils but with fewer dependencies.
We’ve got List::UtilsBy which MoreUtils has also cribbed some functions from.
We’ve got List::AllUtils which attempts to consolidate Util, SomeUtils, and ListBy but has some exceptions to called modules because of the aforementioned duplication between Util and SomeUtils.
We’ve got List::Util::MaybeXS which helps with pure Perl fallbacks in case your version of Util is too old to have a certain function.
We’ve got List::MoreUtils::XS which provides (some?) faster versions of MoreUtils’ functions (but you still have to use MoreUtils).
Am I missing anything? Probably! But these are the ones most associated with being upstream on the CPAN River, so they (or the modules they consolidate) have more projects depending on them.
As a Perl developer, you’re probably aware of the language’s strengths as a text-processing language and how many computing tasks can be broken down into those types of tasks. You might not realize, though, that Perl is also a world-class list processing language and that many problems can be expressed in terms of lists and their transformations.
Chief among Perl’s tools for list processing are the functions map and grep. I can’t count how many times in my twenty-five years as a developer I’ve run into code that could’ve been simplified if only the author was familiar with these two functions. Once you understand map and grep, you’ll start seeing lists everywhere and the opportunity to make your code more succinct and expressive at the same time.
What are lists?
Before we get into functions that manipulate lists, we need to understand what they are. A list is an ordered group of elements, and those elements can be any kind of data you can represent in the language: numbers, strings, objects, regular expressions, references, etc., as long as they’re stored as scalars. You might think of a list as the thing that an array stores, and in fact Perl is fine with using an array where a list can go.
Here we’re assigning the list of numbers from 1 to 3 to the array @foo. The difference between the array and the list is that the list is a fixed collection, while arrays and their elements can be modified by various operations. perlfaq4 has a great discussion on the differences between the two.
Lists are everywhere, man!
Ever wanted to sort some data? You were using a list.
join a bunch of things together into a string? List again.
split a string into pieces? You got a list back (in list context; in scalar context, you got the size of the list.)
Heck, even the humble print function and its cousin say take a list (and an optional filehandle) as arguments; it’s why you can treat Perl as an upscale AWK and feed it scalars to output with a field separator.
You’re using lists all the time and may not even know it.
The map function is devious in its simplicity: It takes two inputs, an expression or block of code, and a list to run it on. For every item in the list, it will alias $_ to it, and then return none, one, or many items in a list based on what happens in the expression or code block. You can call it like this:
my @foo = map bar($_), @list;
Or like this:
my @foo = map { bar($_) } @list;
We’re going to ignore the first way, though because Conway (Perl Best Practices, 2005) tells us that when you specify the first argument as an expression, it’s harder to tell it apart from the remaining arguments, especially if that expression uses a built-in function where the parentheses are optional. So always use a code block!
You should always turn to map (and not, say, a for or foreach loop) when generating a new list from an old list. For example:
When paired with a lookup table, map is also the most efficient way to tell if a member of a list equals a string, especially if that list is static:
use Const::Fast;
const my %IS_EXIT_WORD => map { ($_ => 1) }
qw(q quit bye exit stop done last finish aurevoir);
...
die if $IS_EXIT_WORD{$command};
Here we’re using map’s ability to return multiple items per source element to generate a constant hash, and then testing membership in that hash.
Perl, of course, is really good at regular expressions, but its grep function goes beyond and enables you to match using any expression or code block. Think of it as a partner to map; where map uses a code block to transform a list, grep uses one to filter it down. In fact, other languages typically call this function filter.
You can, of course, use regular expressions with grep, especially because a regexp match in Perl defaults to matching on the $_ variable and grep happens to provide that to its code block argument. So:
my @months_with_a = grep { /[Aa]/ } qw(
January February March
April May June
July August September
October November December
);
But grep really comes into its own when used for its general filtering capabilities; for instance, making sure that you don’t accidentally try to compare an undefined value:
say $_ > 5
? "$_ is bigger"
: "$_ is equal or smaller"
for grep { defined } @numbers;
Or when executing a complicated function that returns true or false depending on its arguments:
my @results = grep { really_large_database_query($_) }
@foo;
You might even consider chaining map and grep together. Here’s an example for getting the JPEG images out of a file list and then lowercasing the results:
When introducing map above I noted that it aliased$_ for every element in the list. I used that term deliberately because modifications to $_ will modify the original element itself, and that is usually an error. Programmers call that a “side effect,” and they can lead to unexpected behavior or at least difficult-to-maintain code. Consider:
my @needs_docs = grep { s/\.pm$/.pod/ && !-e }
@pm_files;
The intent may have been to find files ending in .pm that don’t have a corresponding .pod file, but the actual behavior is replacing the .pm suffix with .pod, then checking whether that filename exists. If it doesn’t, it’s passed through to @needs_docs; regardless, @pm_files has had its contents modified.
If you really do need to modify a copy of each element, assign a variable within your code block like this:
my @needs_docs = grep {
my $file = $_;
$file =~ s/\.pm$/.pod/;
!-e $file
} @pm_files;
But at that point you should probably refactor your multi-line block as a separate function:
my @needs_docs = grep { file_without_docs($_) }
@pm_files;
sub file_without_docs {
my $file = shift;
$file =~ s/\.pm$/.pod/;
return !-e $file;
}
In this case of using the substitution operator s///, you could also do this when using Perl 5.14 or above to get non-destructive substitution:
use v5.14;
my @needs_docs = grep { !-e s/\.pm$/.pod/r }
@pm_files;
And if you do need side effects, just use a for or foreach loop; future code maintainers (i.e., you in six months) will thank you.
Taking you higher
map and grep are examples of higher-order functions, since they take a function (in the form of a code block) as an argument. So congratulations, you just significantly leveled up your knowledge of Perl and computer science. If you’re interested in more such programming techniques, I recommend Mark Jason Dominus’ Higher Order Perl (2005), available for free online.
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