object-oriented programming – The Effective Perler

Use the infix class instance operator

Perl v5.32 adds Paul Evans’s infix isa operator—the “class instance operator” as an experimental feature. It still has some issues to work out which prevent its use at the moment, but it looks promising. It subverts how the UNIVERSAL::isa does its job and breaks that in the process. As an experimental feature, that’s fine, but you shouldn’t use this until that’s worked out.

There’s no word on versions for can or does.

One of the delightful things to note about this is addition is that it is one of the features whose development took place almost entirely through a GitHub issue and pull request. GitHub is now the primary repository for the Perl code, and has been since October 2019. This is a feature that I’ll want to use right away in new production code.

Continue reading “Use the infix class instance operator”

Perl v5.12 adds the package NAME VERSION syntax

Perl v5.12 modifies the package statement to take a version as well as a name. This allows you to implicitly declare the $VERSION variable: Continue reading “Perl v5.12 adds the package NAME VERSION syntax”

Return error objects instead of throwing exceptions

Programmers generally consider two types of error communication: the “modern” and shiny exception throwing, and the old and decrepit return values. When they consider these, they choose one and forsake the other. One is good, and the other is bad. Programmers won’t agree on which is which though. Continue reading “Return error objects instead of throwing exceptions”

Enchant closures for better debugging output

When you’re using code references heavily, you’re going to have a problem figuring out which one of them is having a problem. You define them in possibly several and far-flung parts of your program, but when it comes to using them, you don’t know which one you are using. You can’t really print its value like you would for a scalar, making it more difficult for you to debug things. You can dereference a scalar or an array to see what it is, but you can’t dereference a code reference without making it do something. Continue reading “Enchant closures for better debugging output”

Hide low-level details behind an interface

Perl 5.16 makes the Perl special variable, $$, writeable, but with some magic. That’s the variable that holds the process ID. Why would you ever want to do that? There’s not much to write about with this new feature, but there’s plenty to write against it since it introduces more magic (see commit 9cdac2 on June 13, 2011). Continue reading “Hide low-level details behind an interface”

Override die with END or CORE::GLOBAL::die

Perl lets you override the effects of warn and die by redefining the signals that Perl sends when you call those functions. You probably don’t want to use the signal from die, though, since it might mean a couple of different things. Continue reading “Override die with END or CORE::GLOBAL::die”

Pass the empty subclass test

Is your object-oriented module subclassable? Do you know that from testing or are you just guessing? Setting aside other Perl programmers reaching into your package and redefining your subroutines, there are some basic things you can do to ensure that you’ve made life unhard for the people you want to extend your classes. Continue reading “Pass the empty subclass test”

Use the C3 method resolution order in multiple inheritance

Perl 5.10 introduced a flexible method resolution order mechanism. Instead of Perl’s default order (see Understand Perl’s default inheritance model), you can try something less stupid by using the mro pragma to specify which order perl. Continue reading “Use the C3 method resolution order in multiple inheritance”

Understand Perl’s default inheritance model

Perl’s default inheritance mechanism is a bit weird, and it’s not something that any language designer would want to repeat. However, it is what it is and by knowing its quirks you should have any easier time tracking down inheritance problems. Continue reading “Understand Perl’s default inheritance model”

Use Data::Dump filters for nicer pretty-printing

Data::Dumper, a module that comes in the Standard Library, is one of the great tools knows to Perlers. You give it a big data structure and it pretty prints it for you. If you are one of those people who still believe that the best debugger in the world is print and need to get data structures into a single string with decent formatting, something like Data::Dumper is your best friend. When you get really complex data structures involving complicated objects, though, dumping the entire structure might be too much information. Continue reading “Use Data::Dump filters for nicer pretty-printing”