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All the Perl that's Practical to Extract and Report

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Well, Now I Know (Roles) 17 Comments More | Login | Reply /

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  • [When] the computer can find potential problems -- particularly when it's close to compile time -- this is far better than requiring the programmer to always look for those potential problems.

    This is not a case where the computer can identify problems with certainty. The compiler cannot judge your intent. Did you make a typo in the name of a class-local method such that it collides with the name of a composed method? Did you forget to read the documentation? Did you do it deliberately? Did someone upgrade a role and not tell you?

    The compiler has no access to this information and can only guess.

    That's not a strategy for robust programming, and it's a terrible strategy for user interfaces. (Imagine if Perl warned every time you slurp a file into an array. Sometimes that's a mistake -- perhaps often, it's a mistake. It's not always a mistake.)

    The compiler can only know that it should warn on every class-local method declaration excluding composed methods if you tell it that it should warn. That's easy to do; enable an optional warning.

    • Imagine if Perl warned every time you slurp a file into an array. Sometimes that's a mistake -- perhaps often, it's a mistake. It's not always a mistake.

      Imagine if Perl warned you every time you used a package-scoped variable only once. Sometimes that's a mistake -- perhaps often, it's a mistake. It's not always a mistake.

      Imagine if Perl warned you every time you redefined a function (for example to memoize or inline it). Sometimes that's a mistake -- perhaps often, it's a mistake. It's not always a mista

      • I encourage you to read this insightful blog post [modernperlbooks.com] about why optional warnings are short-sighted.

        I call: The Value of a Warning [modernperlbooks.com].

        If you believe that the only purpose of roles is compile-time, warning-safe mixins, you're missing at least half of their power.

        The purpose of roles -- the real, get your hands dirty, oh wow this is an epiphany moment of roles -- is to produce a type system that tracks the capability of entities without dictating their particular implementation of those capabilities.

        In a true role

        • You're actively discouraging people from taking advantage of allomorphism using well-established and long-recommended design techniques explicitly made safer and more understandable by roles.

          I responded at my blog [blogspot.com] with a description of both sides of the argument. The gist of it is: the warning is gone, and we will support the warning Ovid needs (and much more) as Perl::Critic policies.

    • Please note that I'm not trying to change your opinion here because it's obvious that we strongly disagree here. I provide this for anyone else who may be reading this.

      This is not a case where the computer can identify problems with certainty. The compiler cannot judge your intent.

      And that's why the warning is so desperately needed. If I inherit from A and B and both provide a "foo" method, I usually get the one that I've inherited from first. One could argue that I forgot to read the documentation or that I did it deliberately, but just like the composition problem I list above, there's no way that the compiler can

      • If you mean "override the method", then this implies I knew the method was there and explicitly excluding it takes about two seconds and let's other developers come along and instantly see that there's an excluded method. This is a benefit because it increases the readability of the code.

        Yet when I override multiple methods (or all of the methods) from a role for the purpose of complete allomorphism or delegation, your approach means that I have to exclude every one of them explicitly, which is mere busy

        • For the case of overriding everything or almost everything and the role is not simply an interface, then yes, the work to exclude all of the role's methods would be annoying. That's the only interesting argument I've heard from this entire discussion and had the discussion started out with this, then things might have gone easier.

          The problem is that your solution is still throwing away information, my solution can be cumbersome at times. So the reality is that this is a syntax issue. If a good, clean syn

          • If a good, clean syntax were found which makes overriding a role's methods explicit and non-cumbersome but still warns about ambiguity, then both concerns can be addressed.

            mst has some very nice examples using MooseX::Declare, where you provide a block after applying a role. Any method you define in that block very obviously takes precedence over methods composed from the role.

            ... your solution of ignoring the real, existing, many hours of debugging problem that we've had at the BBC trying to find those

        • Hmm maybe we need some examples here. I am just starting to use Moose, so I must be missing something, but I cannot imagine someone wanting to override all methods of a used role.

          • I can easily imagine this. If you simply want to say "I provide this behavior" but your implementation differs significantly, you might override all of the roles (this is different from an interface because the role can also provide an implementation). The issue is that this is the use case that chromatic wants to support and I'm against silently discarding behavior.

            • I don't understandy why to declare "I provide this behaviour" someone would use a Role. Maybe it comes with more practice - that is why I ask for concrete examples.

              • Let's say that you can serialize an object as HTML. You might use the role Role::Serializable::HTML with a &serialize method. Then someone can ask:

                if ( $object->DOES('Role::Serializable::HTML') ) {
                    print $object->serialize;
                }

                By providing a name for the behavior, you are guaranteeing to someone that you provide a &serialize method and that it will serialize the object as HTML.

                However, and this is chromatic's concern, it's quite reasonable that the object might want to use that r

              • I've written a lot of mock objects, and I've had to work around a lot of code which performs its own type checking. That can interfere with the work of mock objects. I plan to write an article with more concrete examples soon, but for now I hope it's not too abstract to say that the question "Is this entity a member of an inheritance hierarchy at this point or lower?" is much less interesting and much more difficult than the question "Does this entity perform the behavior I expect?"

        • Tell me with a straight face that it's not crazy to apply a role to a class and then immediately list all of the methods you don't want to compose from the role into the class because they appear in the class immediately after the role application.

          Tell me with a straight face that it's not crazy to use a module and then be forced to list all of the methods that you want to import because it didn't define @EXPORT. Particularly when the only reason why someone would want to use that module is to get access t

          • Tell me with a straight face that it's not crazy to use a module and then be forced to list all of the methods that you want to import because it didn't define @EXPORT. Particularly when the only reason why someone would want to use that module is to get access to those functions.

            The important difference is that roles have always had the explicit design goal of not enforcing any particular implementation decision to take advantage of them.

            • The fact that someone has a legitimate design goal that creates action at a distance doesn't necessarily make the decision to create action at a distance a good idea.

              In a similar spirit I would not be opposed to an optional warning if my subclass overrode a parent class's method and didn't, say, provide an attribute that says, "Yes, this is an intentional override, don't warn." You might find having to type :override annoying, but I would find it invaluable. (I also know that I have a snowball's chance in

              • I believe it's easier to argue that inheriting or composing in methods is more action at a distance than declaring them locally. Perhaps prototype OO systems are clearest by this metric.

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