Stackable Traits Pattern in Scala

https://code.sahebmotiani.com/patterns-in-scala-101-5d0fa70aaf3f#.1d15x2akq

by saheb37 • 10 years ago

32 27 10 years ago

code.sahebmotiani.com

lmm2• 10 years ago

I like the way Scala solves the problem with multiple inheritance as described in https://fuhm.net/super-harmful/ : multiple inheritance is allowed, but only classes (not traits) may have constructors, and a class may only have one parent class (i.e. only one parent with a constructor).

For my part I still prefer composition over inheritance and prefer to do composition "by hand" (i.e. explicit members and delegation) where necessary; I wouldn't use this style at all. But Scala's great strength (and great weakness) is that it has absolutely first-class support for traditional OO like this as well as all the functional stuff.

jackweirdy• 10 years ago

Could you expand on why you would say having support for traditional OO is a great weakness?

lmm2• 10 years ago

It means there's a diversity of Scala styles; one person's Scala won't necessarily look like someone else's. More concretely it causes a lot of problems for type inference (particularly when ADTs in Scala are modeled as traits with implementing classes, so e.g. the type of Some(x) ends up as Some rather than Option). Other ML family languages that don't have traditional OO inheritance generally have perfect type inference (at least in the absence of higher kinds) via Hindley-Milner.

oselhn2• 10 years ago

I think that it does not make sense that Jamie extends Tywin etc. This is not "is a" relation.

jinst8gmi• 10 years ago

I think it's more that Joffrey etc. inherits certain traits from his parents rather that is his parents.

acjohnson55• 10 years ago

That's a good point. It's one of the things that confused me about trait stacking when I first started learning Scala. People often effectively use traits to implement composition, in the sense that the intention isn't that you would take advantage of Liskov substitution, but rather you intend to aggregate behavior from a number of sources.

It's one of many ways in Scala where a single abstract construct could have several different semantics applied to it.

akavi• 10 years ago

Yeah, the author is being cute with the word "inheritance", which ends up obscuring the actual point.

saheb378• 10 years ago

I didn't intend to screw up the actual point, maybe I did but I think the actual point was still understandable, isn't it? If not please let me know the actual point where the article lost it in your case.

saheb378• 10 years ago

traits in Scala doesn't always have to be in a "is a" relationship. If it makes you understand then think of them as adding behavior/mixins. And about why the syntax confused you, the first "with" trait has to be extended, if it could replace extends with "with" in case of traits then maybe you would have liked it better.

oselhn2• 10 years ago

Trait in scala represents interface (your type understands some message) or "is a" relation (your type is more specific specialization of type). In your example it is not the case (Tywin is Jamie's father, which should be solved by composition). I do not think that Tywin, Jamie etc. is separate type. In my opinion, those are instances of some type. It is not good practise to call super implementation of method either. You are mixing inheritance and overriding, which in more complex programs always cause trouble (you may break inherited method by changing super implementation). You should solve this by composition (Jamie should not inherit Tywin addToDebt but should depend on Tywin for paying his depts).

I understand what you wanted to show but your example is really confusing and definitely is not correct.

brandonmenc2• 10 years ago

The example is based on characters of Game of Thrones, to make understanding concepts less boring.

For those of us who don't know anything about Game of Thrones, and can't pronounce Elvish names, the example in this article reads like:

Flarg → Twipptch → Glorp → Boop → Fwyxnth → AnyRef → Any

So instead of being less boring, I have to use a decoder ring to figure out what's going on here.

krschultz• 10 years ago

Agreed. Disappointing because I think it's well written beyond that.

saheb378• 10 years ago

Will try to improve in my next post :)

saheb378• 10 years ago

I agree to your point, but was it that difficult to decode or follow till the end? You think A, B, C, D would have been better? Or something more logical names to everyone

brandonmenc2• 10 years ago

No, it wasn't impossible to follow, but the extra layer of indirection made me jump back to the top a few times to make sure I had everything straight.

    //Legacy
    abstract class Lannister

    //Father
    trait Tywin extends Lannister

could have just been:

    abstract class Family

    trait Father extends Family

Otherwise, I appreciate the article - it was informative.

jbooth3• 10 years ago

Alternate view:

https://en.wikipedia.org/wiki/Composition_over_inheritance

twic2• 10 years ago

This sort of is composition. It's like a static version of the Decorator pattern. Here's a vaguely realistic example:

http://blog.ryannedolan.info/2014/03/scala-pattern-stackable...

jbooth3• 10 years ago

Not really, at all. Order the traits differently and you can get different behavior. Inherit one subtrait vs another and you can get different behavior. You can reason about all of this, but I'm not that smart, so I prefer composition.

Composition locks away the behavior you want so that you know exactly what you're getting.

joefkelley• 10 years ago

This seems like more of an antipattern to me. If you have to be thinking about linearization order, your hierarchy is probably too complex.

SolarNet• 10 years ago

I think the point is that if the hierarchy is complex enough that you have to think about it it's simple enough to figure. The cases presented are obviously over complex to describe the behavior.

saheb378• 10 years ago

Maybe to make it more interesting, it turned out to complex as you are saying. But idea was not to use A, B, C etc. Will try to come up with something different next time.

saheb378• 10 years ago

I have explained how linearization works, if you just wanna use it, its very easy. Just goes from right to left and you don't need to remember anything else. About anti pattern, I don't think so. It might not be useful every where, but at some places it makes sense to have stackable behavior.

Stackable Traits Pattern in Scala

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