Beginner questions about type classes

[sangamon]

@jimka

Perhaps someone can suggest a more extensive example more suitable to my problem?

I’m not sure I fully understand your problem and whether type classes (or other implicit usages) might be a part of a solution. It might help if you provided minimal, self-contained examples that highlight the structure of the problem instead of reworked extracts from your actual code base.

• Is the implementation of #treeAggregate() relevant to the problem?
• What other containers would you want to provide #treeAggregate() to, and how - via completely separate implementations? Are the three questions below related to this part at all?
• What’s ClauseAsList, BinaryOperation, etc.? Type aliases are nice if they can be looked up and if the corresponding types are used excessively - not in this kind of example.
• Just wondering: Couldn’t seqop and A be left out in exchange for an upstream #map() invocation?

@jimka

The example from scalac shows how to add a show method to Int and String . (I hesitate to say “add a method” because it doesn’t really add the method to Int nor String ; it just makes the syntax look like a method has been added).

It’s showing how to create a type class Show (that incidentally happens to have one method only) with instances for String and Int. The ShowOps wrapper is just sugar on top and not part of the type class concept as such.

@jimka

def conv(i: Int): Bdd = Bdd(i)
treeAggregate(clause, ident: Bdd, conv, (acc: Bdd, b: Bdd) => op(acc, b)) 

This could probably already be written as

treeAggregate(clause, ident, Bdd(_), op)

@jimka

1. The example starts by defining a trait Show[A] whose type parameter A represents the type of object we’d like to put on the left hand side of the .show . In my case I would like to put a container object on the left hand side. Set(1,2,3).treeAggregate(...) , List("hello","world",...).par.treeAggregate(...) etc.
2. In the example the function show does not have any type parameters. In my case I have two type parameters, [A,B] , one of which A correlates the object on the left hand side of the . to the arguments of the method.

So far this doesn’t invoke the concept of type classes.

implicit class TraversableTreeAggregate[A](val objects: TraversableOnce[A]) extends AnyVal {
  def treeAggregate[B](init: B, seqop: A => B, combop: (B, B) => B): B =
    Aux.treeAggregate(objects, init, seqop, combop)
}

@jimka

The example defines implicit vars intCanShow:Show[Int] and stringCanShow:Show[String] , i.e. having how type parameter variables, rather having fixed types Int and `String. I don’t see how to do that in my case. Do I need a combinatorial explosion of implicit variables?

This one I don’t understand.