Defining implicit view-bounds on Scala traits - scala

I'm doing an exercise to implement a functional binary-search-tree in Scala, following a similar pattern that I've seen used in Haskell. I have a structure that looks something like this:
trait TreeNode[A] {
def isLeaf: Boolean
def traverse: Seq[A]
...
}
case class Branch[A](value: A, left: TreeNode[A], right: TreeNode[A]) extends TreeNode[A] {
def isLeaf: Boolean = false
def traverse: Seq[A] = ...
...
}
case class Leaf[A]() extends TreeNode[A] {
def isLeaf: Boolean = true
def traverse: Seq[A] = Seq[A]()
...
}
I'd like to put a type constraint on A so that it will only accept objects that extend Ordered. It looks like I need to define a view bound on A ([A <% Ordered[A]]) on Branch and Leaf, as well as the TreeNode trait.. I can't do this on the TreeNode trait, however, because view bounds aren't accepted.
As I understand, <%-style view-bounds are syntactic sugar for an implicit definition, so there should be a way to write to define the bound manually within the TreeNode trait. I'm not sure how I'm supposed to do this, though. I've looked around a bit, but haven't gotten much further than that some sort of implicit needs to be defined.
Can anybody point me in the right direction? Am I approaching this from the wrong angle entirely?

The problem is that view bounds as well as context bounds are just syntactic sugar for specific types of implicit parameters. When applied to a type parameter of a generic class (as opposed to when applied to a generic method), these implicits are added to the constructor of the class.
Because traits have no constructor (or rather, only have a single parameterless constructor), there is nowhere to pass these implicit parameters and thus context bounds and view bounds are illegal on generic traits.
The simplest solution would be to turn TreeNode into an abstract class.:
abstract class TreeNode[A <% Ordered[A]]
Note that as advised by Ben James, using a context bound with an Ordering is usually better than a view bound with an Ordered (it is more general). However the problem is still the same: won't work on a trait.
If turning TreeNode into a class is not practical (say you need to mix it at various places in the type hierarchy), you can define an abstract method in TreeNode that will provide the implicit value (of type Ordered[A]) and have all the classes that extend it define it. This unfortunately more verbose and explicit, but you can't do much better in this case:
trait TreeNode[A] {
implicit protected def toOrdered: A => Ordered[A]
}
case class Branch[A<%Ordered[A]](value: A, left: TreeNode[A], right: TreeNode[A]) extends TreeNode[A] {
protected def toOrdered = implicitly[A => Ordered[A]]
}
case class Leaf[A<%Ordered[A]]() extends TreeNode[A] {
protected def toOrdered = implicitly[A => Ordered[A]]
}
Note that for a more concise definition, you could equivalently define Leaf like this:
case class Leaf[A](implicit protected val toOrdered: A => Ordered[A]) extends TreeNode[A]

You could provide the "evidence" that A is Ordered by requiring an abstract member of type Ordered[A] on the trait:
trait TreeNode[A] {
implicit val evidence: Ordered[A]
}
You would then be forced to provide this in any concrete subtypes, this proving that A is Ordered:
case class Leaf[A](value: A)(implicit ev: Ordered[A]) extends TreeNode[A] {
val evidence = ev
}
You might instead want to constrain A to a type which has an implicit Ordering[A] - this is not an inheritance relationship; it is more like a haskell typeclass. But the implementation in terms of the above technique would be the same.

#ben-james's answer is great, I would like improve it a bit to avoid redundant vals in classes.
The idea is to define implicit constructor parameter name the same as it is defined in trait that holds implicit value.
The idea is to avoid this line:
val evidence = ev
Here is a complete example (gist)
trait PrettyPrinted[A] extends (A => String)
object PrettyPrinted {
def apply[A](f: A => String): PrettyPrinted[A] = f(_)
}
trait Printable[A] {
implicit def printer: PrettyPrinted[A]
}
// implicit parameter name is important
case class Person(name: String, age: Int)
(implicit val printer: PrettyPrinted[Person])
extends Printable[Person]
object Person {
implicit val printer: PrettyPrinted[Person] =
PrettyPrinted { p =>
s"Person[name = ${p.name}, age = ${p.age}]"
}
}
// works also with regular classes
class Car(val name: String)
(implicit val printer: PrettyPrinted[Car])
extends Printable[Car]
object Car {
implicit val printer: PrettyPrinted[Car] =
PrettyPrinted { c =>
s"Car[name = ${c.name}]"
}
}

Related

Managing trait instance consumers in a generic way

Let's imagine I have the following base trait and case classes
sealed trait BaseTrait
case class Foo(x: Integer = 1) extends BaseTrait
case class Bar(x: String = "abc") extends BaseTrait
I would like to create a generic interface for classes which can process BaseTrait instances, something like the following
class FooProcessor(val model: FooModel) extends BaseProcessor[Foo] {
val v: Option[Foo] = model.baseVar
}
class BarProcessor(val model: BarModel) extends BaseProcessor[Bar] {
val v: Option[Bar] = model.baseVar
}
For this I have the following traits
trait BaseModel[T <: BaseTrait] {
var baseVar: Option[T] = None
}
trait BaseProcessor[T <: BaseTrait] {
def model: BaseModel[T]
def process(x: T): Unit = model.baseVar = Option(x)
}
The model definitions are the following
class FooModel extends BaseModel[Foo]
class BarModel extends BaseModel[Bar]
Now lets imagine I have the following processors somewhere in my app
val fooProcessor = new FooProcessor(new FooModel)
val barProcessor = new BarProcessor(new BarModel)
I would like to handle them in a somewhat generic way, like this
def func[T <: BaseTrait](p: T) {
val c/*: BaseProcessor[_ >: Foo with Bar <: BaseTrait with Product with Serializable]*/ = p match {
case _: Foo => fooProcessor
case _: Bar => barProcessor
c.process(p)
}
The compiler is not really happy about the last line, it says
type mismatch;
found : T
required: _1
If I understand correctly this is basically the compiler trying to prevent barProcessor.process(Foo()) from happening. I've tried a couple of solutions to get around this and achieve the desired behavior:
the simplest way around this is calling the proper *Processor.process with the proper BaseTrait instance inside the match case, which seems to defy the whole point of handling them in a somewhat generic way
use an abstract type in the BaseModel and BaseProcessor, which one hand got rid of the somewhat unneeded type parameter in BaseModel but the compilers complaint is still valid and I was not able to figure out if it's possible to get that to work
get rid of the type parameter and contraint from the BaseModel, and just do a type cast in the processor to get the proper type, but the explicit type cast also isn't really what I was hoping for
Like so:
trait BaseModel {
var baseVar: Option[BaseTrait] = None
}
trait BaseProcessor[T <: BaseTrait] {
def model: BaseModel
def process(x: T): Unit = model.baseVar = Some(x)
def getBaseValue: T = model.baseVar.map(_.asInstanceOf[T])
}
I guess one could also somehow convince the compiler that the two types (T of the Processor and T of the func parameter p) are equivalent, but that also seems like an overkill (and I'm also not really sure how it can be done).
So my question is the following: is it possible to do what I'm trying to achieve here (managing processors in a uniform way + each processor knows their specific type of BaseTrait) in a somewhat easy fashion? Is there a better model for this which I'm missing?
Update
As per Tim's answer making the controllers implicit solves the problem, however if you want to have a class where you define your controllers + have 'func' on it's interface the compiler no longer seems to properly resolve the implicits. So if I try to do something like this
class ProcessorContainer {
implicit val fooProcessor = new FooProcessor(new FooModel)
implicit val barProcessor = new BarProcessor(new BarModel)
def func[T <: BaseTrait](p: T) = typedFunc(p)
private def typedFunc[T <: BaseTrait](p: T)(implicit processor: BaseProcessor[T]) =
processor.process(p)
}
class Test {
val processorContainer = new ProcessorContainer
processorContainer.func(Foo())
processorContainer.func(Bar())
}
I get the following compile error (one for Foo and one for Bar):
could not find implicit value for parameter processor: BaseProcessor[Foo]
not enough arguments for method
Is there a way around this? I could of course expose the controllers so they can be passed in implicitly, however I'd prefer not doing that.
You can create a simple typeclass by making the processors implicit and passing them as an extra argument to func:
implicit val fooProcessor = new FooProcessor(new FooModel)
implicit val barProcessor = new BarProcessor(new BarModel)
def func[T <: BaseTrait](p: T)(implicit processor: BaseProcessor[T]) =
processor.process(p)
If you pass a Foo to func it will call FooProcessor.process on it, and if you pass a Bar to func it will call BarProcessor on it.

How to parameterize an instance of a class with a view bound?

My motivation is I want an abstract class for sorting functions. Right now that looks like:
abstract class Sorter[A <% Ordered[A]]{
def apply(xs: Seq[A]): Seq[A]
}
But I'm not sure how to make a concrete instance of it, i.e.:
val mySort = new Sorter {
def apply(xs: Seq[A]) = { ... }
}
I suppose my question is two-fold:
Is my abstract class thing even correct?
How do I parameterize mySort?
View bounds are deprecated so if you don't have to use them you could write something simple like:
trait Sorter[A] {
def sort(xs: Seq[A])(implicit ord: Ordering[A]): Seq[A]
}
new Sorter[Int] {
override def sort(xs: Seq[X])(implicit ord: Ordering[X]): Seq[X] = {
xs.sorted
}
}
Sorter is a trait but could well be an abstract class. Parameterizing instance is done right before the brackets as shown in the example.
On a side note, in Scala I don't see the need for an abstract class (or trait) for ordering things.
The collections already define the sorted and sortBy methods. So when you have a collection col of A you can sort it with col.sorted provided you have an implicit Ordering[A] in scope.
It's up to you to figure out if Ordering is what you want or if abstract class and concrete implementation is required.
Keep in mind you can always implement you custom Ordering like:
new Ordering[Int] {
override def compare(x: X, y: X) = ???
}

In Scala, How to reference the ClassTag of implemented abstract type in all subclasses?

If I have 1 trait and 2 objects:
trait MyClass {
type T <: MyClass
def foo(): ClassTag[T] = {...}
}
object ChildClass1 extends MyClass {
type T = String
}
object ChildClass2 extends MyClass {
type T = Option[String]
}
is it possible to implement foo() in MyClass, such that ChildClass1.foo() yields ClassTag[String], and ChildClass2.foo() yields ClassTag[Option].
If not, what's the easiest way to bypass it? It should be noted that the implementation of T may be inner classes/objects, so hacking reflection may have some side effects.
Ok, it's possible that I don't completely understand your goal, but from what I can see, you are trying to create a trait with an upper bound type, and you also want to be able to get the type at runtime, correct?
So let's assume you have a Foo trait:
class MyClass // not important
trait Foo[T <: MyClass] {
def foo: ClassTag[T]
}
If you want an object implementation, the solution is trivial, since you know the type at compile time:
class MyClassSubclass extends MyClass // also not important
object FooObject extends Foo[MyClassSubclass] {
def foo: ClassTag[MyClassSubclass] = ClassTag(classOf[MyClassSubclass])
}
But if you want a class, then you can solve the problem with the implicitly + context bound combo in a pretty readable way:
class FooImpl[T <: MyClass : ClassTag] extends Foo[T] {
def foo: ClassTag[T] = implicitly[ClassTag[T]]
}
I would like to propose a dirty and impaired answer, please advice me if you have any better idea:
lazy val mf: ClassTag[T] = {
val clazz = this.getClass
val name = clazz.getName
val modifiedName = name + "T"
val reprClazz = Utils.classForName(modifiedName)
Manifest.classType(reprClazz)
}
it only works if the subclass is a singleton object.

How might I create a Typed Configuration Map similar to SBT's new configuration?

I am new to Scala and still wrapping my head around the type system and how flexible it is / is not. I'm having some issues understanding some of the type scoping and what type parameters I can and cannot refer to when defining parameters in various locations - especially with "higher kinded types" where I'm nesting types at two levels, ie: Foo[T <: Bar[U]](x: U) // U does not seem to be accessible here as a type for x
What I'd like to be able to do is define a Config type which is typesafe but generic enough to be used for multiple types (Config[T]), such that I can pass in Config[T](c: Map[AttributeKey[Attribute[T]], AttributeValue[Attribute[T]]]) which enforces that only AttributeKeys for T can be passed in, and, similarly, only AttributeValues fora specific Attribute for T.
What I have defined below does not work, and I have a feeling I may be going down the wrong path. I would really love if someone could help me detangle it and understand it a bit more. I've tried taking a look at the SBT source, but haven't been able to distill it down to the parts that I need.
package foo.conf
package object conf {
case object Provider {
sealed trait Type {
def name: String
override def toString = name
}
sealed trait MyProvider extends Type {
val name = "MyProvider"
}
}
import Provider._
case class Provider[T <: Type](config: Config[T])
case object Config {
sealed trait Attribute[T <: Type, VT] {
def name: String
def value: VT
override def toString = name
}
final case class AttributeKey[T <: Attribute](name: String)
case object AttributeKey {
implicit def attributeToKey[T <: Attribute](a: T): AttributeKey[T] = AttributeKey(a.name)
}
final case class AttributeValue[VT, T: Attribute[_, VT]](value: VT)
case object AttributeValue {
implicit def attributeToValue[T](a: Attribute[_, T]): AttributeValue[T] = AttributeValue(a.value)
}
case class MyProviderSpecificAttr[T: String](value: T) extends Attribute[MyProvider, T] {
val name = "my-provider-specific-attr"
}
case object `my-provider-specific-attr-val-1` extends MyProviderSpecificAttr("my-provider-specific-attr-val-1")
case object `my-provider-specific-attr-val-2` extends MyProviderSpecificAttr("my-provider-specific-attr-val-2")
}
import Config._
final case class Config[T <: Type](c: Map[AttributeKey[Attribute[T, _]], AttributeValue])
val test = Config[MyProvider](Map(
MyProviderSpecificAttr -> `my-provider-specific-attr-val-1`
))
}

Scala: illegal inheritance; self-type Foo[T] does not conform to Foo[T]'s selftype T

I have the following code snippet:
abstract class Foo[T <: Foo[T]] { self: T =>
def bar(x: T): T
def newFoo: Foo[T] = {
new Foo[T] { self: T =>
// ...
}
}
}
I have a need to generate a new instance of Foo within a method of my abstract class. Can anyone advise me on how best to approach this?
Thanks,
Hadil
The self-type self: T => implies that your Foo[T] must also be a T. new Foo[T] { ... } isn't an instance of T for any arbitrary T that makes up Foo[T]. You also can't add a self-type to an anonymous class like new Foo[T] { ... }, because it doesn't make sense. Either the concrete class is or isn't a T at that point.
Constructing a method like def newFoo: Foo[T] in a type-safe way isn't really possible with the self-type in place, because you'd need to know how to construct an arbitrary T. You might be able to do what you want with reflection, when each T has the same constructor.
import scala.reflect._
abstract class Foo[T <: Foo[T] : ClassTag] { self: T =>
def bar(x: T): T
def newFoo: Foo[T] = classTag[T].runtimeClass.newInstance.asInstanceOf[T]
}
class Bar extends Foo[Bar] {
def bar(x: Bar): Bar = x
}
scala> val b = new Bar
b: Bar = Bar#2a2d45ba
scala> b.newFoo
res1: Foo[Bar] = Bar#146ba0ac
This ceases to work when there are constructor parameters:
case class Baz(i: Int) extends Foo[Baz] {
def bar(x: Baz): Baz = x
}
scala> val baz = new Baz(0)
baz: Baz = Baz(0)
scala> baz.newFoo
java.lang.InstantiationException: Baz
at java.lang.Class.newInstance(Class.java:427)
at Foo.newFoo(<console>:16)
Well, you do not know the concrete class where Foo will be inherited in the future. So especially, you do not know which constructor parameters this class will have and how to supply arguments for them. If you really would want to do this, you would have to make some assumptions (that you cannot enforce at compile time) to achieve this.
So what you probably should do, is, leave this method abstract and implement it in a subclass. If this is not an option, you probably have some issues with your overall class design. Better present here what you want to model and ask for help.
If you assume, that the constructor of the concrete class won't have any parameters, you can implement newFoo just as
def newFoo = this.getClass.newInstance
There is no need for classTags or other fancy stuff.