Override toString behavior for Option[_] in scala - scala

I would prefer to see just the value of the Option (if it were not None) , instead of the following additional Some() noise:
List((Some(OP(_)),Some(share),3), (Some(OP(D)),Some(shaara),4), (Some(OP(I)),Some(shaaee),4))
Now, I could write a method that handles this for List[Option[_]] .. But there are many other structures in which Options appear - so this approach of addressing explicitly each one is cumbersome.
Due to implicits having lower precedence the following code simply gets ignored:
implicit def toString(myopt : Option[_]) = if (myopt == None) "None" else myopt.get
The concern is that - although implementing for example a toString(List[Option_]]) method that handles this in the desired manner, that is still a one-off. What about a
Map[Option,Option] => def toString(Map[Option,Option]) = { .. }
It seems We would still need to implement an explicit toString() for each collection type..

I guess you can't override this behavior for toString, but you could use shows (and show) methods from scalaz. You could override behavior of these methods:
import scalaz._, Scalaz._
val l = List(1.some, none, 3.some, 4.some, none, 6.some)
l.shows
// res0: String = [Some(1),None,Some(3),Some(4),None,Some(6)]
implicit def optionShow[T: Show]: Show[Option[T]] =
Show.show{ _.map{_.show}.getOrElse(Cord("<none>")) }
l.shows
// res1: String = [1,<none>,3,4,<none>,6]
It works for all types with Show:
1.some.node(none.node(2.some.leaf)).drawTree
// 1
// |
// `- <none>
// |
// `- 2
Map(1.some -> 2.some, none[Int] -> 3.some).shows
// Map[1->2, <none>->3]

Well i would just write a Wrapper similar to your implicit.
class OW[T](val option : Option[T]) {
override def toString = if (option.isEmpty) "None" else option.get.toString
}
Then when i want the pretty toString, i would just map any collection to instances of OW.
println(List(Some(3), Some("Hello"), None).map(new OW(_)))
which prints: List(3, Hello, None)
Don't really see a better way.

Following takes care of the cases that came to mind:
def show(obj: Any) : String = {
obj match {
case o: Option[_] =>
if (o == None) {
"<none>"
} else {
show(o.get)
}
case i: Iterable[_] =>
i.map(show).mkString("[", ",", "]")
case m: Map[_, _] =>
m.map {
case (a, b) => List(show(a), show(b)).mkString(":")
}.mkString("{", ",", "}")
case e: Enumeration =>
e.toString
case c : Product if !c.getClass.getMethods.map(_.getName).contains("copy$default$2") =>
c.toString
case t: Product =>
t.productIterator.map(a => show(a)).mkString("(", ",", ")")
case _ =>
if (obj.isInstanceOf[AnyRef])
obj.asInstanceOf[AnyRef].toString
else
"" + obj
}
}
I opted to code this up to avoid adding scalaz dependency (assuming their Show class supports similar feature)

Related

How to negate types in pattern match in Scala?

With this code println will be executed only for specified exception. I'm wondering if it's possible to negate that line to make it executed for all other exceptions that are not specified. I know it's possible using 2 cases, but I want to know if it can be done with one case.
val myHandler: PartialFunction[Throwable, Unit] = {
case e # (_: MappingException | _: ParseException | _: SomeOtherException) =>
println("Got it")
}
AFAIk you can not do this with a single match, but you can create your own custom Extractor in case you need to replicate this behaviour in multiple places.
import scala.reflect.ClassTag
final class Not[A : ClassTag] {
def unapply(any: Any): Boolean = any match {
case _: A => false
case _ => true
}
}
object Not {
def apply[A : ClassTag]: Not[A] = new Not
}
which you can use like this:
final val NotAnInt = Not[Int]
10 match {
case NotAnInt() => false
case _ => true
}
// res: Boolean = true
"10" match {
case NotAnInt() => true
case _ => false
}
// res: Boolean = true
However, keep in mind this will have all the limitation of any type check, like not being able to differentiate between a List[Int] from a List[String] due erasure; and being considered a bad practice.
I would suggest looking into a typeclass approach, for example, I believe Shapeless provides a negation one.
You can see the code running here.
Well you've already identified what is probably the more readable way to do it.
val myHandler: PartialFunction[Throwable, Unit] = {
case e # (_: MappingException | _: ParseException | _: SomeOtherException) =>
throw e
case _ =>
println("Got it")
}
This is probably how I'd write this in actual production code. It's sensible and it's clear at a glance.
But you asked for one case, so let's give that a go. Since we want to check against several types, we'll need to be able to represent them as a list. There are countless Scala libraries that make this prettier, but for our purposes we'll just roll our own.
trait TList {
def isMember(x: Any): Boolean
}
object Nil extends TList {
def isMember(x: Any) = false
}
case class Cons[H : ClassTag](val tail: TList) extends TList {
def isMember(x: Any) = {
x match {
case _: H => true
case _ => tail.isMember(x)
}
}
}
So we can represent classical Lisp-style singly-linked lists and check whether an arbitrary Any value has a type anywhere in the list. Now let's negate it and write an unapply method.
case class NotMember(val types: TList) {
def unapply(elem: Any): Boolean =
!types.isMember(elem)
}
Then our handler looks like
val test = NotMember(
Cons[MappingException](Cons[ParseException](Cons[SomeOtherException](Nil)))
)
val myHandler: PartialFunction[Throwable, Unit] = {
case test() =>
println("Got it")
}
Again, if you really want to go down this road, you'll want to grab a library to make the type-level stuff manageable. But it's definitely possible. The only question is whether it's worth it for your use case.

How to implement something like the strategy pattern in scala

I would like to achieve something akin to the strategy pattern in scala without resorting to pattern matching with a long list of case statements. Here is roughly what I have in mind:
trait HandlerTrait {
def handlerA(...): Unit
def handlerB(...): Unit
}
SomeClass1 extends HandlerTrait {
override def handlerA(...) {...}
override def handlerB(...) {...}
}
SomeClass2 extends HandlerTrait {
override def handlerA(...) {...}
override def handlerB(...) {...}
}
object MyApp extends App {
// 1. define bindings for these implementations
val myBindings = Map(x -> someClass1, y -> someClass2)
// 2. Such that implementation of someMethod targeting handlerA implementations could look like this:
def someMethod(object: ObjectType): Unit = {
myBindings.get(object.x) match {
case Some(entry) => entry.handlerA(object)
case None => ()
}
}
}
A few more things:
I don't know how many SomeClassXXX I will have. I will add as needed
to provide customizations on how to handle A/B/C ...
Given a key, I want to dispatch to the correct class and execute the targeted handler.
Is there a better more concise way of achieving this in scala?
I think one way to reduce boilerplate is to use type system rather than inheritance. For instance, if your handler is of type T => Unit then any function that satisfies this type can be a handler, there is no need to officially declare HandlerTrait and even someMethod.
Whether to use a Map or cases to map from a key to a handler is up to you. Both can be extended to handle new cases.
Here is an example to sum up what I'm proposing:
val currentlyDefinedStrategies: PartialFunction[String, Unit] = {
case "1" => println(1)
case "2" => println(2)
}
val newStrategies: PartialFunction[String, Unit] = {
case "3" => println(3)
}
val defaultStrategy: PartialFunction[String, Unit] = {
case _ => println("default")
}
And usage:
scala> currentlyDefinedStrategies("1")
1
scala> currentlyDefinedStrategies("3")
scala.MatchError: 3 (of class java.lang.String) ...
scala> currentlyDefinedStrategies.orElse(newStrategies)("3")
3
scala> currentlyDefinedStrategies.orElse(newStrategies)("4")
scala.MatchError: 4 (of class java.lang.String)
scala> currentlyDefinedStrategies.orElse(newStrategies).orElse(defaultStrategy)("4")
default
You can achieve similar pattern with Map or using other FP techniques. The main point is to keep the most relevant code and get rid of boilerplate. Of course, HandlerTrait might be useful to you for structuring your code and thinking in terms of classes rather than functions, but the idea is the same.
See also: https://pavelfatin.com/design-patterns-in-scala/#strategy
The above example is a bit simplified and you actually want to pass parameters to handler (println in our case). Here is how:
val currentlyDefinedStrategies: Int => PartialFunction[String, Unit] = (x) => {
case "1" => println("1: " + x)
case "2" => println("2: " + x)
case _ => println("default: " + x)
}
You can fix the argument without choosing a strategy:
scala> val noStrategy = currentlyDefinedStrategies(1)
noStrategy: PartialFunction[String,Unit] = <function1>
... and provide strategy afterwards:
scala> noStrategy("1")
1: 1
Or apply the strategy right away:
scala> currentlyDefinedStrategies(1)("1")
1: 1
You can also decide on your strategy first and then pass an argument:
val currentlyDefinedStrategies: PartialFunction[String, Int => Unit] = {
case "1" => x => println("1: " + x)
case "2" => x => println("2: " + x)
case _ => x => println("default: " + x)
}
scala> val handlerWithChosenStrategy = currentlyDefinedStrategies("1")
handlerWithChosenStrategy: Int => Unit = $anonfun$1$$Lambda$1374/666224848#59a9f3eb
scala> handlerWithChosenStrategy(1)
1: 1
I think the point is that FP is so rich and flexible that strategy pattern is really not a thing. It's basically just some function type that suits your convenience like type Strategy[T, -A, +B] = PartialFunction[T, A => B]. Example:
scala> type Strategy[T, -A, +B] = PartialFunction[T, A => B]
defined type alias Strategy
val currentlyDefinedStrategies: Strategy[String, Int, Unit] = {
case "1" => x => println("1: " + x)
case "2" => x => println("2: " + x)
case _ => x => println("default: " + x)
}
currentlyDefinedStrategies: Strategy[String,Int,Unit] = <function1>
scala> currentlyDefinedStrategies("1")(1)
1: 1
A more advanced concept would be an Expression Problem (here) where you need to extend both the operations you can do on types as well as adding new types.

scalaz, Disjunction.sequence returning a list of lefts

In scalaz 7.2.6, I want to implement sequence on Disjunction, such that if there is one or more lefts, it returns a list of those, instead of taking only the first one (as in Disjunction.sequenceU):
import scalaz._, Scalaz._
List(1.right, 2.right, 3.right).sequence
res1: \/-(List(1, 2, 3))
List(1.right, "error2".left, "error3".left).sequence
res2: -\/(List(error2, error3))
I've implemented it as follows and it works, but it looks ugly. Is there a getRight method (such as in scala Either class, Right[String, Int](3).right.get)? And how to improve this code?
implicit class RichSequence[L, R](val l: List[\/[L, R]]) {
def getLeft(v: \/[L, R]):L = v match { case -\/(x) => x }
def getRight(v: \/[L, R]):R = v match { case \/-(x) => x }
def sequence: \/[List[L], List[R]] =
if (l.forall(_.isRight)) {
l.map(e => getRight(e)).right
} else {
l.filter(_.isLeft).map(e => getLeft(e)).left
}
}
Playing around I've implemented a recursive function for that, but the best option would be to use separate:
implicit class RichSequence[L, R](val l: List[\/[L, R]]) {
def sequence: \/[List[L], List[R]] = {
def seqLoop(left: List[L], right: List[R], list: List[\/[L, R]]): \/[List[L], List[R]] =
list match {
case (h :: t) =>
h match {
case -\/(e) => seqLoop(left :+ e, right, t)
case \/-(s) => seqLoop(left, right :+ s, t)
}
case Nil =>
if(left.isEmpty) \/-(right)
else -\/(left)
}
seqLoop(List(), List(), l)
}
def sequenceSeparate: \/[List[L], List[R]] = {
val (left, right) = l.separate[\/[L, R], L, R]
if(left.isEmpty) \/-(right)
else -\/(left)
}
}
The first one just collects results and at the end decide what to do with those, the second its basically the same with the exception that the recursive function is much simpler, I didn't think about performance here, I've used :+, if you care use prepend or some other collection.
You may also want to take a look at Validation and ValidationNEL which unlike Disjunction accumulate failures.

Specialization of Scala methods to a specific tags

I have a generic map with values, some of which can be in turn lists of values.
I'm trying to process a given key and convert the results to the type expected by an outside caller, like this:
// A map with some values being other collections.
val map: Map[String, Any] = Map("foo" -> 1, "bar" -> Seq('a', 'b'. 'a'))
// A generic method with a "specialization" for collections (pseudocode)
def cast[T](key: String) = map.get(key).map(_.asInstanceOf[T])
def cast[C <: Iterable[T]](key: String) = map.get(key).map(list => list.to[C].map(_.asIntanceOf[T]))
// Expected usage
cast[Int]("foo") // Should return 1:Int
cast[Set[Char]]("bar") // Should return Set[Char]('a', 'b')
This is to show what I would like to do, but it does not work. The compiler error complains (correctly, about 2 possible matches). I've also tried to make this a single function with some sort of pattern match on the type to no avail.
I've been reading on #specialized, TypeTag, CanBuildFrom and other scala functionality, but I failed to find a simple way to put it all together. Separate examples I've found address different pieces and some ugly workarounds, but nothing that would simply allow an external user to call cast and get an exception is the cast was invalid. Some stuff is also old, I'm using Scala 2.10.5.
This appears to work but it has a some problems.
def cast[T](m: Map[String, Any], k: String):T = m(k) match {
case x: T => x
}
With the right input you get the correct output.
scala> cast[Int](map,"foo")
res18: Int = 1
scala> cast[Set[Char]](map,"bar")
res19: Set[Char] = Set(a, b)
But it throws if the type is wrong for the key or if the map has no such key (of course).
You can do this via implicit parameters:
val map: Map[String, Any] = Map("foo" -> 1, "bar" -> Set('a', 'b'))
abstract class Casts[B] {def cast(a: Any): B}
implicit val doubleCast = new Casts[Double] {
override def cast(a: Any): Double = a match {
case x: Int => x.toDouble
}
}
implicit val intCast = new Casts[Int] {
override def cast(a: Any): Int = a match {
case x: Int => x
case x: Double => x.toInt
}
}
implicit val seqCharCast = new Casts[Seq[Char]] {
override def cast(a: Any): Seq[Char] = a match {
case x: Set[Char] => x.toSeq
case x: Seq[Char] => x
}
}
def cast[T](key: String)(implicit p:Casts[T]) = p.cast(map(key))
println(cast[Double]("foo")) // <- 1.0
println(cast[Int]("foo")) // <- 1
println(cast[Seq[Char]]("bar")) // <- ArrayBuffer(a, b) which is Seq(a, b)
But you still need to iterate over all type-to-type options, which is reasonable as Set('a', 'b').asInstanceOf[Seq[Char]] throws, and you cannot use a universal cast, so you need to handle such cases differently.
Still it sounds like an overkill, and you may need to review your approach from global perspective

Scala, pattern matching on a tuple of generic trait, checking if types are equal

I know a lot of questions exist about type erasure and pattern matching on generic types, but I could not understand what should I do in my case from answers to those, and I could not explain it better in title.
Following code pieces are simplified to present my case.
So I have a trait
trait Feature[T] {
value T
def sub(other: Feature[T]): Double
}
// implicits for int,float,double etc to Feature with sub mapped to - function
...
Then I have a class
class Data(val features: IndexedSeq[Feature[_]]) {
def sub(other: Data): IndexedSeq[Double] = {
features.zip(other.features).map {
case(e1: Feature[t], e2: Feature[y]) => e1 sub e2.asInstanceOf[Feature[t]]
}
}
}
And I have a test case like this
case class TestFeature(val value: String) extends Feature[String] {
def sub(other: Feature[String]): Double = value.length - other.length
}
val testData1 = new Data(IndexedSeq(8, 8.3f, 8.232d, TestFeature("abcd"))
val testData2 = new Data(IndexedSeq(10, 10.1f, 10.123d, TestFeature("efg"))
testData1.sub(testData2).zipWithIndex.foreach {
case (res, 0) => res should be (8 - 10)
case (res, 1) => res should be (8.3f - 10.1f)
case (res, 2) => res should be (8.232d - 10.123d)
case (res, 3) => res should be (1)
}
This somehow works. If I try sub operation with instances of Data that have different types in same index of features, I get a ClassCastException. This actually satisfies my requirements, but if possible I would like to use Option instead of throwing an exception. How can I make following code work?
class Data(val features: IndexedSeq[Feature[_]]) {
def sub(other: Data): IndexedSeq[Double] = {
features.zip(other.features).map {
// of course this does not work, just to give idea
case(e1: Feature[t], e2: Feature[y]) if t == y => e1 sub e2.asInstanceOf[Feature[t]]
}
}
}
Also I am really inexperienced in Scala, so I would like to get feedback on this type of structure. Are there another ways to do this and which way would make most sense?
Generics don't exist at runtime, and an IndexedSeq[Feature[_]] has forgotten what the type parameter is even at compile time (#Jatin's answer won't allow you to construct a Data with a list of mixed types of Feature[_]). The easiest answer might be just to catch the exception (using catching and opt from scala.util.control.Exception). But, to answer the question as written:
You could check the classes at runtime:
case (e1: Feature[t], e2: Feature[y]) if e1.value.getClass ==
e2.value.getClass => ...
Or include the type information in the Feature:
trait Feature[T] {
val value: T
val valueType: ClassTag[T] // write classOf[T] in subclasses
def maybeSub(other: Feature[_]) = other.value match {
case valueType(v) => Some(actual subtraction)
case _ => None
}
}
The more complex "proper" solution is probably to use Shapeless HList to preserve the type information in your lists:
// note the type includes the type of all the elements
val l1: Feature[Int] :: Feature[String] :: HNil = f1 :: f2 :: HNil
val l2 = ...
// a 2-argument function that's defined for particular types
// this can be applied to `Feature[T], Feature[T]` for any `T`
object subtract extends Poly2 {
implicit def caseFeatureT[T] =
at[Feature[T], Feature[T]]{_ sub _}
}
// apply our function to the given HLists, getting a HList
// you would probably inline this
// could follow up with .toList[Double]
// since the resulting HList is going to be only Doubles
def subAll[L1 <: HList, L2 <: HList](l1: L1, l2: L2)(
implicit zw: ZipWith[L1, L2, subtract.type]) =
l1.zipWith(l2)(subtract)
That way subAll can only be called for l1 and l2 all of whose elements match, and this is enforced at compile time. (If you really want to do Options you can have two ats in the subtract, one for same-typed Feature[T]s and one for different-typed Feature[_]s, but ruling it out entirely seems like a better solution)
You could do something like this:
class Data[T: TypeTag](val features: IndexedSeq[Feature[T]]) {
val t = implicitly[TypeTag[T]]
def sub[E: TypeTag](other: Data[E]): IndexedSeq[Double] = {
val e = implicitly[TypeTag[E]]
features.zip(other.features).flatMap{
case(e1, e2: Feature[y]) if e.tpe == t.tpe => Some(e1 sub e2.asInstanceOf[Feature[T]])
case _ => None
}
}
}
And then:
case class IntFeature(val value: Int) extends Feature[Int] {
def sub(other: Feature[Int]): Double = value - other.value
}
val testData3 = new Data(IndexedSeq(TestFeature("abcd")))
val testData4 = new Data(IndexedSeq(IntFeature(1)))
println(testData3.sub(testData4).zipWithIndex)
gives Vector()