Is there really no simple idiomatic way of checking whether an object is a certain class, other than using .isInstanceOf[ClassName] or a full match expression?
A match for a single class may avoid the need for using .AsInstanceOf (when necessary for acting on class-unique members), but is very verbose in general and in requiring to explicitly cover for the default case...
Perhaps there would be some library for working with class hierarchies that simplifies assigning classes to objects and checking for them, to bypass the scala core class matcher limitations. I even considered implementing my own type assignment, hierarchies and checking solution, using plain collections, rather than using scala classes, for rich hierarchical type modeling that feels more idiomatic.
No, there is not. The syntax was deliberately made verbose because having to use this feature often indicates a bad design.
It is possible, however, to write a function yourself:
implicit class WithAs[T](x: T) {
import scala.reflect._
def as[O: ClassTag]: Option[O] = x match {
case o: O => Some(o)
case _ => None
}
}
live demo
Related
I am writing by hand a recursive-descent parser for a small language. In my lexer I have:
trait Token{def position:Int}
trait Keyword extends Token
trait Operator extends Token
case class Identifier(position:Int, txt:String) extends Token
case class If (position:Int) extends Keyword
case class Plus (position:Int) extends Operator
/* etcetera; one case class per token type */
My parser works well, and now I would like to incorporate some error recovery: replacing, inserting or discarding tokens until some synchronization point.
For that, it would be handy to have a function that, in invalid Scala, would look something like this
def scanFor(tokenSet:Set[TokenClass], lookahead:Int) = {
lexer.upcomingTokens.take(lookahead).find{ token =>
tokenSet.exists(tokenClass => token.isInstanceOf[tokenClass])
}
}
which I would call, for example: scanFor(Set(Plus, Minus, Times, DividedBy), 4)
However TokenClass, of course, is not a valid type, and I don't know how to create the previous set.
As alternatives:
I could just create a new trait and make all the token classes in the token set I want to check against extend that trait, and then just do an instanceOf check against that trait. However, I may have several of those sets, which could make them hard to name, and the code hard to maintain later on.
I could create isXXX:Token=>Boolean functions, and make sets of those, but it seems unelegant
Any suggestions?
I actually recommend, if there are only a handful of such combinations, using an additional trait. It's easy to write and understand, and it will be fast at runtime. It's not really so bad to say
case class Plus(position: Int)
extends Operator with Arithmetic with Precedence7 with Unary
But there are a wide range of alternatives.
If you don't mind a finicky manual maintenance process and need something really fast, defining an ID number (which you must manually keep distinct) for each token type will allow you to use Set[Int] or BitSet or even just a Long to select those classes you like. You can then do set operations (union, intersection) to build up these selectors from each other. It's not hard to write unit tests to help make the finicky bit a little more reliable. If you can at least manage to list all your types:
val everyone = Seq(Plus, Times, If /* etc */)
assert(everyone.length == everyone.map(_.id).toSet.size)
So you shouldn't be too alarmed by this approach if you decide the performance and composability are essential.
You can also write custom extractors that can (more slowly) pull out the right subset of tokens by pattern matching. For example,
object ArithOp {
def unapply(t: Token): Option[Operator] = t match {
case o: Operator => o match {
case _: Plus | _: Minus | _: Times | _: DividedBy => Some(o)
case _ => None
}
case _ => None
}
}
will give you None if it's not the right type of operation. (In this case, I'm assuming there's no parent other than Operator.)
Finally, you could probably express your types as unions and HLists and pick them out that way using Shapeless, but I don't have experience personally doing that with a parser, so I'm not sure of what difficulties you might encounter.
Reading Martin's book, chapter about equality Object equality chapter one may notice that properly implementing equals in scala (as actually in any other language) is not very straightforward. However scala is extremely powerful and agile, and I cannot believe it could not simplify things a bit. I know that scala generates proper equals for case classes, so I wonder why couldn't it generate simplifications for normal classes?
To show my point, I wrote an example of how would I see this should look like. It probably has flaws, and I had to use ClassTag which I know is very wrong for such basic thing as equals due to performance (any tip how could I do it without ClassTag?), but thinking that scala can generate proper equals for case classes, I'd say it should be able to generate proper code for normal classes, giving that developer provides the Key which should be used to compare objects.
trait Equality[T] extends Equals {
val ttag: ClassTag[T]
def Key: Seq[Any]
def canEqual(other: Any): Boolean = other match {
case that: Equality[_] if that.ttag == ttag => true
case _ => false
}
override def equals(other: Any): Boolean =
other match {
case that: Equality[T] => canEqual(that) && Key == that.Key
case _ => false
}
override def hashCode = Key.foldLeft(1)((x, y) => 41 * x + y.hashCode)
}
Then you can use it like this:
class Point(val x: Int, val y: Int)(implicit val ttag: ClassTag[Point]) extends Equality[Point]{
override def Key: Seq[Any] = Seq(x, y)
}
I'm not very much into ClassTags, so I might have made it wrong, but it seems to be working. Still that's not what I am asking - I want to know is there any serious reasons, why scala itself do not simplify implementing equality checks?
It is an interesting idea, and I can see from a comment that scalaz has something like this. I'm not sure I have a complete answer, but some factors to consider are:
Case classes are a bit unique in that you're not supposed to inherit from them. Assuming they're not sub-classed, canEqual isn't even required.
There's something elegant about the idea that even '+' in scala is a function not a 'language feature.' It's not necessarily better to have this as a language feature instead of a library (and there are a number of utilities even in Java to help with implementing hashcode/equals). The existing "equals" method isn't a language feature, it's just a method on the parent class Object, inherited from Java.
Scala does still need to play-nice with Java, and that could be one barrier to radically changing how equals works. Interface requirements can't be imposed on existing Java classes that scala might want to inherit from.
When you do consider what the syntax might look like if it were a language feature, I'm not sure what could actually be eliminated or changed.
For example, the developer still has to specify the Key you suggest. Also, to be able to support, for example, anonymous subclasses and trait mix-ins where canEqual won't change, with a language feature you'd still need to explicitly define your class tag.
Maybe the analysis could be more interesting if you provide what, syntactically, it might look like if it were a language feature instead of a helper library. I might be missing some aspects of how this would work.
There is no such thing "proper equals" since it depends on the use cases.
For simple cases what you suggest may work, but in such case, using case class with also work. The problem is that while for simple Data classes it works, the same isn't true for other cases. For example, when there are inheritance - what is the right thing to do? if class have private members - does it should be part of the equals?? does public getters should be ? What will happen when when there are cyclic dependencies ?
One of the main reasons for the case classes cannot be inherit by other case class is the equality.
As far as I understand value classes in Scala are just there to wrap primitive types like Int or Boolean into another type without introducing additional memory usage. So they are basically used as a lightweight alternative to ordinary classes.
That reminds me of Haskell's newtype notation which is also used to wrap existing types in new ones, thus introducing a new interface to some data without consuming additional space (to see the similarity of both languages consider for instance the restriction to one "constructor" with one field both in Haskell and in Scala).
What I am wondering is why the concept of introducing new types that get inlined by the compiler is not generalized to Haskell's approach of having zero-overhead type wrappers for any kind of type. Why did the Scala guys stick to primitive types (aka AnyVal) here?
Or is there already a way in Scala to also define such wrappers for Scala.AnyRef types?
They're not limited to AnyVal.
implicit class RichOptionPair[A,B](val o: Option[(A,B)]) extends AnyVal {
def ofold[C](f: (A,B) => C) = o map { case (a,b) => f(a,b) }
}
scala> Some("fish",5).ofold(_ * _)
res0: Option[String] = Some(fishfishfishfishfish)
There are various limitations on value classes that make them act like lightweight wrappers, but only being able to wrap primitives is not one of them.
The reasoning is documented as Scala Improvement Process (SIP)-15. As Alexey Romanov pointed out in his comment, the idea was to look for an expression using existing keywords that would allow the compiler to determine this situation.
In order for the compiler to perform the inlining, several constraints apply, such as the wrapping class being "ephemeral" (no field or object members, constructor body etc.). Your suggestion of automatically generating inlining classes has at least two problems:
The compiler would need to go through the whole list of constraints for each class. And because the status as value class is implicit, it may flip by adding members to the class at a later point, breaking binary compatibility
More constraints are added by the compiler, e.g. the value class becomes final prohibiting inheritance. So you would have to add these constraints to any class who want to be inlineable that way, and then you gain nothing but extra verbosity.
One could think of other hypothetical constructs, e.g. val class Meter(underlying: Double) { ... }, but the advantage of extends AnyVal IMO is that no syntactic extensions are needed. Also all primitive types are extending AnyVal, so there is a nice analogy (no reference, no inheritance, effective representation etc.)
Recently I read following SO question :
Is there any use cases for employing the Visitor Pattern in Scala?
Should I use Pattern Matching in Scala every time I would have used
the Visitor Pattern in Java?
The link to the question with title:
Visitor Pattern in Scala. The accepted answer begins with
Yes, you should probably start off with pattern matching instead of
the visitor pattern. See this
http://www.artima.com/scalazine/articles/pattern_matching.html
My question (inspired by above mentioned question) is which GOF Design pattern(s) has entirely different implementation in Scala? Where should I be careful and not follow java based programming model of Design Patterns (Gang of Four), if I am programming in Scala?
Creational patterns
Abstract Factory
Builder
Factory Method
Prototype
Singleton : Directly create an Object (scala)
Structural patterns
Adapter
Bridge
Composite
Decorator
Facade
Flyweight
Proxy
Behavioral patterns
Chain of responsibility
Command
Interpreter
Iterator
Mediator
Memento
Observer
State
Strategy
Template method
Visitor : Patten Matching (scala)
For almost all of these, there are Scala alternatives that cover some but not all of the use cases for these patterns. All of this is IMO, of course, but:
Creational Patterns
Builder
Scala can do this more elegantly with generic types than can Java, but the general idea is the same. In Scala, the pattern is most simply implemented as follows:
trait Status
trait Done extends Status
trait Need extends Status
case class Built(a: Int, b: String) {}
class Builder[A <: Status, B <: Status] private () {
private var built = Built(0,"")
def setA(a0: Int) = { built = built.copy(a = a0); this.asInstanceOf[Builder[Done,B]] }
def setB(b0: String) = { built = built.copy(b = b0); this.asInstanceOf[Builder[A,Done]] }
def result(implicit ev: Builder[A,B] <:< Builder[Done,Done]) = built
}
object Builder {
def apply() = new Builder[Need, Need]
}
(If you try this in the REPL, make sure that the class and object Builder are defined in the same block, i.e. use :paste.) The combination of checking types with <:<, generic type arguments, and the copy method of case classes make a very powerful combination.
Factory Method (and Abstract Factory Method)
Factory methods' main use is to keep your types straight; otherwise you may as well use constructors. With Scala's powerful type system, you don't need help keeping your types straight, so you may as well use the constructor or an apply method in the companion object to your class and create things that way. In the companion-object case in particular, it is no harder to keep that interface consistent than it is to keep the interface in the factory object consistent. Thus, most of the motivation for factory objects is gone.
Similarly, many cases of abstract factory methods can be replaced by having a companion object inherit from an appropriate trait.
Prototype
Of course overridden methods and the like have their place in Scala. However, the examples used for the Prototype pattern on the Design Patterns web site are rather inadvisable in Scala (or Java IMO). However, if you wish to have a superclass select actions based on its subclasses rather than letting them decide for themselves, you should use match rather than the clunky instanceof tests.
Singleton
Scala embraces these with object. They are singletons--use and enjoy!
Structural Patterns
Adapter
Scala's trait provides much more power here--rather than creating a class that implements an interface, for example, you can create a trait which implements only part of the interface, leaving the rest for you to define. For example, java.awt.event.MouseMotionListener requires you to fill in two methods:
def mouseDragged(me: java.awt.event.MouseEvent)
def mouseMoved(me: java.awt.event.MouseEvent)
Maybe you want to ignore dragging. Then you write a trait:
trait MouseMoveListener extends java.awt.event.MouseMotionListener {
def mouseDragged(me: java.awt.event.MouseEvent) {}
}
Now you can implement only mouseMoved when you inherit from this. So: similar pattern, but much more power with Scala.
Bridge
You can write bridges in Scala. It's a huge amount of boilerplate, though not quite as bad as in Java. I wouldn't recommend routinely using this as a method of abstraction; think about your interfaces carefully first. Keep in mind that with the increased power of traits that you can often use those to simplify a more elaborate interface in a place where otherwise you might be tempted to write a bridge.
In some cases, you may wish to write an interface transformer instead of the Java bridge pattern. For example, perhaps you want to treat drags and moves of the mouse using the same interface with only a boolean flag distinguishing them. Then you can
trait MouseMotioner extends java.awt.event.MouseMotionListener {
def mouseMotion(me: java.awt.event.MouseEvent, drag: Boolean): Unit
def mouseMoved(me: java.awt.event.MouseEvent) { mouseMotion(me, false) }
def mouseDragged(me: java.awt.event.MouseEvent) { mouseMotion(me, true) }
}
This lets you skip the majority of the bridge pattern boilerplate while accomplishing a high degree of implementation independence and still letting your classes obey the original interface (so you don't have to keep wrapping and unwrapping them).
Composite
The composite pattern is particularly easy to achieve with case classes, though making updates is rather arduous. It is equally valuable in Scala and Java.
Decorator
Decorators are awkward. You usually don't want to use the same methods on a different class in the case where inheritance isn't exactly what you want; what you really want is a different method on the same class which does what you want instead of the default thing. The enrich-my-library pattern is often a superior substitute.
Facade
Facade works better in Scala than in Java because you can have traits carry partial implementations around so you don't have to do all the work yourself when you combine them.
Flyweight
Although the flyweight idea is as valid in Scala as Java, you have a couple more tools at your disposal to implement it: lazy val, where a variable is not created unless it's actually needed (and thereafter is reused), and by-name parameters, where you only do the work required to create a function argument if the function actually uses that value. That said, in some cases the Java pattern stands unchanged.
Proxy
Works the same way in Scala as Java.
Behavioral Patterns
Chain of responsibility
In those cases where you can list the responsible parties in order, you can
xs.find(_.handleMessage(m))
assuming that everyone has a handleMessage method that returns true if the message was handled. If you want to mutate the message as it goes, use a fold instead.
Since it's easy to drop responsible parties into a Buffer of some sort, the elaborate framework used in Java solutions rarely has a place in Scala.
Command
This pattern is almost entirely superseded by functions. For example, instead of all of
public interface ChangeListener extends EventListener {
void stateChanged(ChangeEvent e)
}
...
void addChangeListener(ChangeListener listener) { ... }
you simply
def onChange(f: ChangeEvent => Unit)
Interpreter
Scala provides parser combinators which are dramatically more powerful than the simple interpreter suggested as a Design Pattern.
Iterator
Scala has Iterator built into its standard library. It is almost trivial to make your own class extend Iterator or Iterable; the latter is usually better since it makes reuse trivial. Definitely a good idea, but so straightforward I'd hardly call it a pattern.
Mediator
This works fine in Scala, but is generally useful for mutable data, and even mediators can fall afoul of race conditions and such if not used carefully. Instead, try when possible to have your related data all stored in one immutable collection, case class, or whatever, and when making an update that requires coordinated changes, change all things at the same time. This won't help you interface with javax.swing, but is otherwise widely applicable:
case class Entry(s: String, d: Double, notes: Option[String]) {}
def parse(s0: String, old: Entry) = {
try { old.copy(s = s0, d = s0.toDouble) }
catch { case e: Exception => old }
}
Save the mediator pattern for when you need to handle multiple different relationships (one mediator for each), or when you have mutable data.
Memento
lazy val is nearly ideal for many of the simplest applications of the memento pattern, e.g.
class OneRandom {
lazy val value = scala.util.Random.nextInt
}
val r = new OneRandom
r.value // Evaluated here
r.value // Same value returned again
You may wish to create a small class specifically for lazy evaluation:
class Lazily[A](a: => A) {
lazy val value = a
}
val r = Lazily(scala.util.Random.nextInt)
// not actually called until/unless we ask for r.value
Observer
This is a fragile pattern at best. Favor, whenever possible, either keeping immutable state (see Mediator), or using actors where one actor sends messages to all others regarding the state change, but where each actor can cope with being out of date.
State
This is equally useful in Scala, and is actually the favored way to create enumerations when applied to methodless traits:
sealed trait DayOfWeek
final trait Sunday extends DayOfWeek
...
final trait Saturday extends DayOfWeek
(often you'd want the weekdays to do something to justify this amount of boilerplate).
Strategy
This is almost entirely replaced by having methods take functions that implement a strategy, and providing functions to choose from.
def printElapsedTime(t: Long, rounding: Double => Long = math.round) {
println(rounding(t*0.001))
}
printElapsedTime(1700, math.floor) // Change strategy
Template Method
Traits offer so many more possibilities here that it's best to just consider them another pattern. You can fill in as much code as you can from as much information as you have at your level of abstraction. I wouldn't really want to call it the same thing.
Visitor
Between structural typing and implicit conversion, Scala has astoundingly more capability than Java's typical visitor pattern. There's no point using the original pattern; you'll just get distracted from the right way to do it. Many of the examples are really just wishing there was a function defined on the thing being visited, which Scala can do for you trivially (i.e. convert an arbitrary method to a function).
Ok, let's have a brief look at these patterns. I'm looking at all these patterns purely from a functional programming point of view, and leaving out many things that Scala can improve from an OO point of view. Rex Kerr answer provides an interesting counter-point to my own answers (I only read his answer after writing my own).
With that in mind, I'd like to say that it is important to study persistent data structures (functionally pure data structures) and monads. If you want to go deep, I think category theory basics are important -- category theory can formally describe all program structures, including imperative ones.
Creational Patterns
A constructor is nothing more than a function. A parameterless constructor for type T is nothing more than a function () => T, for example. In fact, Scala's syntactical sugar for functions is taken advantage on case classes:
case class T(x: Int)
That is equivalent to:
class T(val x: Int) { /* bunch of methods */ }
object T {
def apply(x: Int) = new T(x)
/* other stuff */
}
So that you can instantiate T with T(n) instead of new T(n). You could even write it like this:
object T extends Int => T {
def apply(x: Int) = new T(x)
/* other stuff */
}
Which turns T into a formal function, without changing any code.
This is the important point to keep in mind when thinking of creational patterns. So let's look at them:
Abstract Factory
This one is unlikely to change much. A class can be thought of as a group of closely related functions, so a group of closely related functions is easily implemented through a class, which is what this pattern does for constructors.
Builder
Builder patterns can be replaced by curried functions or partial function applications.
def makeCar: Size => Engine => Luxuries => Car = ???
def makeLargeCars = makeCar(Size.Large) _
def makeCar: (Size, Engine, Luxuries) => Car = ???
def makeLargeCars = makeCar(Size.Large, _: Engine, _: Luxuries)
Factory Method
Becomes obsolete if you discard subclassing.
Prototype
Doesn't change -- in fact, this is a common way of creating data in functional data structures. See case classes copy method, or all non-mutable methods on collections which return collections.
Singleton
Singletons are not particularly useful when your data is immutable, but Scala object implements this pattern is a safe manner.
Structural Patterns
This is mostly related to data structures, and the important point on functional programming is that the data structures are usually immutable. You'd be better off looking at persistent data structures, monads and related concepts than trying to translate these patterns.
Not that some patterns here are not relevant. I'm just saying that, as a general rule, you should look into the things above instead of trying to translate structural patterns into functional equivalents.
Adapter
This pattern is related to classes (nominal typing), so it remains important as long as you have that, and is irrelevant when you don't.
Bridge
Related to OO architecture, so the same as above.
Composite
Lot at Lenses and Zippers.
Decorator
A Decorator is just function composition. If you are decorating a whole class, that may not apply. But if you provide your functionality as functions, then composing a function while maintaining its type is a decorator.
Facade
Same comment as for Bridge.
Flyweight
If you think of constructors as functions, think of flyweight as function memoization. Also, Flyweight is intrinsic related to how persistent data structures are built, and benefits a lot from immutability.
Proxy
Same comment as for Adapter.
Behavioral Patterns
This is all over the place. Some of them are completely useless, while others are as relevant as always in a functional setting.
Chain of Responsibility
Like Decorator, this is function composition.
Command
This is a function. The undo part is not necessary if your data is immutable. Otherwise, just keep a pair of function and its reverse. See also Lenses.
Interpreter
This is a monad.
Iterator
It can be rendered obsolete by just passing a function to the collection. That's what Traversable does with foreach, in fact. Also, see Iteratee.
Mediator
Still relevant.
Memento
Useless with immutable objects. Also, its point is keeping encapsulation, which is not a major concern in FP.
Note that this pattern is not serialization, which is still relevant.
Observer
Relevant, but see Functional Reactive Programming.
State
This is a monad.
Strategy
A strategy is a function.
Template Method
This is an OO design pattern, so it's relevant for OO designs.
Visitor
A visitor is just a method receiving a function. In fact, that's what Traversable's foreach does.
In Scala, it can also be replaced with extractors.
I suppose, Command pattern not needed in functional languages at all. Instead of encapsulation command function inside object and then selecting appropriate object, just use appropriate function itself.
Flyweight is just cache, and has default implementation in most functional languages (memoize in clojure)
Even Template method, Strategy and State can be implemented with just passing appropriate function in method.
So, I recommend to not go deep in Design Patterns when you tries yourself in functional style but reading some books about functional concepts (high-order functions, laziness, currying, and so on)
Case classes in Scala are standard classes enhanced with pattern-matching, equals, ... (or am I wrong?). Moreover they require no "new" keyword for their instanciation. It seems to me that they are simpler to define than regular classes (or am I again wrong?).
There are lots of web pages telling where they should be used (mostly about pattern matchin). But where should they be avoided ? Why don't we use them everywhere ?
There are many places where case classes are not adequate:
When one wishes to hide the data structure.
As part of a type hierarchy of more than two or three levels.
When the constructor requires special considerations.
When the extractor requires special considerations.
When equality and hash code requires special considerations.
Sometimes these requirements show up late in the design, and requires one to convert a case class into a normal class. Since the benefits of a case class really aren't all that great -- aside from the few special cases they were specially made for -- my own recommendation is not to make anything a case class unless there's a clear use for it.
Or, in other words, do not overdesign.
Inheriting from case classes is problematic. Suppose you have code like so:
case class Person(name: String) { }
case class Homeowner(address: String,override val name: String)
extends Person(name) { }
scala> Person("John") == Homeowner("1 Main St","John")
res0: Boolean = true
scala> Homeowner("1 Main St","John") == Person("John")
res1: Boolean = false
Perhaps this is what you want, but usually you want a==b if and only if b==a. Unfortunately, the compiler can't sensibly fix this for you automatically.
This gets even worse because the hashCode of Person("John") is not the same as the hashCode of Homeowner("1 Main St","John"), so now equals acts weird and hashCode acts weird.
As long as you know what to expect, inheriting from case classes can give comprehensible results, but it has come to be viewed as bad form (and thus has been deprecated in 2.8).
One downside that is mentioned in Programming in Scala is that due to the things automatically generated for case classes the objects get larger than for normal classes, so if memory efficiency is important, you might want to use regular classes.
It can be tempting to use case classes because you want free toString/equals/hashCode. This can cause problems, so avoid doing that.
I do wish there were an annotation that let you get those handy things without making a case class, but maybe that's harder than it sounds.