I need something to store a LIFO. No need for traversing and other functions except for push and pop.
I've found special class in scala collection for creating a stack. But it lacks Nil object in pattern matching and other handy scala idioms. Immutable lists suit well at first glance, they have cons for construction and for extraction and that is all needed from LIFO.
Is there any reason behind scala.collection.immutable.Stack existence? Why I should prefer to use it, what are use cases to show its benefits?
From the API documentation:
Note: This class exists only for historical reason and as an analogue
of mutable stacks. Instead of an immutable stack you can just use a
list.
And in a little more detail:
Immutable stacks are used rarely in Scala programs because their
functionality is subsumed by lists: A push on an immutable stack is
the same as a :: on a list and a pop on a stack is the same as a tail on
a list.
So to answer your questions:
Yes, there's a reason for its existence.
No, you shouldn't prefer it over lists.
Related
I'm currently porting some code from traditional Scala to Scalaz style.
It's fairly common through most of my code to use the Seq trait in my exposed API signatures rather than a concrete type (i.e. List, Vector) directly. However, this poses some problem with Scalaz, since it doesn't provide an implementation of a Bind[Seq] typeclass.
i.e. This will work correctly.
List(1,2,3,4) >>= bindOperation
But this will not
Seq(1,2,3,4) >>= bindOperation
failing with the error could not find implicit value for parameter F0: scalaz.Bind[Seq]
I assume this is an intentional design decision in Scalaz - however am unsure about intended/best practice on how to precede.
Should I instead write my code directly to List/Vector as appropriate instead of using the more flexible Seq interface? Or should I simply define my own Bind[Seq] typeclass?
The collections library does backflips to accommodate subtyping: when you use map on a specific collection type (list, map, etc.), you'll (usually) get the same type back. It manages this through the use of an extremely complex inheritance hierarchy together with type classes like CanBuildFrom. It gets the job done (at least arguably), but the complexity doesn't feel very principled. It's a mess. Lots of people hate it.
The complexity is generally pretty easy to avoid as a library user, but for a library designer it's a nightmare. If I provide a monad instance for Seq, that means all of my users' types get bumped up the hierarchy to Seq every type they use a monadic operation.
Scalaz folks tend not to like subtyping very much, anyway, so for the most part Scalaz stays around the leaves of the hierarchy—List, Vector, etc. You can see some discussion of this decision on the mailing list, for example.
When I first started using Scalaz I wrote a lot of utility code that tried to provide instances for Seq, etc. and make them usable with CanBuildFrom. Then I stopped, and now I tend to follow Scalaz in only ever using List, Vector, Map, and Set in my own code. If you're committed to "Scalaz style", you should do that as well (or even adopt Scalaz's own IList, ISet, ==>>, etc.). You're not going to find clear agreement on best practices more generally, though, and both approaches can be made to work, so you'll just need to experiment to find which you prefer.
I'm new to scala(just start learning it), but have figured out smth strange for me: there are classes Array and List, they both have such methods/functions as foreach, forall, map etc. But any of these methods aren't inherited from some special class(trait). From java perspective if Array and List provide some contract, that contract have to be declared in interface and partially implemented in abstract classes. Why do in scala each type(Array and List) declares own set of methods? Why do not they have some common type?
But any of these methods aren't inherited from some special class(trait)
That simply not true.
If you open scaladoc and lookup say .map method of Array and List and then click on it you'll see where it is defined:
For list:
For array:
See also info about Traversable and Iterable both of which define most of the contracts in scala collections (but some collections may re-implement methods defined in Traversable/Iterable, e.g. for efficiency).
You may also want to look at relations between collections (scroll to the two diagrams) in general.
I'll extend om-nom-nom answer here.
Scala doesn't have an Array -- that's Java Array, and Java Array doesn't implement any interface. In fact, it isn't even a proper class, if I'm not mistaken, and it certainly is implemented through special mechanisms at the bytecode level.
On Scala, however, everything is a class -- an Int (Java's int) is a class, and so is Array. But in these cases, where the actual class comes from Java, Scala is limited by the type hierarchy provided by Java.
Now, going back to foreach, map, etc, they are not methods present in Java. However, Scala allows one to add implicit conversions from one class to another, and, through that mechanism, add methods. When you call arr.foreach(println), what is really done is Predef.refArrayOps(arr).foreach(println), which means foreach belongs to the ArrayOps class -- as you can see in the scaladoc documentation.
I was wondering if there is a good reason to use Collection.empty[T] instead of new Collection[T]() (or the inverse) ? Or is it just a personal preference ?
Thanks.
Calling new Collection[T]() will create a new instance every time. On the other hand, Collection.empty[T] will most likely always return the same singleton object, usually defined somewhere as
object Empty extends Collection[Nothing] ...
which will be much faster. Edit: This is only possible for immutable collections, mutable collections have to return a new instance every time empty is called.
You should always prefer Collection.empty[Type].
In addition to Collection.empty[T] being clearer on the intent, you should favour it for the same reason that you should favour factory methods in general when instantiating a collection: because thoses factories abstract away some implementation details that you might not (or should not) care about.
By example, when you do Seq.empty[String] you actually get an instance of List[String]. You could directly instantiate a List[String] but if all you care about is to have some Seq you would introduce a needless dependency to List (well OK, actually you cannot as it stands, because List is already abstract, but let's pretend we can for the sake of the argument)
The whole point of factories is precisely to have some amount of separation of concern and not bother with unnecessary instantiation details.
As another more elaborate example, let's talk about collection.immutable.HashMap. This one is very much a concrete class so you might think there is no need for a factory here. Except that for optimization purpose the factory in the companion object collection.immutable.HashMap will actually create different sub-classes depending on the number of elements that you initialize the map with (see this question: Scala: how to make a Hash(Trie)Map from a Map (via Anorm in Play)). Obviously, if you directly instantiate collection.immutable.HashMap you will lose this optimization.
Another common optimization for empty is to always return (when it is an immutable collection) the same instance, yet another useful optimization that you would lose by directly instantiating the collection.
So as a rule of thumb, as far as you can you should use the factories that are provided by the various collection companion objects, so as to shield yourself from unneeded dependencies while at the same time benefiting from potential optimizations provided by the collection framework.
empty is just a special case of factory, and so the same logic applies.
Being new to scala and a current java developer, scala was designed to encourage the use of immutability to class design.
How does this translate practically to the design of classes? The only thing that is brought to my mind is case classes. Are case classes strongly encouraged for defining data? Example? How else is immutability encouraged in Scala design of classes?
As a java developer, classes defining data were mutable. The equivalent Scala classes should be defined as case classes?
Well, case classes certainly help, but the biggest contributor is probably the collection library. The default collections are immutable, and the methods are geared toward manipulating collections by producing new ones instead of mutating. Since the immutable collections are persistent, that doesn't require copying the whole collection, which is something one often has to do in Java.
Beyond that, for-comprehensions are monadic comprehensions, which is helpful in doing immutable tasks, there's tail recursion optimization, which is very important in immutable algorithms, and general attention to immutability in many libraries, such as parser combinators and xml.
Finally, note that you have to ask for a var to get some mutability. Parameters are immutable, and val is just as short as var. Contrast this with Java, where parameters are mutable, and you need to add a final keyword to get immutability. Whereas in Scala it is as easy or easier to stay immutable, in Java it is easier to stay mutable.
Addendum
Persistent data structures are data structures that share parts between modified versions of it. This might be a bit difficult to understand, so let's consider Scala's List, which is pretty basic and easy to understand.
A Scala List is composed of two classes, known as cons and Nil. The former is actually written :: in Scala, but I'll refer to it by the traditional name.
Nil is the empty list. It doesn't contain anything. Methods that depend on the list not being empty, such as head and tail throw exceptions, while others work ok.
Naturally, cons must then represent a non-empty list. In fact, cons has exactly two elements: a value, and a list. These elements are known as head and tail.
So a list with three elements is composed of three cons, since each cons will hold only one value, plus a Nil. It must have a Nil because a cons must point to a list. As lists are not circular, then one of the cons must point to something other than a cons.
One example of such list is this:
val list = 1 :: 2 :: 3 :: Nil
Now, the components of a Scala List are immutable. One cannot change neither the value nor the list of a cons. One benefit of immutability is that you never need to copy the collection before passing or after receiving it from some other method: you know that list cannot change.
Now, let's consider what would happen if I modified that list. Let's consider two modifications: removing the first element and prepending a new element.
We can remove one element with the method tail, whose name is not a coincidence at all. So, we write:
val list2 = list.tail
And list2 will point to the same list that list's tail is pointing. Nothing at all was created: we simply reused part of list. So, let's prepend an element to list2 then:
val list3 = 0 :: list2
We created a new cons there. This new cons has a value (a head) equal to 0, and its tail points to list2. Note that both list and list3 point to the same list2. These elements are being shared by both list and list3.
There are many other persistent data structures. The very fact that the data you are manipulating is immutable makes it easy to share components.
One can find more information about this subject on the book by Chris Okasaki, Purely Functional Data Structures, or on his freely available thesis by the same name.
Scala has both a mutable and an immutable Map ,
but it has only an immutable List.
If you want a mutable List you need a ListBuffer.
I don't understand why this is so.
Any one knows?.
You can choose between these:
scala.collection.mutable.DoubleLinkedList
scala.collection.mutable.LinkedList
scala.collection.mutable.ListBuffer
scala.collection.mutable.MutableList
So, yes, Scala has mutable lists :-)
I hope that this article may be of some use to you. The diagram at the bottom of the page is particularly useful in providing the mutable and immutable classes.
http://www.scala-lang.org/docu/files/collections-api/collections_1.html
There is a mutable List, but it is called Buffer. The article linked by Graham goes into more depth, but I thought there should be a specific answer to the question as well.
Map is a trait -- like Java's interface --, while List is a class, a concrete implementation of a Seq. There are mutable and immutable Seq, just like for Map.
This may be confusing to Java programmers because, in Java, List is an interface, whose (main) implementations are ArrayList and LinkedList. Alas, Java naming is atrocious. First, ArrayList is not a List by any stretch of imagination. Also, the interface has methods that are not really related to any traditional list.
So, if you want mutable/immutable equivalence, look to concrete subclass implementations of Seq.