Scala pattern match not working on Stream?

Question

I do a group-by and I get key -> Stream(of values)

I then need to do a pattern match on the stream collection to access the last item but the pattern match doesn't work.

When I manually build the list of values using Seq the same pattern match code works

So my question is there a way to convert the Stream to Seq or List?

The IDE says toSeq is redundant

How are you doing the pattern match?, Which Stream are you talking about, the scala one, the java one, the akka one?, Can you please share a [**MCVE**](https://stackoverflow.com/help/reprex). - BTW, if you are trying to pattern match the stream itself using `Nil` & `x :: xs` that won't work, first, those are specific to `List`, second usually streams are not ADTs, thus they are not pattern matcheable. — Luis Miguel Mejía Suárez, May 15 '19 at 14:09

Valy Dia · Accepted Answer · 2019-05-15T14:20:05.850

4

When I manually build the list of values using Seq the same pattern match code works

In scala 2.12, Seq (or sequence) are defaulted to List, see this question:

scala> Seq(1,2,3)
res3: Seq[Int] = List(1, 2, 3)

This is probably why the pattern matching works on your sequence (which happens to be a List) but not an a Stream, see this question.

The IDE says toSeq is redundant

Stream are indeed Seq:

Stream(1,2,3).toSeq
res4: collection.immutable.Seq[Int] = Stream(1, 2, 3)

So my question is there a way to convert the Stream to Seq or List?

To transform a Stream into a List, you can call the .toList method:

Stream(1,2,3).toList
res5: List[Int] = List(1, 2, 3)

edited May 15 '19 at 14:20

answered May 15 '19 at 14:12

Valy Dia

What collection is mostly used in scala List or Seq? – jakstack May 15 '19 at 17:24
1

`List` are a specific type of a `Seq` btw, (see [this question](https://stackoverflow.com/questions/10866639/difference-between-a-seq-and-a-list-in-scala))... So which type of `Seq` to use, really depends on your use case... `List` are quite common, and are standard, if you need an access to the head in a contant time... – Valy Dia May 15 '19 at 17:53

pme · Answer 2 · 2019-05-15T14:28:15.873

2

Or with this Answer you don't have to transform to List:

val n: Seq[Any] = Stream(..)
n match {
    case Nil => "Empty"
    case h :: t => "Non-empty"
    case h #:: t => "Non-empty stream"
  }

For Stream, the concat symbol should be #::, the pattern match should like:

Make sure you reverse the Stream - so you get the last element, here an example:

n.reverse match {
    case Nil => "Empty"
    case h #:: _ => s"last element is $h"
  }

Check it here ScalaFiddle

edited May 15 '19 at 14:28

answered May 15 '19 at 14:21

pme

1

`Nil` does not work for streams, you should use `Stream.Empty`, see [this](https://stackoverflow.com/questions/28530794/why-cant-i-pattern-match-on-stream-empty-in-scala). - BTW, reversing a _possibly_ **infinite** collection does not seems like a good idea to. – Luis Miguel Mejía Suárez May 15 '19 at 14:36
@LuisMiguelMejíaSuárez: we pattern match to a `Seq` not `Stream` (check https://scalafiddle.io/sf/UdJCMhM/1). jakstack requirement is to match the last element - so I assumed the `Stream` is not infinite - otherwise you are right of course ;) . – pme May 15 '19 at 14:51

2 Answers2