What's the difference between scala.List and scala.collection.immutable.List?

Question

I'm new to Scala. And I noticed that List exists in both scala and scala.collection.immutable. At the beginning, I thought scala.List is just an alias for scala.collection.immutable.List. But later I found that:

scala> typeOf[scala.List[A]]
res1: reflect.runtime.universe.Type = scala.List[A]

scala> typeOf[scala.collection.immutable.List[A]]
res2: reflect.runtime.universe.Type = List[A]

As shown above, the typeof on these two Lists give different results, which make me doubious about my judgement.

I would like to know: Are scala.List and scala.collection.immutable.List the same thing? If yes, why typeOf gives different results as shown above? If no, what are the differences?

Thanks!

API documentation is invaluable here: scala.List is a type alias for scala.collection.immutable.List — Patryk Ćwiek
– Patryk Ćwiek, Commented Sep 19, 2016 at 8:58
If you look at source code of package scala it has these two members type List[+A] = scala.collection.immutable.List[A] and val List = scala.collection.immutable.List. So yes... scala.List is just an alias for scala.collection.immutable.List — sarveshseri
– sarveshseri, Commented Sep 19, 2016 at 9:00
@SarveshKumarSingh Thanks. But then why typeOf gives different results on these two types? — Lifu Huang
– Lifu Huang, Commented Sep 19, 2016 at 9:03

Jatin · Accepted Answer · 2016-09-19 09:15:26Z

2

As @Patryk mentioned in comment, it is an alias for scala.collection.immutable.List.

If you look at scala\package.scala:

  type List[+A] = scala.collection.immutable.List[A]
  val List = scala.collection.immutable.List

So fundamentally they are the same:

scala> implicitly[scala.List[Int] =:= scala.collection.immutable.List[Int]]
res19: =:=[List[Int],List[Int]] = <function1>

scala> :type List
scala.collection.immutable.List.type

scala> :type scala.collection.immutable.List
collection.immutable.List.type

What you are using is I believe typeOf from import scala.reflect.runtime.universe._. Which is a reflective representation of the type parameter and hence it gave you just scala.List (which is correct behavior in my opinion)

answered Sep 19, 2016 at 9:15

Jatin

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Comments

Nagarjuna Pamu · Accepted Answer · 2016-09-19 09:05:24Z

0

scala.List is type alias for scala.collection.immutable.List

package scala

type List[+A] = scala.collection.immutable.List[A]

Anything under the package scala is by default available you don't have to explicitly import it.

As List is something which is widely used it makes perfect sense if it is available by default in the context rather than importing. That is why a type alias of the scala.collection.immutable.List is in package scala.

typeOf must be showing the higher preference package by default.

edited Sep 19, 2016 at 9:05

answered Sep 19, 2016 at 8:59

Nagarjuna Pamu

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