In scala, How could Map[_,_] and scala.collection.immutable.Map[_,_] have different TypeTag?

Question

They refer to the same thing, yet when I compare 2 type tags:

val ttg1 = typeTag[Map[_,_]]
val ttg2 = typeTag[immutable.Map[_,_]]

assert(ttg1.tpe == ttg2.tpe)

I got:

Map[_, _] did not equal scala.collection.immutable.Map[_,_]
ScalaTestFailureLocation: 
Expected :scala.collection.immutable.Map[_,_]
Actual   :Map[_, _]

How could this happen in a typed language? How do I make them identical?

UPDATE: for List this is more confusing:

val ttg1 = typeTag[List[_]]
val ttg2 = typeTag[immutable.List[_]]

assert(ttg1.tpe == ttg2.tpe)

List[_] did not equal List[_]
ScalaTestFailureLocation: 
Expected :List[_]
Actual   :List[_]

They are exactly the same thing! Yet reflection choose to ignore it.

@Dima I have `scala.collection.immutable` imported, sorry forgot to copy the import, but you got my point — tribbloid, Dec 30 '17 at 19:21

score 6 · Accepted Answer · answered Dec 30 '17 at 05:35

6

You are comparing type identity when it seems you really want type equivalence. In

type A = B

A and B are two different types that are equivalent (which is why they have different type tags). Try changing your test to

assert(ttg1.tpe =:= ttg2.tpe)

=:= on types tests equivalence.

answered Dec 30 '17 at 05:35

Joe Pallas

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Thanks a lot! Though it looks not simple for common programmers – tribbloid Dec 30 '17 at 19:21
I think it is safe to say that reflection is an intermediate-to-advanced topic. Most programmers should not need to use it. – Joe Pallas Dec 31 '17 at 00:15

In scala, How could Map[_,_] and scala.collection.immutable.Map[_,_] have different TypeTag?

1 Answers1