Question

the other day I was wondering why scala.collection.Map defines its unzip method as

def unzip [A1, A2] (implicit asPair: ((A, B)) ⇒ (A1, A2)): (Iterable[A1], Iterable[A2])

Since the method returns “only” a pair of Iterable instead of a pair of Seq it is not guaranteed that the key/value pairs in the original map occur at matching indices in the returned sequences since Iterable doesn’t guarantee the order of traversal. So if I had a

Map((1,A), (2,B))

, then after calling

Map((1,A), (2,B)) unzip

I might end up with

... = (List(1, 2),List(A, B))

just as well as with

... = (List(2, 1),List(B, A))

While I can imagine storage-related reasons behind this (think of HashMaps, for example) I wonder what you guys think about this behavior. It might appear to users of the Map.unzip method that the items were returned in the same pair order (and I bet this is probably almost always the case) yet since there’s no guarantee this might in turn yield hard-to-find bugs in the library user’s code.

Maybe that behavior should be expressed more explicitly in the accompanying scaladoc?

EDIT: Please note that I’m not referring to maps as ordered collections. I’m only interested in “matching” sequences after unzip, i.e. for

val (keys, values) = someMap.unzip

it holds for all i that (keys(i), values(i)) is an element of the original mapping.

Answer 1

Actually, the examples you gave will not occur. The Map will always be unzipped in a pair-wise fashion. Your statement that Iterable does not guarantee the ordering, is not entirely true. It is more accurate to say that any given Iterable does not have to guarantee the ordering, but this is dependent on implementation. In the case of Map.unzip, the ordering of pairs is not guaranteed, but items in the pairs will not change they way they are matched up — that matching is a fundamental property of the Map. You can read the source to GenericTraversableTemplate to verify this is the case.