CODEWITHSUNDEEP
scalasyntactic-sugarassociativity
William Allcock
William Allcock

Reputation: 134

++: does not appear to be right associative?

I am trying to learn scala. I was looking at the documentation for queues (https://www.scala-lang.org/api/current/scala/collection/immutable/Queue.html).

It is my understanding that methods that end in a colon are right associative. However, to me, the ++: does not apear to do so:

import scala.collection.immutable.Queue
val q0 = Queue(0)
val q1 = Queue(1)
q0 ++ q1 // yields Queue(0,1) as I expected
q0 ++: q1 // yields Queue(0,1) as well;  I expected Queue(1,0)

Both the documentation and experimentation seem to indicate that ++: is not right associative. The documentation for both ++ and ++: say left followed by the right and that is what happens above, I just don't understand why. Clearly, there is something I am missing. Could someone please clarify this for me?

Upvotes: 0

Views: 109

Answers (1)

Andrey Tyukin
Andrey Tyukin

Reputation: 44957

A very simple experiment:

case class A(s: String) { def ++:(a: A) = A(s"(${a.s} + ${s})") }
A("x") ++: A("y") ++: A("z")

gives:

A((x + (y + z)))

so it's x + (y + z) and not (x + y) + z. Thus, the ++: is right-associative, as advertised.

Note that it's ${a.s} + ${s} and not ${s} + ${a.s}. In the case of Queue, it's probably analogous, something like:

def ++:(left: Queue[A]): Queue[A] = left ++ this

so the order of elements appears "as it should be" when you write q0 ++: q1 which desugars into q1.++:(q0) and then expands into q0 ++ q1.

Upvotes: 4

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