Question

In REPL:

import collection.mutable.{ HashSet, SynchronizedSet }

var myPool = new HashSet[String] with SynchronizedSet[String]
myPool += "oh"
myPool += "yes"
myPool = myPool.tail

and I get:

error: type mismatch;
 found   : scala.collection.mutable.HashSet[String]
 required: scala.collection.mutable.HashSet[String] with scala.collection.mutable.SynchronizedSet[String]
   myPool = myPool.tail
                   ^

What am I doing wrong?

Answer 1

Just what the message says, myPool.tail has type HashSet[String], and your variable MyPool is declared HashSet[String] with SynchronizedSet[String]

You just need to declare the type you want to avoid the too precise inferred one.

var myPool : HashSet[String] = new HashSet[String] with SynchronizedSet[String]

Note that on a mutable set, tail is a costly operation and returns you a new Set. That might not be what you want. (Moreover, the spec is mute as to which element will be removed)