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    <p>Scala sequences have three operators that produce new sequences by adding something to an old sequence: <code>++</code>, <code>+:</code> and <code>:+</code>. The <code>++</code> operator simply concatenates a Scala sequence with another (or a traversable). The other two prepend and append elements, respectively. </p> <p>The peculiar syntax of <code>+:</code> and <code>:+</code> is due to the way they are used. Any operator ending with <code>:</code> applies to the object on the <em>right</em>, not on the left. That is:</p> <pre><code>1 +: Seq.empty == Seq.empty.+:(1) </code></pre> <p>By symmetry, the other operator is <code>:+</code>, though the colon is meaningless in that case. This let you write things like this:</p> <pre><code>scala&gt; 1 +: 2 +: 3 +: Seq.empty :+ 4 :+ 5 :+ 6 res2: Seq[Int] = List(1, 2, 3, 4, 5, 6) </code></pre> <p>Note how the elements being added end up in the exact same position as they appear in the expression. This makes it easier to visualize what's happening.</p> <p>Now, you have <code>:+=</code>, not any of the above. As it happens, Scala allows one to concatenate any operator with <code>=</code> to make up a get-and-set operation. So the common increment expression:</p> <pre><code>x += 1 </code></pre> <p>Actually means</p> <pre><code>x = x + 1 </code></pre> <p>Likewise, </p> <pre><code>v1 :+= "" </code></pre> <p>means</p> <pre><code>v1 = v1 :+ "" </code></pre> <p>which creates a new vector by appending the empty string to the old vector, and then assigns it to <code>v1</code>.</p>
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    1. POWhat does :+= method do defined for scala.collection.immutable.Vector?
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    1. USDaniel C. Sobral
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    1. POWhat does :+= method do defined for scala.collection.immutable.Vector?
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    1. COExcellent answer. This commit makes them extractors/unapply(), as well as sequence composers: https://github.com/scala/scala/commit/b3efb3d493605d1c7e106e5f0a697b52ebb3d97c
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    2. COThanks, got it! Scala is really crazy, once i understood something, it feels like never wanting to go back to java .... but lets see ;-)
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    3. COFor completeness, there is nothing like `+=:` ?
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