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    <p>Well, you could do:</p> <pre><code>mapList reduce (_ ++ _) </code></pre> <p>except for the special requirement for collision.</p> <p>Since you do have that special requirement, perhaps the best would be doing something like this (2.8):</p> <pre><code>def combine(m1: Map, m2: Map): Map = { val k1 = Set(m1.keysIterator.toList: _*) val k2 = Set(m2.keysIterator.toList: _*) val intersection = k1 &amp; k2 val r1 = for(key &lt;- intersection) yield (key -&gt; (m1(key) + m2(key))) val r2 = m1.filterKeys(!intersection.contains(_)) ++ m2.filterKeys(!intersection.contains(_)) r2 ++ r1 } </code></pre> <p>You can then add this method to the map class through the Pimp My Library pattern, and use it in the original example instead of "<code>++</code>":</p> <pre><code>class CombiningMap(m1: Map[Symbol, Double]) { def combine(m2: Map[Symbol, Double]) = { val k1 = Set(m1.keysIterator.toList: _*) val k2 = Set(m2.keysIterator.toList: _*) val intersection = k1 &amp; k2 val r1 = for(key &lt;- intersection) yield (key -&gt; (m1(key) + m2(key))) val r2 = m1.filterKeys(!intersection.contains(_)) ++ m2.filterKeys(!intersection.contains(_)) r2 ++ r1 } } // Then use this: implicit def toCombining(m: Map[Symbol, Double]) = new CombiningMap(m) // And finish with: mapList reduce (_ combine _) </code></pre> <p>While this was written in 2.8, so <code>keysIterator</code> becomes <code>keys</code> for 2.7, <code>filterKeys</code> might need to be written in terms of <code>filter</code> and <code>map</code>, <code>&amp;</code> becomes <code>**</code>, and so on, it shouldn't be too different.</p>
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    1. POScala: how to merge a collection of Maps
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    1. USDaniel C. Sobral
    1. USDaniel C. Sobral
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    1. COKinda defeats the point to ignore that requirement.
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      1. USJeff
    2. COThat's why I expanded upon it.
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      1. USDaniel C. Sobral
    3. COWith modern Scala: val k1 = m1.keysIterator.toSet
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      1. USqerub
 

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