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    <p>If you have <code>Option[T]</code> and if there's a <code>Monoid</code> for <code>T</code>, then there's a <code>Monoid[Option[T]]</code>:</p> <pre><code>implicit def optionTIsMonoid[T : Monoid]: Monoid[Option[T]] = new Monoid[Option[T]] { val monoid = implicitly[Monoid[T]] val zero = None def append(o1: Option[T], o2: =&gt;Option[T]) = (o1, o2) match { case (Some(a), Some(b)) =&gt; Some(monoid.append(a, b)) case (Some(a), _) =&gt; o1 case (_, Some(b)) =&gt; o2 case _ =&gt; zero } } </code></pre> <p>Once you are equipped with this, you can just use <code>sum</code> (better than <code>foldMap(identity)</code>, as suggested by @missingfaktor):</p> <pre><code> List(Some(1), None, Some(2), Some(3), None).asMA.sum === Some(6) </code></pre> <p><strong>UPDATE</strong></p> <p>We can actually use applicatives to simplify the code above:</p> <pre><code>implicit def optionTIsMonoid[T : Monoid]: Monoid[Option[T]] = new Monoid[Option[T]] { val monoid = implicitly[Monoid[T]] val zero = None def append(o1: Option[T], o2: =&gt;Option[T]) = (o1 |@| o2)(monoid.append(_, _)) } </code></pre> <p>which makes me think that we can maybe even generalize further to:</p> <pre><code>implicit def applicativeOfMonoidIsMonoid[F[_] : Applicative, T : Monoid]: Monoid[F[T]] = new Monoid[F[T]] { val applic = implicitly[Applicative[F]] val monoid = implicitly[Monoid[T]] val zero = applic.point(monoid.zero) def append(o1: F[T], o2: =&gt;F[T]) = (o1 |@| o2)(monoid.append(_, _)) } </code></pre> <p>Like that you would even be able to sum Lists of Lists, Lists of Trees,...</p> <p><strong>UPDATE2</strong></p> <p>The question clarification makes me realize that the <strong>UPDATE</strong> above is incorrect!</p> <p>First of all <code>optionTIsMonoid</code>, as refactored, is not equivalent to the first definition, since the first definition will skip <code>None</code> values while the second one will return <code>None</code> as soon as there's a <code>None</code> in the input list. But in that case, this is not a <code>Monoid</code>! Indeed, a <code>Monoid[T]</code> must respect the Monoid laws, and <code>zero</code> must be an <a href="http://en.wikipedia.org/wiki/Identity_element">identity</a> element. </p> <p>We should have:</p> <pre><code>zero |+| Some(a) = Some(a) Some(a) |+| zero = Some(a) </code></pre> <p>But when I proposed the definition for the <code>Monoid[Option[T]]</code> using the <code>Applicative</code> for <code>Option</code>, this was not the case:</p> <pre><code>None |+| Some(a) = None None |+| None = None =&gt; zero |+| a != a Some(a) |+| None = zero None |+| None = zero =&gt; a |+| zero != a </code></pre> <p>The fix is not hard, we need to change the definition of <code>zero</code>:</p> <pre><code>// the definition is renamed for clarity implicit def optionTIsFailFastMonoid[T : Monoid]: Monoid[Option[T]] = new Monoid[Option[T]] { monoid = implicitly[Monoid[T]] val zero = Some(monoid.zero) append(o1: Option[T], o2: =&gt;Option[T]) = (o1 |@| o2)(monoid.append(_, _)) } </code></pre> <p>In this case we will have (with <code>T</code> as <code>Int</code>):</p> <pre><code>Some(0) |+| Some(i) = Some(i) Some(0) |+| None = None =&gt; zero |+| a = a Some(i) |+| Some(0) = Some(i) None |+| Some(0) = None =&gt; a |+| zero = zero </code></pre> <p>Which proves that the identity law is verified (we should also verify that the <a href="http://en.wikipedia.org/wiki/Associative">associative</a> law is respected,...).</p> <p>Now we have 2 <code>Monoid[Option[T]]</code> which we can use at will, depending on the behavior we want when summing the list: skipping <code>None</code>s or "failing fast".</p>
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    1. POSumming a List of Options with Applicative Functors
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    1. COJust after posting this, I realize that I'm not really answering the question, since I'm not using any Applicative. Just consider this as one of the numerous alternatives,...
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    2. COI don't understand how that works... why is there no `+` anywhere?
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    3. CO`.foldMap(identity)` can be replaced by `.asMA.sum`.
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