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    <p>As pointed out by Ben James, you are looking for Miles Sabin's <a href="http://halcat0x15a.github.com/scalaz/core/target/scala-2.9.2/api/scalaz/Unapply.html" rel="noreferrer"><code>Unapply</code> trick</a>. <a href="https://github.com/scalaz/scalaz/blob/scalaz-seven/core/src/main/scala/scalaz/Unapply.scala" rel="noreferrer">Here</a> it is in scalaz repo. <a href="https://github.com/scalaz/scalaz/blob/scalaz-seven/core/src/main/scala/scalaz/Traverse.scala#L49" rel="noreferrer">Here's</a> <code>traverseU</code>, implemented with its help. <a href="https://github.com/scalaz/scalaz/blob/scalaz-seven/example/src/main/scala/scalaz/example/UnapplyInference.scala" rel="noreferrer">Here</a> are some example usages. And here's my sketchy (hopefully correct) implementation for your particular case (note: I've renamed your <code>Applicative</code> to <code>ApplicativeTest</code> not to interfere with <code>Applicative</code>, defined in scalaz):</p> <pre><code>scalaz&gt; core/console [warn] Credentials file /home/folone/.ivy2/.credentials does not exist [info] Starting scala interpreter... [info] Welcome to Scala version 2.9.2 (OpenJDK 64-Bit Server VM, Java 1.7.0_15). Type in expressions to have them evaluated. Type :help for more information. scala&gt; :paste // Entering paste mode (ctrl-D to finish) import scalaz._ trait ApplicativeTest[AF[_]] { def ap[A, B](a: AF[A])(f: AF[A =&gt; B]): AF[B] def pure[A](a: A): AF[A] def fmap[A, B](a: AF[A])(f: A =&gt; B): AF[B] } def traverse_[AP, A](xs: Iterable[A])(f: A =&gt; AP)(implicit G: Unapply[ApplicativeTest, AP]): G.M[Unit] = { (xs :\ G.TC.pure(())) { (x, acc) =&gt; val apFunc = G.TC.fmap(G(f(x)))(a =&gt; identity[Unit] _) G.TC.ap(acc)(apFunc) } } implicit def optionAp = new ApplicativeTest[Option] { def ap[A, B](a: Option[A])(f: Option[A =&gt; B]): Option[B] = f flatMap (a map _) def pure[A](a: A) = Some(a) def fmap[A, B](a: Option[A])(f: A =&gt; B) = a map f } implicit def eitherAp[L]: ApplicativeTest[({type l[x]=Either[L, x]})#l] = new ApplicativeTest[({type l[x]=Either[L, x]})#l] { def ap[A, B](a: Either[L, A])(f: Either[L, A =&gt; B]): Either[L, B] = f.right flatMap (a.right map _) def pure[A](a: A) = Right(a) def fmap[A, B](a: Either[L, A])(f: A =&gt; B) = a.right map f } implicit def iterAp = new ApplicativeTest[Iterable] { def ap[A, B](a: Iterable[A])(f: Iterable[A ⇒ B]): Iterable[B] = f flatMap(a map _) def pure[A](a: A) = Iterable(a) def fmap[A, B](a: Iterable[A])(f: A ⇒ B) = a map f } // Exiting paste mode, now interpreting. import scalaz._ defined trait ApplicativeTest traverse_: [AP, A](xs: Iterable[A])(f: A =&gt; AP)(implicit G: scalaz.Unapply[ApplicativeTest,AP])G.M[Unit] optionAp: java.lang.Object with ApplicativeTest[Option]{def pure[A](a: A): Some[A]} eitherAp: [L]=&gt; ApplicativeTest[[x]Either[L,x]] iterAp: java.lang.Object with ApplicativeTest[Iterable] scala&gt; val x = traverse_(1 to 10) { | case 5 =&gt; None | case _ =&gt; Some(()) | } x: Option[Unit] = None scala&gt; val y = traverse_(1 to 10) { | case 5 =&gt; Left("x"): Either[String, Unit] | case _ =&gt; Right(()) | } y: Either[String,Unit] = Left(x) </code></pre> <p>I still don't know how to make it infer <code>Either[String, Unit]</code> instead of <code>Product with Serializable with scala.util.Either[String,Unit]</code> other than strictly specify type in one of the cases like I did on this line: <code>case 5 =&gt; Left("x"): Either[String, Unit]</code>.</p>
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    1. POIs is possible to improve type inference for partially applied types in Scala?
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    1. COHere is another example of `Unapply` in action: http://stackoverflow.com/questions/14924707/how-to-write-a-scalaz-isempty-parameter-for-generic-types
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    2. COPerfect, thanks! How Miles comes up with this stuff I'll never know.
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    3. COHere's another one: http://stackoverflow.com/a/16095159/1011414
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