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POIs is possible to improve type inference for partially applied types in Scala?
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  1. POIs is possible to improve type inference for partially applied types in Scala?
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    copied!<p>I'm trying to improve the type inference of the <code>traverse_</code> function in the code below:</p> <pre><code>import scala.language.higherKinds trait Applicative[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[_]: Applicative, A](xs: Iterable[A])(f: A =&gt; AP[Unit]): AP[Unit] = { val ap = implicitly[Applicative[AP]] (xs :\ ap.pure(())) { (x, acc) =&gt; val apFunc = ap.fmap(f(x))(a =&gt; identity[Unit] _) ap.ap(acc)(apFunc) } } implicit def optionAp = new Applicative[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] = new Applicative[({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 } // silly, but compiles val x = traverse_(1 to 10) { case 5 =&gt; None case _ =&gt; Some(()) } println(x) // also silly, but does not compile val y = traverse_(1 to 10) { case 5 =&gt; Left("x") case _ =&gt; Right(()) } println(y) </code></pre> <p>Running the above gives:</p> <pre><code>/Users/lodea/tmp/traverse.scala:49: error: no type parameters for method traverse_: (f: Int =&gt; AP[Unit])(implicit evidence$1: this.Applicative[AP])AP[Unit] exist so that it can be applied to arguments (Int =&gt; Product with Serializable with scala.util.Either[String,Unit]) --- because --- argument expression's type is not compatible with formal parameter type; found : Int =&gt; Product with Serializable with scala.util.Either[String,Unit] required: Int =&gt; ?AP val y = traverse_(1 to 10) { ^ /Users/lodea/tmp/traverse.scala:49: error: type mismatch; found : Int =&gt; Product with Serializable with scala.util.Either[String,Unit] required: Int =&gt; AP[Unit] val y = traverse_(1 to 10) { ^ two errors found </code></pre> <p>To get it to compile, I have to specify the type arguments to <code>traverse_</code>:</p> <pre><code>val y = traverse_[({type l[x]=Either[String, x]})#l, Int](1 to 10) { case 5 =&gt; Left("x") case _ =&gt; Right(()) } </code></pre> <p>Is there a way I can restructure <code>traverse_</code>, or any other part of the code, to make the type inference work? When the types start getting more complex, this gets annoying fast.</p>
 

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