Scala Kleisli throws an error in IntelliJ

@sinanspd was right. In Dotty the code seems to compile: https://scastie.scala-lang.org/n17APWgMQkWqy93ct2cghw

Manually resolved

val safeRootRecip: Partial[Double, Double] = KleisliOps(safeRoot).>=>(safeRecip) 

compiles but compiler doesn't find this conversion itself

Information: KleisliOps{<null>} is not a valid implicit value for App.safeRoot.type => ?{def >=> : ?} because: type mismatch; found : App.safeRoot.type (with underlying type App.Partial[Double,Double]) required: App.Partial[A,Double] (which expands to) A => Option[Double] val safeRootRecip: Partial[Double, Double] = safeRoot.>=>(safeRecip) 

It seems type parameter A is not inferred.

(By the way, here Martin Odersky explains why presence of implicit conversions in language makes type inference worse: https://contributors.scala-lang.org/t/can-we-wean-scala-off-implicit-conversions/4388)

Try to make Partial covariant with respect to B and (especially) contravariant with respect to A (similarly to A => Option[B] being covariant with respect to B and contravariant with respect to A)

type Partial[-A, +B] = A => Option[B] 

Then the code seems to compile.

Another workaround is to replace implicit conversions (X => Y, KleisliOps) with a type class (MyTransform) and implicit conversion (myConversion) defined in terms of this type class (sometimes this helps with implicit conversions)

trait MyTransform[X, Y] { def transform(x: X): Y } implicit def myConversion[X, Y](x: X)(implicit mt: MyTransform[X, Y]): Y = mt.transform(x) type Partial[A, B] = A => Option[B] implicit def partialToKleisliOps[A, B]: MyTransform[Partial[A, B], KleisliOps[A, B]] = f1 => new KleisliOps(f1) class KleisliOps[A, B](f1: Partial[A, B]) { def >=>[C](f2: Partial[B, C]): Partial[A, C] = (x: A) => for { y <- f1(x) z <- f2(y) } yield z def identity(f: Partial[A, B]): Partial[A, B] = x => f(x) }