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scalascalazfunction-compositionkleisli

Using scalaz kleisli without explicit wrapping the function before

Let's say I have two functions val f: A => M[B] and val g: B => M[C] where M is monadic. Thus I want to combine them by using kleisli. What I currently do is this: kleisliU(f) andThenK g But I have not found a way to execute this combination without manually wrapping into kleisli first.

How can write something like f <???> g so that f is wrapped into kleisli automatically and then combined with g? I hope something in scalaz already exists for that and that I don't need to write my own implicit class / conversion.

Just for sake of completeness, that should also work with more functions e.g. f <???> g <???> h.

Solution

  • Once I wanted the same thing and I did not find it in scalaz, so I just wrote it myself:

    implicit def toKleisliK[M[_], A, B]: (A => M[B]) => Kleisli[M, A, B] = f => { 
    kleisli[M, A, B](a => f(a)) 
}
// then for example you can write such:
val f: Int => Option[String] = ???
val g: String => Option[Double] = ???
val result = f andThenK g // inferred type is ReaderT[Option, Int, Double]