import scala.annotation.tailrec //very poor performance pseudo monad sealed trait Tail[+A] { import Tail.* def flatMap[B](f: A => Tail[B]): Tail[B]^{this, f} = this match case Now(value) => ...

def foo(src: String): Option[List[Int] -> Int] = None def traverse[A, B](xs: List[A], f: A => Option[B]): Option[List[B]] = None val weird = for actions <- traverse(List[String](), foo) _ = acti...

import cats.syntax.all.given val buf = StringBuffer() def foo(src: String): Either[String, StringBuffer -> Unit] = for x <- Right(1) yield _.append(x) val strs = List("a", "b", "c") val weird ...