Part 2: Followup of the FP meetup "Abstractions of higher kind" - using Writer

example-with-traverse-and-writer.scala

import cats._
import cats.data._
import cats.implicits._

case class User(age: Int)

// notice F: FlatMap; fetchUser returning G[F[User]], and the use of flatTraverse
def complicatedFunction[F[_]: Traverse : FlatMap, G[_]: Applicative, A: Monoid](ids: F[Int], fetchUser: Int => G[F[User]], consume: User => A): G[A] = {
  ids.flatTraverse(fetchUser)
    .map(_.foldMap(consume))
}

type ErrorsAnd[T] = Writer[List[String], T]

def fetchUserFromMem(i: Int): ErrorsAnd[List[User]] = {
  i match {
    case 1 => Writer(List(), List(User(25)))
    case 2 => Writer(List(), List(User(25)))
    case 3 => Writer(List(), List(User(33)))
    case _ => Writer(List(s"User with id $i not found"), List())
  }
}

val validIds = List(1, 2, 3)
val invalidIds = List(21, 22, 1, 2, 3)

complicatedFunction(validIds, fetchUserFromMem, u => Map(u.age -> 1))
// ErrorsAnd[scala.collection.immutable.Map[Int,Int]] = WriterT((List(),Map(25 -> 2, 33 -> 1)))

complicatedFunction(invalidIds, fetchUserFromMem, u => Map(u.age -> 1))
// ErrorsAnd[scala.collection.immutable.Map[Int,Int]] = WriterT((List(User with id 21 not found, User with id 22 not found),Map(25 -> 2, 33 -> 1)))