How to use functors with scala/cats

Question

Aloha! :)

Could please somebody point me to a useful scala/cats tutorial? I’m struggling with making a class a functor for the last few days and I’m about to punch a hole in my display. All the documentation I’ve found up until now was not of help for me.

Maybe I should give Eta a try... =D

Here is the class I would like to turn into a functor. Besides 'show' doesn't behave the way I expected also.

package org.hudelundpfusch.utilites.decisions.data

import cats.{Functor, Show}
import cats.kernel.Eq
import cats.syntax.functor._
import cats.syntax.show._

import scala.reflect.runtime.universe
import scala.reflect.runtime.universe._

case class Fact[T <: Any] (name: String, value: T) (implicit private val paramTypeTagT: WeakTypeTag[T])
  extends Equals {

  val paramType: universe.Type = paramTypeTagT.tpe

  val paramTypeClass: Option[Class[_ <: T]] = if (value != null) {
    Some(value.getClass)
  } else {
    None
  }

  def map[A, B](fa: Fact[A])(f: A => B): Fact[B] = Fact[B](fa.name, f(fa.value))

  override def canEqual(other: Any): Boolean = other.isInstanceOf[Fact[_]]

  override def equals(other: Any): Boolean = other match {
    case that: Fact[_] =>
      (that canEqual this) &&
        name == that.name
        paramType == that.paramType &&
        paramTypeClass == that.paramTypeClass &&
        value == that.value
    case _ => false
  }

  override def hashCode(): Int = {
    val state = Seq(name, paramType, paramTypeClass, value)
    state.map(_.hashCode()).foldLeft(0)((a, b) => 31 * a + b)
  }

  override def toString = s"Fact(name=${name}, paramType=$paramType, paramTypeClass=$paramTypeClass, value=$value)"

}

case object Fact extends Show[Fact[_]] {

  override def show(t: Fact[_]): String = t.toString

}

Thanks in advance

Have a nice day

Alex

Update:

package org.hudelundpfusch.utilites.decisions.data

import cats.{Functor, Show}
import cats.kernel.Eq
import cats.syntax.functor._
import cats.syntax.show._

import scala.reflect.runtime.universe
import scala.reflect.runtime.universe._

case class Fact[T <: Any] (name: String, value: T) (implicit private val paramTypeTagT: WeakTypeTag[T])
  extends Functor[Fact]
    with Equals {

  val paramType: universe.Type = paramTypeTagT.tpe

  val paramTypeClass: Option[Class[_ <: T]] = if (value != null) {
    Some(value.getClass)
  } else {
    None
  }

  def map[A, B](fa: Fact[A])(f: A => B): Fact[B] = Fact[B](fa.name, f(fa.value))

  override def canEqual(other: Any): Boolean = other.isInstanceOf[Fact[_]]

  override def equals(other: Any): Boolean = other match {
    case that: Fact[_] =>
      (that canEqual this) &&
        name == that.name
        paramType == that.paramType &&
        paramTypeClass == that.paramTypeClass &&
        value == that.value
    case _ => false
  }

  override def hashCode(): Int = {
    val state = Seq(name, paramType, paramTypeClass, value)
    state.map(_.hashCode()).foldLeft(0)((a, b) => 31 * a + b)
  }

  override def toString = s"Fact(name=${name}, paramType=$paramType, paramTypeClass=$paramTypeClass, value=$value)"

}

Okay, I tried this now:

object Fact {
  implicit val factFunctor: Functor[Fact] = new Functor[Fact] {
    override def map[A, B](fa: Fact[A])(f: A => B): Fact[B] = Fact[B](fa.name, f(fa.value))
  }

  implicit def factShow[T]: Show[Fact[T]] = new Show[Fact[T]] {
    override def show(t: Fact[T]): String = this.toString
  }
}

Unfortunately the call to the mapping function looks a bit cumbersome:

package org.hudelundpfusch.utilites.decisions.data

object Fuddel {
  def main(args: Array[String]): Unit = {
    val fact1: Fact[Int] = Fact("Fact-1", 23)
    val fact2 = Fact.factFunctor.map(fact1){x: Int => x * 2}

    println(s"$fact2")
  }
}

Answer 1

With Cats you do not extend Show, Functor traits (OOP way), you create implicit instances of Show, Functor (FP way).

http://eed3si9n.com/herding-cats/Functor.html

https://typelevel.org/cats/typeclasses/functor.html

implicit def factShow[T]: Show[Fact[T]] = new Show[Fact[T]] {
  override def show(t: Fact[T]): String = ???
}

//  implicit object factFunctor extends Functor[Fact] {
//    override def map[A, B](fa: Fact[A])(f: A => B): Fact[B] = ???
//  }

implicit val factFunctor: Functor[Fact] = new Functor[Fact] {
  override def map[A, B](fa: Fact[A])(f: A => B): Fact[B] = ???
}

Show[Fact[Int]].show(Fact("a", 1))
Functor[Fact].map(Fact("a", 1))(_ + 1)

import cats.syntax.show._
Fact("a", 1).show

import cats.syntax.functor._
Fact("a", 1).map(_ + 1)

Just in case, Fact[_] in your case object Fact extends Show[Fact[_]] is existential type, and Fact in my implicit object factFunctor extends Functor[Fact] is higher-kinded type.