Scala Typeclasses

Question

I'm trying to implement simple type class pattern. It suppose to work similarly to scalaz's typeclasses. Unfortunately I can't get it to work. I have trait Str

trait Str[T] {
  def str(t: T): String
}

object Str {
  def apply[T](implicit instance: Str[T]) : Str[T] = instance
}

And in my and implicit instance of it.

object Temp extends App {

  implicit val intStr = new Str[Int] {
    def str(i: Int) = i.toString
  }

  1.str //error: value str is not a member of Int

}

I would appreciate any insight.

Shouldn't it be Str(1).str? You were not defining an implicit class, just an object with apply. — Gábor Bakos
– Gábor Bakos, Commented Apr 18, 2016 at 16:45

Łukasz · Accepted Answer · 2016-04-18 16:57:43Z

8

Everything you can do now is

Str[Int].str(1)

to use 1.str you need to introduce implicit conversion.

You can for example use this approach:

implicit class StrOps[A](val self: A) extends AnyVal {
    def str(implicit S: Str[A]) = S.str(self)
}

Which gives:

scala> 1.str
res2: String = 1

edited Apr 18, 2016 at 16:57

answered Apr 18, 2016 at 16:48

Łukasz

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