Making our own typeclass with simulacrum 

LYAHFGG:

In JavaScript and some other weakly typed languages, you can put almost anything inside an if expression. …. Even though strictly using Bool for boolean semantics works better in Haskell, let’s try and implement that JavaScript-ish behavior anyway. For fun!

The conventional steps of defining a modular typeclass in Scala used to look like:

1. Define typeclass contract trait Foo.
2. Define a companion object Foo with a helper method apply that acts like implicitly, and a way of defining Foo instances typically from a function.
3. Define FooOps class that defines unary or binary operators.
4. Define FooSyntax trait that implicitly provides FooOps from a Foo instance.

Frankly, these steps are mostly copy-paste boilerplate except for the first one. Enter Michael Pilquist (@mpilquist)’s simulacrum. simulacrum magically generates most of steps 2-4 just by putting @typeclass annotation. By chance, Stew O’Connor (@stewoconnor/@stew)’s #294 got merged, which refactors Cats to use it.

Yes-No typeclass 

In any case, let’s see if we can make our own truthy value typeclass. Note the @typeclass annotation:

scala> import simulacrum._
scala> :paste
@typeclass trait CanTruthy[A] { self =>
  /** Return true, if `a` is truthy. */
  def truthy(a: A): Boolean
}
object CanTruthy {
  def fromTruthy[A](f: A => Boolean): CanTruthy[A] = new CanTruthy[A] {
    def truthy(a: A): Boolean = f(a)
  }
}

According to the README, the macro will generate all the operator enrichment stuff:

// This is the supposed generated code. You don't have to write it!
object CanTruthy {
  def fromTruthy[A](f: A => Boolean): CanTruthy[A] = new CanTruthy[A] {
    def truthy(a: A): Boolean = f(a)
  }

  def apply[A](implicit instance: CanTruthy[A]): CanTruthy[A] = instance

  trait Ops[A] {
    def typeClassInstance: CanTruthy[A]
    def self: A
    def truthy: A = typeClassInstance.truthy(self)
  }

  trait ToCanTruthyOps {
    implicit def toCanTruthyOps[A](target: A)(implicit tc: CanTruthy[A]): Ops[A] = new Ops[A] {
      val self = target
      val typeClassInstance = tc
    }
  }

  trait AllOps[A] extends Ops[A] {
    def typeClassInstance: CanTruthy[A]
  }

  object ops {
    implicit def toAllCanTruthyOps[A](target: A)(implicit tc: CanTruthy[A]): AllOps[A] = new AllOps[A] {
      val self = target
      val typeClassInstance = tc
    }
  }
}

To make sure it works, let’s define an instance for Int and use it. The eventual goal is to get 1.truthy to return true:

scala> implicit val intCanTruthy: CanTruthy[Int] = CanTruthy.fromTruthy({
         case 0 => false
         case _ => true
       })
scala> import CanTruthy.ops._
scala> 10.truthy

It works. This is quite nifty. One caveat is that this requires Macro Paradise plugin to compile. Once it’s compiled the user of CanTruthy can use it without Macro Paradise.

Symbolic operators 

For CanTruthy the injected operator happened to be unary, and it matched the name of the function on the typeclass contract. simulacrum can also define operator with symbolic names using @op annotation:

scala> @typeclass trait CanAppend[A] {
  @op("|+|") def append(a1: A, a2: A): A
}
scala> implicit val intCanAppend: CanAppend[Int] = new CanAppend[Int] {
  def append(a1: Int, a2: Int): Int = a1 + a2
}
scala> import CanAppend.ops._
scala> 1 |+| 2