a Yes-No typeclass 

Let’s see if we can make our own truthy value typeclass in the style of Scalaz. Except I am going to add my twist to it for the naming convention. Scalaz calls three or four different things using the name of the typeclass like Show, show, and show, which is a bit confusing.

I like to prefix the typeclass name with Can borrowing from CanBuildFrom, and name its method as verb + s, borrowing from sjson/sbinary. Since yesno doesn’t make much sense, let’s call ours truthy. Eventual goal is to get 1.truthy to return true. The downside is that the extra s gets appended if we want to use typeclass instances as functions like CanTruthy[Int].truthys(1).

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait CanTruthy[A] { self =>
  /** @return true, if `a` is truthy. */
  def truthys(a: A): Boolean
object CanTruthy {
  def apply[A](implicit ev: CanTruthy[A]): CanTruthy[A] = ev
  def truthys[A](f: A => Boolean): CanTruthy[A] = new CanTruthy[A] {
    def truthys(a: A): Boolean = f(a)
trait CanTruthyOps[A] {
  def self: A
  implicit def F: CanTruthy[A]
  final def truthy: Boolean = F.truthys(self)
object ToCanIsTruthyOps {
  implicit def toCanIsTruthyOps[A](v: A)(implicit ev: CanTruthy[A]) =
    new CanTruthyOps[A] {
      def self = v
      implicit def F: CanTruthy[A] = ev

// Exiting paste mode, now interpreting.

defined trait CanTruthy
defined module CanTruthy
defined trait CanTruthyOps
defined module ToCanIsTruthyOps

scala> import ToCanIsTruthyOps._
import ToCanIsTruthyOps._

Here’s how we can define typeclass instances for Int:

scala> implicit val intCanTruthy: CanTruthy[Int] = CanTruthy.truthys({
         case 0 => false
         case _ => true
intCanTruthy: CanTruthy[Int] = CanTruthy$$anon$1@71780051

scala> 10.truthy
res6: Boolean = true

Next is for List[A]:

scala> implicit def listCanTruthy[A]: CanTruthy[List[A]] = CanTruthy.truthys({
         case Nil => false
         case _   => true  
listCanTruthy: [A]=> CanTruthy[List[A]]

scala> List("foo").truthy
res7: Boolean = true

scala> Nil.truthy
<console>:23: error: could not find implicit value for parameter ev: CanTruthy[scala.collection.immutable.Nil.type]

It looks like we need to treat Nil specially because of the nonvariance.

scala> implicit val nilCanTruthy: CanTruthy[scala.collection.immutable.Nil.type] = CanTruthy.truthys(_ => false)
nilCanTruthy: CanTruthy[collection.immutable.Nil.type] = CanTruthy$$anon$1@1e5f0fd7

scala> Nil.truthy
res8: Boolean = false

And for Boolean using identity:

scala> implicit val booleanCanTruthy: CanTruthy[Boolean] = CanTruthy.truthys(identity)
booleanCanTruthy: CanTruthy[Boolean] = CanTruthy$$anon$1@334b4cb

scala> false.truthy
res11: Boolean = false

Using CanTruthy typeclass, let’s define truthyIf like LYAHFGG:

Now let’s make a function that mimics the if statement, but that works with YesNo values.

To delay the evaluation of the passed arguments, we can use pass-by-name:

scala> :paste
// Entering paste mode (ctrl-D to finish)

def truthyIf[A: CanTruthy, B, C](cond: A)(ifyes: => B)(ifno: => C) =
  if (cond.truthy) ifyes
  else ifno

// Exiting paste mode, now interpreting.

truthyIf: [A, B, C](cond: A)(ifyes: => B)(ifno: => C)(implicit evidence$1: CanTruthy[A])Any

Here’s how we can use it:

scala> truthyIf (Nil) {"YEAH!"} {"NO!"}
res12: Any = NO!

scala> truthyIf (2 :: 3 :: 4 :: Nil) {"YEAH!"} {"NO!"}
res13: Any = YEAH!

scala> truthyIf (true) {"YEAH!"} {"NO!"}
res14: Any = YEAH!

We’ll pick it from here later.