CODEWITHSUNDEEP
scala
A_P
A_P

Reputation: 336

"Missing implementations" when using ClassTag

I'm trying to implement the following trait:

trait Monad[M[_]] {
  def pure[A](a: A): M[A]
  def flatMap[A, B](ma: M[A], f: A => M[B]): M[B]
}

for type Array:

def arrayMonad: Monad[Array] = new Monad[Array] {
  def pure[A](a: A): Array[A] = Array(a)
  def flatMap[A, B](a: Array[A], f: A => Array[B]): Array[B] = a flatMap f
}

The compiler complains that there are no ClassTags for A and B. If I add them (either with an implicit parameter or "[A: ClassTag]" style), I'm told that I'm missing implementations:

Missing implementations for 2 members. Stub implementations follow:
  def flatMap[A, B](ma: Array[A], f: A => Array[B]): Array[B] = ???
  def pure[A](a: A): Array[A] = ???

There may be something obvious I'm missing, but I'm not sure what it is.

Upvotes: 1

Views: 323

Answers (2)

Mario Galic
Mario Galic

Reputation: 48400

Perhaps emulate Luis' contribution to cats Adding instances for ArraySeq #3273 and use untagged factory to avoid having to provide implicit ClassTag:

def arrayMonad: Monad[ArraySeq] = new Monad[ArraySeq] {
  def pure[A](a: A): ArraySeq[A] = ArraySeq.untagged(a)
  def flatMap[A, B](a: ArraySeq[A], f: A => ArraySeq[B]): ArraySeq[B] = a flatMap f
}

arrayMonad.pure(42)
// res0: scala.collection.immutable.ArraySeq[Int] = ArraySeq(42)

Upvotes: 1

Alexey Romanov
Alexey Romanov

Reputation: 170713

If you add them, then you change signature and are no longer overriding the original methods (add override and you'll see "overrides nothing" error).

You simply can't implement Monad[Array] for this definition of Monad.

Upvotes: 3

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