Calculating the first n Fibonacci numbers using Effectful with the ST monad.

effecfulFibST.scala

import effectful._
import scalaz._
import Scalaz._
import effect._
import ST._
import std.indexedSeq._

def fibST(n: Int): List[Int] = {
  type ForallST[A] = Forall[({type λ[S] = ST[S, A]})#λ]

  def compute[S] = effectfully {
    val arr = newArr[S, Int](n, 1).!
    
    for (i <- 2 until n)
      arr.write(i, arr.read(i-2).! + arr.read(i-1).!).!
    
    arr.freeze.!.toArray.toList
  }

  runST(new ForallST[List[Int]] { def apply[S] = compute[S] })
}

// fibST(20) ==> List(1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765)