Trampolining scalaz' Monad.whileM_ to prevent stack overflow - monads

Question

I'm using scalaz' Monad.whileM_ to implement a while loop in a functional way as follows:
object Main {
import scalaz._
import Scalaz._
import scala.language.higherKinds
case class IState(s: Int)
type IStateT[A] = StateT[Id, IState, A]
type MTransT[S[_], A] = EitherT[S, String, A]
type MTrans[A] = MTransT[IStateT, A]
def eval(k: Int): MTrans[Int] = {
for {
state <- get[IState].liftM[MTransT]
_ <- put(state.copy(s = (state.s + 1) % k)).liftM[MTransT]
} yield (k + 1)
}
def evalCond(): MTrans[Boolean] = {
for {
state <- get[IState].liftM[MTransT]
} yield (state.s != 0)
}
def run() = {
val k = 10
eval(k).whileM_(evalCond()).run(IState(1))
}
}
While this works for small k, it results in a StackOverflow error for large k (e.g. 1000000). Is there a way to trampoline whileM_ or is there a better way to be stack safe?

Answer 1

Use scalaz.Free.Trampoline instead of scalaz.Id.Id.
type IStateT[A] = StateT[Trampoline, IState, A]
The state operations used here return State[S, A] which is just an alias for StateT[Id, S, A]. You need to use the lift[M[_]] function defined on StateT to lift StateT[Id, S, A] to StateT[Trampoline, S, A].
def eval(k: Int): MTrans[Int] = {
for {
state <- get[IState].lift[Trampoline].liftM[MTransT]
_ <- put(state.copy(s = (state.s + 1) % k)).lift[Trampoline].liftM[MTransT]
} yield (k + 1)
}
def evalCond(): MTrans[Boolean] = {
for {
state <- get[IState].lift[Trampoline].liftM[MTransT]
} yield (state.s != 0)
}
Finally, calling .run(IState(1)) now results in Trampoline[(IState, String \/ Unit)]. You must additionally run this as well.
eval(k).whileM_(evalCond()).run(IState(1)).run