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@forzaken hw3 #33

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57 changes: 57 additions & 0 deletions src/main/scala/fpspeedrun/Hw3.scala
Original file line number Diff line number Diff line change
@@ -0,0 +1,57 @@
package fpspeedrun

import cats.Eval

trait Foldable2[F[_]] {
type LazyMonoid[A] = Monoid[Eval[A]]
type LazyEndo[A] = Eval[A] => Eval[A]

def endoLazyMonoid[A] = new LazyMonoid[LazyEndo[A]] {
override def empty: Eval[LazyEndo[A]] = Eval.now(x => x)
override def combine(x: Eval[LazyEndo[A]], y: Eval[LazyEndo[A]]): Eval[LazyEndo[A]] = {
Eval.later(t => {
(for {
xv <- x
yv <- y
} yield yv andThen xv).flatMap(_(t))
})
}
}

def foldMapLazy[A, B: LazyMonoid](fa: F[A])(f: A => Eval[B]): Eval[B]

def foldRightLazy[A, B](fa: F[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = {
val fu: A => Eval[Eval[B] => Eval[B]] = a => Eval.later(f.curried(a))
foldMapLazy[A, Eval[B] => Eval[B]](fa)(fu)(endoLazyMonoid)
.flatMap(_(lb))
}
}

object StreamFoldable2 extends Foldable2[Stream] {
override def foldMapLazy[A, B: StreamFoldable2.LazyMonoid](fa: Stream[A])(f: A => Eval[B]): Eval[B] = {
Eval.now(fa).flatMap { s =>
val m = implicitly[StreamFoldable2.LazyMonoid[B]]
if (s.isEmpty) m.empty else m.combine(f(s.head), Eval.defer(foldMapLazy(s.tail)(f)))
}
}
}

object Main extends App {
def sumN(xs: Stream[Int])(n: Int): Int = {
StreamFoldable2.foldRightLazy[Int, Int => Eval[Int]](xs, Eval.now(_ => Eval.now(0))) { case (el, cntToSum) =>
Eval.later(cnt => {
if (cnt <= 0) {
Eval.now(0)
}
else {
for {
f <- cntToSum
v <- f(cnt - 1)
} yield el + v
}
})
}.flatMap(_(n)).value
}

println(sumN(Stream.from(1))(10000)) //50005000
}