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Sep 25, 2024
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Add Defer.recursiveFn to aid in recursion #4656

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025867d
Add Defer.recursiveFn to aid in recursion
johnynek Sep 22, 2024
e45b590
actually, there is no reason to make loopFn lazy
johnynek Sep 22, 2024
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23 changes: 23 additions & 0 deletions core/src/main/scala/cats/Defer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -72,6 +72,29 @@ trait Defer[F[_]] extends Serializable {
lazy val res: F[A] = fn(defer(res))
res
}

/**
* Useful when you want a recursive function that returns F where
* F[_]: Defer. Examples include IO, Eval, or transformers such
* as EitherT or OptionT.
*
* example:
*
* val sumTo: Int => Eval[Int] =
* Defer[Eval].recursiveFn[Int, Int] { recur =>
*
* { i =>
* if (i > 0) recur(i - 1).map(_ + i)
* else Eval.now(0)
* }
* }
*/
def recursiveFn[A, B](fn: (A => F[B]) => (A => F[B])): A => F[B] =
new Function1[A, F[B]] { self =>
val loopFn: A => F[B] = fn(self)

def apply(a: A): F[B] = defer(loopFn(a))
}
}

object Defer {
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13 changes: 13 additions & 0 deletions tests/shared/src/test/scala/cats/tests/EvalSuite.scala
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Expand Up @@ -297,4 +297,17 @@ class EvalSuite extends CatsSuite {
assert(n2 == 1)
}
}

test("test Defer.recursiveFn example") {
val sumTo: Int => Eval[Int] =
cats.Defer[Eval].recursiveFn[Int, Int] { recur =>

{ i =>
if (i > 0) recur(i - 1).map(_ + i)
else Eval.now(0)
}
}

assert(sumTo(300000).value == (0 to 300000).sum)
}
}