Eval

Eval is a data type for controlling synchronous evaluation. Its implementation is designed to provide stack-safety at all times using a technique called trampolining.

There are two different factors that play into evaluation: memoization and laziness.

Memoized evaluation evaluates an expression only once and then remembers (memoizes) that value. Lazy evaluation refers to when the expression is evaluated. We talk about eager evaluation if the expression is immediately evaluated when defined and about lazy evaluation if the expression is evaluated when it’s first used.

For example, in Scala, a lazy val is both lazy and memoized, a method definition def is lazy, but not memoized, since the body will be evaluated on every call. A normal val evaluates eagerly and also memoizes the result.

Eval is able to express all of these evaluation strategies and allows us to chain computations using its Monad instance.

Eval.now

First of the strategies is eager evaluation, we can construct an Eval eagerly using Eval.now:

import cats.Eval
// import cats.Eval

import cats.implicits._
// import cats.implicits._

val eager = Eval.now {
  println("Running expensive calculation...")
  1 + 2 * 3
}
// Running expensive calculation...
// eager: cats.Eval[Int] = Now(7)

We can run the computation using the given evaluation strategy anytime by using the value method.

eager.value
// res0: Int = 7

Eval.later

If we want lazy evaluation, we can use Eval.later:

val lazyEval = Eval.later {
  println("Running expensive calculation...")
  1 + 2 * 3
}
// lazyEval: cats.Eval[Int] = cats.Later@35bb79b7

lazyEval.value
// Running expensive calculation...
// res1: Int = 7

lazyEval.value
// res2: Int = 7

Notice that “Running expensive calculation” is printed only once, since the value was memoized internally. Eval.later is different to using a lazy val in a few different ways. First, it allows the runtime to perform garbage collection of the thunk after evaluation, leading to more memory being freed earlier. Secondly, when lazy vals are evaluated, in order to preserve thread-safety, the Scala compiler will lock the whole surrounding class, whereas Eval will only lock itself.

Eval.always

If we want lazy evaluation, but without memoization akin to Function0, we can use Eval.always

val always = Eval.always {
  println("Running expensive calculation...")
  1 + 2 * 3
}
// always: cats.Eval[Int] = cats.Always@334891da

always.value
// Running expensive calculation...
// res3: Int = 7

always.value
// Running expensive calculation...
// res4: Int = 7

Here we can see, that the expression is evaluated every time we call .value.

Chaining lazy computations

One of the most useful applications of Eval is its ability to chain together computations in a stack-safe way. You can see one such usage when looking at the foldRight method found in Foldable. Another great example are mutual tail-recursive calls:

object MutualRecursion {
  def even(n: Int): Eval[Boolean] =
    Eval.always(n == 0).flatMap {
      case true => Eval.now(true)
      case false => odd(n - 1)
    }

  def odd(n: Int): Eval[Boolean] =
    Eval.always(n == 0).flatMap {
      case true => Eval.now(false)
      case false => even(n - 1)
    }
}
// defined object MutualRecursion

MutualRecursion.odd(199999).value
// res5: Boolean = true

Because Eval guarantees stack-safety, we can chain a lot of computations together using flatMap without fear of blowing up the stack.

You can also use Eval.defer to defer any computation that will return an Eval[A]. This is useful, because nesting a call to .value inside any of the Eval creation methods can be unsafe.