Evaluation models

• Eager computations happen immediately
• Lazy computations happen on access.
• Memoized computations are run once on first access, after which the results are cached.

val is eager and memoized:

Usage

val x = {
  println("Computing X")
  math.random
}

// Computing X
// x: Double = 0.2800286720619266
x // first access
// res0: Double = 0.2800286720619266
x // second access
// res1: Double = 0.2800286720619266

def is lazy and not memoized:

Usage

def y = {
  println("Computing Y")
  math.random
}

// y: Double
y // first access
// Computing Y
// res0: Double = 0.8868971918220552
y // second access
// Computing Y
// res1: Double = 0.3816438919323032

lazy val is lazy and memoized. The code is not run until we access it for the first time (lazy). The result is then cached and re-used on subsequent accesses (memoized):

Usage

lazy val z = {
  println("Computing Z")
  math.random
 }

// z: Double = <lazy>
z // first access
// Computing Z
// res4: Double = 0.9968902519993859
z // second access
// res5: Double = 0.9968902519993859

Recap

Scala	Cats	Properties
val	Now	eager, memoized
lazy val	Later	lazy, memoized
def	Always	lazy, not memoized