Eval

cats.Eval

See theEval companion object

sealed abstract class Eval[+A] extends Serializable

Eval is a monad which controls evaluation.

This type wraps a value (or a computation that produces a value) and can produce it on command via the .value method.

There are three basic evaluation strategies:

- Now: evaluated immediately - Later: evaluated once when value is needed - Always: evaluated every time value is needed

The Later and Always are both lazy strategies while Now is eager. Later and Always are distinguished from each other only by memoization: once evaluated Later will save the value to be returned immediately if it is needed again. Always will run its computation every time.

Eval supports stack-safe lazy computation via the .map and .flatMap methods, which use an internal trampoline to avoid stack overflows. Computation done within .map and .flatMap is always done lazily, even when applied to a Now instance.

It is not generally good style to pattern-match on Eval instances. Rather, use .map and .flatMap to chain computation, and use .value to get the result when needed. It is also not good style to create Eval instances whose computation involves calling .value on another Eval instance -- this can defeat the trampolining and lead to stack overflows.

Attributes

Companion: object
Source: Eval.scala
Graph
Supertypes: trait Serializable

class Object

trait Matchable

class Any
Known subtypes: class Defer[A]

class FlatMap[A]

class Leaf[A]

class Always[A]

class Later[A]

class Now[A]
Show all
Self type: Eval[A]

Members list

Value members

Abstract methods

Ensure that the result of the computation (if any) will be memoized.

Practically, this means that when called on an Always[A] a Later[A] with an equivalent computation will be returned.

Attributes

Source: Eval.scala

Evaluate the computation and return an A value.

For lazy instances (Later, Always), any necessary computation will be performed at this point. For eager instances (Now), a value will be immediately returned.

Attributes

Source: Eval.scala

Concrete methods

Lazily perform a computation based on an Eval[A], using the function f to produce an Eval[B] given an A.

This call is stack-safe -- many .flatMap calls may be chained without consumed additional stack during evaluation. It is also written to avoid left-association problems, so that repeated calls to .flatMap will be efficiently applied.

Computation performed in f is always lazy, even when called on an eager (Now) instance.

Attributes

Source: Eval.scala

Transform an Eval[A] into an Eval[B] given the transformation function f.

This call is stack-safe -- many .map calls may be chained without consumed additional stack during evaluation.

Computation performed in f is always lazy, even when called on an eager (Now) instance.

Attributes

Source: Eval.scala

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