//arrow-core/arrow.core/Eval

Eval

common sealed class Eval<out A>

Eval is a monad which controls evaluation of a value or a computation that produces a value.

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.

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.

Example of stack safety:

import arrow.core.Eval

//sampleStart
fun even(n: Int): Eval<Boolean> =
  Eval.always { n == 0 }.flatMap {
    if(it == true) Eval.now(true)
    else odd(n - 1)
  }

fun odd(n: Int): Eval<Boolean> =
  Eval.always { n == 0 }.flatMap {
    if(it == true) Eval.now(false)
    else even(n - 1)
  }

// if not wrapped in eval this type of computation would blow the stack and result in a StackOverflowError
fun main() {
  println(odd(100000).value())
}
//sampleEnd

Types

Name Summary
Always common data class Always<out A>(f: () -> A) : Eval<A>
Construct a lazy Eval instance.
Companion common object Companion
Defer common data class Defer<out A>(thunk: () -> Eval<A>) : Eval<A>
Defer is a type of Eval that is used to defer computations which produce Eval.
FlatMap common abstract class FlatMap<out A> : Eval<A>
FlatMap is a type of Eval that is used to chain computations involving .map and .flatMap.
Later common data class Later<out A>(f: () -> A) : Eval<A>
Construct a lazy Eval instance.
Now common data class Now<out A>(value: A) : Eval<A>
Construct an eager Eval instance.

Functions

Name Summary
coflatMap common inline fun <B> coflatMap(crossinline f: (Eval<A>) -> B): Eval<B>
flatMap common fun <B> flatMap(f: (A) -> Eval<B>): Eval<B>
map common inline fun <B> map(crossinline f: (A) -> B): Eval<B>
memoize common abstract fun memoize(): Eval<A>
toString common open override fun toString(): String
value common abstract fun value(): A

Inheritors

Name
Eval
Eval
Eval
Eval
Eval

Extensions

Name Summary
replicate common fun <A> Eval<A>.replicate(n: Int): Eval<List<A»
fun <A> Eval<A>.replicate(n: Int, MA: Monoid<A>): Eval<A>
zip common fun <A, B, Z> Eval<A>.zip(b: Eval<B>, map: (A, B) -> Z): Eval<Z>
fun <A, B> Eval<A>.zip(b: Eval<B>): Eval<Pair<A, B»
fun <A, B, C, D> Eval<A>.zip(b: Eval<B>, c: Eval<C>, map: (A, B, C) -> D): Eval<D>
fun <A, B, C, D, E> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, map: (A, B, C, D) -> E): Eval<E>
fun <A, B, C, D, E, F> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, map: (A, B, C, D, E) -> F): Eval<F>
fun <A, B, C, D, E, F, G> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, f: Eval<F>, map: (A, B, C, D, E, F) -> G): Eval<G>
fun <A, B, C, D, E, F, G, H> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, f: Eval<F>, g: Eval<G>, map: (A, B, C, D, E, F, G) -> H): Eval<H>
fun <A, B, C, D, E, F, G, H, I> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, f: Eval<F>, g: Eval<G>, h: Eval<H>, map: (A, B, C, D, E, F, G, H) -> I): Eval<I>
fun <A, B, C, D, E, F, G, H, I, J> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, f: Eval<F>, g: Eval<G>, h: Eval<H>, i: Eval<I>, map: (A, B, C, D, E, F, G, H, I) -> J): Eval<J>
fun <A, B, C, D, E, F, G, H, I, J, K> Eval<A>.zip(b: Eval<B>, c: Eval<C>, d: Eval<D>, e: Eval<E>, f: Eval<F>, g: Eval<G>, h: Eval<H>, i: Eval<I>, j: Eval<J>, map: (A, B, C, D, E, F, G, H, I, J) -> K): Eval<K>

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