Defer

cats.Defer

See theDefer companion object

Defer is a type class that shows the ability to defer creation inside of the type constructor F[_].

This comes up with F[_] types that are implemented with a trampoline or are based on function application.

The law is that defer(fa) is equivalent to fa, but not evaluated immediately, so

scala> import cats._
scala> import cats.syntax.all._

scala> var evaluated = false
scala> val dfa = Defer[Eval].defer {
    |   evaluated = true
    |   Eval.now(21)
    | }

scala> evaluated
res0: Boolean = false

scala> Eq[Eval[Int]].eqv(dfa, Eval.now(21))
res1: Boolean = true

Attributes

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

class Object

trait Matchable

class Any

Members list

Value members

Abstract methods

Attributes

Source: Defer.scala

Concrete methods

Defer instances, like functions, parsers, generators, IO, etc...

Defer instances, like functions, parsers, generators, IO, etc... often are used in recursive settings where this function is useful

fix(fn) == fn(fix(fn))

example:

val parser: P[Int] = Defer[P].fix[Int] { rec => CharsIn("0123456789") | P("(") ~ rec ~ P(")") }

Note, fn may not yield a terminating value in which case both of the above F[A] run forever.

Attributes

Source: Defer.scala

In this article

Generated with