trait Defer[F[_]] extends Serializable
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
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- Defer.scala
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- abstract def defer[A](fa: => F[A]): F[A]
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- def fix[A](fn: (F[A]) => F[A]): F[A]
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.
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