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.implicits._
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
- Companion:
- object
Value members
Concrete methods
Defer instances, like functions, parsers, generators, IO, etc... often are used in recursive settings where this function is useful
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.