Simple CircuitBreaker example with Kotlin & Arrow Fx

CircuitBreaker.kt

import arrow.fx.extensions.io.applicative.map
import arrow.fx.extensions.io.monad.flatMap
import arrow.fx.extensions.io.monadDefer.monadDefer
import java.lang.RuntimeException
​
enum class CBState {
  CLOSED, OPEN, DISABLED;
}
​
class CallNotPermitedException(val state: CBState):
  RuntimeException("CircuitBreaker was closed with state: $state")
​
class CircuitBreaker private constructor(val ref: Ref<ForIO, CBState>) {
​
  fun <A> decorateIO(io: IO<A>): IO<A> =
    ref.get().flatMap { state ->
      when(state) {
        CBState.OPEN -> io
        CBState.DISABLED -> io
​
        CBState.CLOSED -> IO.raiseError(CallNotPermitedException(state))
      }
    }
​
  companion object {
    operator fun invoke(initial: CBState = CBState.OPEN): IO<CircuitBreaker> =
      Ref.invoke(IO.monadDefer(), initial).map { ref -> CircuitBreaker(ref) }
  }