kt-defer: A Kotlin repository from saantiaguilera

Deferrable

Toy project trying out Kotlin's contexts introduced in 1.6.X. This projects mirrors golang's defer feature as much as we can allowing flexible try-catches + function defers to close stuff or operate after the end of it regardless of a potential error being thrown

Feel free to copy this and do whatever you like with it, although (as kotlin also suggests) I highly discourage using it in production since the context API is experimental

Implementation

Pretty straightforward implementation where we create a scope function that will have as context a DeferrableScope which, as its name suggests, allows us to defer functions (that will be invoked once the scope ends regardless of an error thrown or not).

When deferring a func, we will also have another scope called RecoverableScope since all deferred functions can gracefully recover from a thrown error. The first function to recover from it will consume it (hence, further calls to recover will yield no value). If no function consumes the thrown error, the thrown error will be broadcasted as a normal one (hence you will have to try-catch it)

Behavior should be close-to-exact as go. Everything is the same besides two things that are outside of our scope because they are language limitations:

We cannot name return parameters, hence we can't alter return values inside a defer directly.

fun problem(): ReturnValue {
    deferrable {
        defer { return ReturnValue() } // Bad. We cannot return inside a defer because language + contract's API limitations in interfaces/classes
        // do stuff that may throw errors
        return ReturnValue() // OK. We can return inside a deferrable, this works fine
    }
    return ReturnValue() // OK. This is needed because if an error is thrown and we recover, we will reach here.
}

fun solution(): ReturnValue {
    var returnValue: ReturnValue? // name the return value at the beginning.. "similar" to go named return values
    deferrable {
        defer { recover()?.let { err -> returnValue = ErrorValue(err) } }
        // do stuff that may throw errors OR assign returnValue
    }
    return returnValue
}

We cannot pass invocations as defers instead of high order functions

fun problem() = deferrable {
    defer aFunction(param) // Bad. We can't pass an invocation
    defer { aFunction(param) } // OK.
}

Besides this two minimal limitations (which don't even pose a problem), everything is mirrored as-is:

internal panics cant be recovered, thus are signaled
multiple recovers behave in a first come first serve fashion
cannot recover from a panic after a recover
stack invocation ordering
context visibility
etc

Code

Code is super straightforward as said before. This is the whole implementation:

typealias Deferrable = context(DeferrableScope) () -> Unit
typealias Recoverable = context(RecoverableScope) () -> Unit

interface DeferrableScope {
  fun defer(block: Recoverable)
}

interface RecoverableScope {
  fun recover(): Throwable?
}

class GoDeferrableScope : DeferrableScope {

  private val defers: ArrayDeque<Recoverable> = ArrayDeque()

  override fun defer(block: Recoverable) {
    defers.add(block)
  }

  fun isEmpty(): Boolean = defers.isEmpty()

  fun pop(): Recoverable = defers.removeLast()
}

class GoRecoverableScope(private var err: Throwable?) : RecoverableScope {

  override fun recover(): Throwable? {
    val ret = err
    err = null
    return ret
  }

  fun recovered(): Boolean = err == null
}

@Throws(Throwable::class)
@ExperimentalContracts
inline fun deferrable(block: Deferrable) {
  contract {
    callsInPlace(block, InvocationKind.AT_MOST_ONCE) 
  }
  val scope = GoDeferrableScope()
  var err: Throwable? = null
  try {
    block(scope)
  } catch (e: Throwable) {
    err = e
  } finally {
    val recoverableScope = GoRecoverableScope(err)
    while (!scope.isEmpty()) {
      scope.pop()(recoverableScope)
    }

    // if there was an error and it wasn't recovered, throw it.
    if (!recoverableScope.recovered()) {
      throw err!!
    }
  }
}

Examples

Try-Catch

Catch an error more gracefully without needing to pollute the whole method

fun main() = deferrable {
  defer { recover()?.let { err -> /* handle error */ } }
  // do stuff
  throw Error("oops!")
}

Another example might be

// Imagine a REST API controller for a GET endpoint 
fun restGet(request: Request): Response {
  var response: Any? = null
  deferrable {
    defer { recover()?.let { err -> response = handleError(request, err) } }
    // do stuff
  }

  return response // in case an error is thrown, we recover and return here.
}

Or even, in a more complex fashion you can create your own APM contextual function, eg:

context(RecoverableScope)
fun customRecover() {
  val err = recover()
  if (err != null) {
    // track in newrelic / your APM
    // log it?
    // do stuff.
    println(err)
  }
}

fun tryCatchComplexExample() = deferrable {
  defer { customRecover() }
  throw Error("oops!") // do stuff
}

Closeables

Stuff that should be closed after usage to zero it

context(DeferrableScope)
fun File.open(): InputStream? {
  return try {
    val fis = inputStream()
    defer { fis.close() } // will still be closed even if an error is thrown
    fis
  } catch (ignored: Exception) {
    null // handle it better in your implementation. this is an example :)
  }
}

fun main() = deferrable {
  val fis = File("test.txt").open()
  // use it, it will be closed afterwards
}

Countdown Latch / Semaphore

Concurrent stuff that has to be signaled after usage to release them

fun semaphore(s: Semaphore) = deferrable {
  s.acquire()
  defer { s.release() }

  // do stuff, even throwing an error still releases the acquired object
  throw Error("oops")
}

Elapsed time

Getting the elapsed time for an operation to complete, even if it throws an error

context(DeferrableScope)
fun trace(fn: () -> Unit) {
  val start = System.currentTimeMillis()
  defer { println("execution of fn took ${System.currentTimeMillis() - start} ms") }
  fn()
}

fun main() = deferrable {
  trace {
    // do stuff and measure them
  }
}

saantiaguilera/kt-defer