Given an Option, what is the idiomatic way to get its value or throw an exception trying?
def foo() : String = {
val x : Option[String] = ...
x.getOrException()
}
A throw "statement" is really an expression in Scala, and it has type Nothing, which is a subtype of every other type. This means you can just use plain old getOrElse:
def myGet[A](oa: Option[A]) = oa.getOrElse(throw new RuntimeException("Can't."))
You really, really shouldn't be doing this, though.
(EDIT: this is not the best or most idiomatic way to do it. I wrote it when I was not familiar with Scala. I leave it here for an example of how not to do it. Nowadays I would do as #TravisBrown)
I think it really boils down to two things:
how sure are you that the value is there?
how do you want to react if it isn't?
If at that point in your code you expect the value to be there, and in the remote case that it isn't you want your program to fail fast, then I would only do a normal get and let Scala throw a NoSuchElementException if there was no value:
def foo() : String = {
val x : Option[String] = ...
x.get
}
If you want to handle the case differently (throw your own exception) I think a more elegant way would look like this:
def foo(): String = {
val x: Option[String] = None
x match {
case Some(value) => value
case None => throw new MyRuntimeException("blah")
}
}
And of course if you want to supply your own alternative value for the case that the Option is None you would just use getOrElse:
def foo(): String = {
val x: Option[String] = None
x.getOrElse("my alternative value")
}
I hope this will help you to understand how to represent errors (and generally effects) using types.
Error handling strategies in functional Scala
Use Option to return optional values. For example - fail to find entity in storage.
Use Option(possiblyNull) to avoid instances of Some(null).
Use Either[Error, T] to report expected failure. For example - email format is wrong, cannot parse a string to a number, etc.
Model your errors as ADTs (simply speaking kind of type hierarchies) to use it, for example, on the Left of the Either to represent more complex error scenarios.
Throw Exception only to signal unexpected and not-recoverable failures. Like missing config file.
Use Either.catchOnly or Try or Cats.IO (advanced) rather than a catch block for handling unexpected failures. Hint: You can still use ADT but extend them from throwables. More about Either vs Try.
Use Validated data-type from Cats lib to accumulate errors rather than fail-fast (Either), but prefer Either's on module-level to simplify the composition of the program (to have the same types). For example - form data validation, parsing errors accumulation.
Use mentioned types and don't optimize program preemptively - since most probably, bottle-necks would be in business logic, not in effect types.
Such an approach will simplify maintenance and updates of your code since you can reason about it without going to implementation specifics (aka local-reasoning). Also - reduce bugs - you cannot miss an error in the type. And compose the program easier (with help of map, flatMap and other combinators) - since it's simpler on type level, rather than with non-local exceptions and side-effects.
More about learning functional Scala.
But be aware that sometimes with this approach types could stack up and it could become harder to compose things. Given, for example: x: Future[Either[Error, Option[T]]] What you can do:
Use map and flatMap in combination with pattern-matching to compose different values of such types, for example:
x.faltMap { case Right(Some(v)) => anotherFuture(v); case Left(er) => ... }
If it doesn't help you can try to use MonadTransformers (don't be scared of the name, it's just wrappers around the effect types like Either and Future)
Also, an option is to simplify your errors ADT by extending them from the Throwable to unify it with Future, then it'll be Future[Option[T]]
And finally, in your case one option will be:
def foo() : Either[Error, String] = {
val x : Option[String] = ...
x match {
case Some(v) => Right(v)
case None => Left(Error(reason))
}
}
Just use the .get method.
def get[T](o:Option[T]) = o.get
It will throw a NoSuchElementException if o is an instance of None.
Basically, I would work with options like this:
def addPrint(oi:Option[Int]) = oi.map(_+1).foreach(println)
addPrint(Some(41))
addPrint(Some(1336))
addPrint(None)
to avoid your specific question.
Scala now support this operation on maps using getOrElse() method, see documentation here
As pointed out already, throwing an exception in Scala is an expression as well.
So you can do the following:
myMap.getOrElse(myKey, throw new MyCustomException("Custom Message HERE")
Related
What is the idiomatic way in Scala to type an API that may or may not return results?
I can simply type to Future[Option[A]] and return a None if there is no results, but this will make things a little more verbose for API clients I think because it has to do some pattern matching (or other techniques like folding) over the Option returned.
trait SomeAPI {
def fetch(): Future[Option[String]]
}
object APIImpl {
def asyncDBCall(): Future[List[String]] = ???
override def fetch(): Future[Option[String]] = asyncDBCall().map(r => if (r.isEmpty) None else Some(r.head)
}
However, Futures already give us a way to say that there are no results making it fail. But I don't know if this is a proper or expressive way of say that the call doesn't return results.
trait SomeAPI {
def fetch(): Future[String]
}
class NoResultsException extends Exception
object APIImpl extends SomeAPI {
def asyncDBCall(): Future[List[String]] = ???
override def fetch(): Future[String] = asyncDBCall().map(r => if (r.isEmpty) throw new NoResultsException() else r.head)
}
At first it seems that both options are acceptable and the final decision may be just about personal preference, but maybe I'm missing something and make a Future fail is not a good option.
Exceptions should be reserved for errors. If the API may or may not returns results, and both cases are considered normal, then the API should use Option.
If that makes it more verbose by forcing the client to handle both situations, that's good, because the client should handle both situations. Exceptions are good when you don't want to force the client code to handle an exceptional case (unexpected error) which should probably cascade and be handled by an error handler.
Now, if lack of results truly indicates that something went wrong, then an exception would be appropriate.
I'd expect clients want to treat "success, no results" and failure differently, generally speaking (e.g. retry in the second case, but not the first). If you denote the first with a failed future, you make verbosity for the clients worse, not better: they just need to check the exception instead of the success, and the API isn't helping them to be aware of that like Future[Option[A]] would.
If you really worry about verbosity, you could add custom extractors or methods for the clients to use if they want, e.g.
implicit class FutureOption[A](future: Future[Option[A]]) {
def mapOpt[B](handleSuccessWithResult: A => B, handleSuccessWithNoResult: () => B) = Future.map {
case Some(x) => onSuccessWithResult(x)
case None => onSuccessWithNoResult()
}
}
The others are right. Failures are not a good way to describe the lack of a concrete result, if this is part of regular operation.
Scala gives you another possibility to explicitly note, that there may be no results, sealed traits.
sealed trait MyOperationReturn
case object OkNoResult
case class OkWithStringResponse(data: String) extends MyOperationReturn
case class OkWithIntResponse(data: Int) extends MyOperationReturn
The user can then just pattern match. This option is especially handy, if you have either multiple return types (not only two) and no natural empty return type (like Seq.empty)
I am a new to Scala coming from Java background, currently confused about the best practice considering Option[T].
I feel like using Option.map is just more functional and beautiful, but this is not a good argument to convince other people. Sometimes, isEmpty check feels more straight forward thus more readable. Is there any objective advantages, or is it just personal preference?
Example:
Variation 1:
someOption.map{ value =>
{
//some lines of code
}
} orElse(foo)
Variation 2:
if(someOption.isEmpty){
foo
} else{
val value = someOption.get
//some lines of code
}
I intentionally excluded the options to use fold or pattern matching. I am simply not pleased by the idea of treating Option as a collection right now, and using pattern matching for a simple isEmpty check is an abuse of pattern matching IMHO. But no matter why I dislike these options, I want to keep the scope of this question to be the above two variations as named in the title.
Is there any objective advantages, or is it just personal preference?
I think there's a thin line between objective advantages and personal preference. You cannot make one believe there is an absolute truth to either one.
The biggest advantage one gains from using the monadic nature of Scala constructs is composition. The ability to chain operations together without having to "worry" about the internal value is powerful, not only with Option[T], but also working with Future[T], Try[T], Either[A, B] and going back and forth between them (also see Monad Transformers).
Let's try and see how using predefined methods on Option[T] can help with control flow. For example, consider a case where you have an Option[Int] which you want to multiply only if it's greater than a value, otherwise return -1. In the imperative approach, we get:
val option: Option[Int] = generateOptionValue
var res: Int = if (option.isDefined) {
val value = option.get
if (value > 40) value * 2 else -1
} else -1
Using collections style method on Option, an equivalent would look like:
val result: Int = option
.filter(_ > 40)
.map(_ * 2)
.getOrElse(-1)
Let's now consider a case for composition. Let's say we have an operation which might throw an exception. Additionaly, this operation may or may not yield a value. If it returns a value, we want to query a database with that value, otherwise, return an empty string.
A look at the imperative approach with a try-catch block:
var result: String = _
try {
val maybeResult = dangerousMethod()
if (maybeResult.isDefined) {
result = queryDatabase(maybeResult.get)
} else result = ""
}
catch {
case NonFatal(e) => result = ""
}
Now let's consider using scala.util.Try along with an Option[String] and composing both together:
val result: String = Try(dangerousMethod())
.toOption
.flatten
.map(queryDatabase)
.getOrElse("")
I think this eventually boils down to which one can help you create clear control flow of your operations. Getting used to working with Option[T].map rather than Option[T].get will make your code safer.
To wrap up, I don't believe there's a single truth. I do believe that composition can lead to beautiful, readable, side effect deferring safe code and I'm all for it. I think the best way to show other people what you feel is by giving them examples as we just saw, and letting them feel for themselves the power they can leverage with these sets of tools.
using pattern matching for a simple isEmpty check is an abuse of pattern matching IMHO
If you do just want an isEmpty check, isEmpty/isDefined is perfectly fine. But in your case you also want to get the value. And using pattern matching for this is not abuse; it's precisely the basic use-case. Using get allows to very easily make errors like forgetting to check isDefined or making the wrong check:
if(someOption.isEmpty){
val value = someOption.get
//some lines of code
} else{
//some other lines
}
Hopefully testing would catch it, but there's no reason to settle for "hopefully".
Combinators (map and friends) are better than get for the same reason pattern matching is: they don't allow you to make this kind of mistake. Choosing between pattern matching and combinators is a different question. Generally combinators are preferred because they are more composable (as Yuval's answer explains). If you want to do something covered by a single combinator, I'd generally choose them; if you need a combination like map ... getOrElse, or a fold with multi-line branches, it depends on the specific case.
It seems similar to you in case of Option but just consider the case of Future. You will not be able to interact with the future's value after going out of Future monad.
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Promise
import scala.util.{Success, Try}
// create a promise which we will complete after sometime
val promise = Promise[String]();
// Now lets consider the future contained in this promise
val future = promise.future;
val byGet = if (!future.value.isEmpty) {
val valTry = future.value.get
valTry match {
case Success(v) => v + " :: Added"
case _ => "DEFAULT :: Added"
}
} else "DEFAULT :: Added"
val byMap = future.map(s => s + " :: Added")
// promise was completed now
promise.complete(Try("PROMISE"))
//Now lets print both values
println(byGet)
// DEFAULT :: Added
println(byMap)
// Success(PROMISE :: Added)
I'm new to Scala, being slightly confused at the various ways to handle exceptions and looking for best-practice advice on the topic. I'm writing a simple method to retrieve a Customer using an existing blocking SDK. The possible outcomes are:
Customer is found
Customer is not found (comes back as a NotFoundException from the SDK)
Error while talking to remote server (SDK throws some other exception)
So I want my method to have a return type Future[Option[Customer]], and return for each case above:
Successful Future: Some(customer)
Successful Future: None
Failed Future
Here is what I wrote using try/catch:
private def findCustomer(userId: Long): Future[Option[Customer]] = future {
try {
Some(gateway.find(userId))
} catch {
case _: NotFoundException => None
}
}
This works fine and seems clean to me, but doesn't seem to be really the "Scala way" to do it - I've been told to avoid using try/catch. So I've been looking for a way to rewrite it using a Try instead.
Here are 2 variants that (I think) behave exactly the same, using a Try.
Variant A:
private def findCustomer(userId: Long): Future[Option[Customer]] = future {
Try(
Some(gateway.find(userId))
).recover {
case _: NotFoundException => None
}.get
}
Variant B:
private def findCustomer(userId: Long): Future[Option[Customer]] = future {
Try(
Some(gateway.find(userId))
).recover {
case _: NotFoundException => None
}
} flatMap {
case Success(s) => Future.successful(s)
case Failure(f) => Future.failed(f)
}
I'm not a huge fan of A (even though it's more concise than B) since the .get seems a bit treacherous. B is definitely the most explicit but mapping the Try cases to corresponding Future outcomes seems boring.
How would an experienced Scala programmer write this?
I think your initial version using try/catch is perfectly fine, because it's a wrapper around an existing SDK.
Alternatively, you can use the recover method on Future:
def findCustomer(userId: Long): Future[Option[Customer]] =
Future(Option(gateway.find(userId))).recover {
case e: NotFoundException => None
}
One option is to avoid chaining Try and Future. In a sense, a future is an asynchronous Try.
You could use Future[Customer] directly and consider NotFoundException as something to recover from rather than a None value. Normally, you would chain operation on the future itself, with no need to deal with the failure cases (by mapping it, etc). It all depends on how you are going to use it (how your logic branches when the future is completed). In other words, maybe it seems convoluted to you because it is, and you are forcing it to be by forcing the Future[Option[Customer]] return type.
The ability to do chain multiple operations and carry on with computing if and only if all if going all right is the nice Scala feature here (especially on for comprehensions, etc).
You might be looking for:
Future.fromTry(myTry)
So for your example you might do:
Future.fromTry {
Try(Some(gateway.find(userId))).recover {
case _: NotFoundException => None
}
}
To only catch the NotFoundException, as in your solution. The only problem here is that the method won't be executed asynchronously. If that is necessary, consider using Future.recover, as suggested by Ionut.
Another idiomatic option worth mentioning here is to use Either[F, S], where S is the type of the successful return, and F can hold the errors (which you might want to propagate). So you could use Either[ Exception, Option[String]] or Either[String, Option[String]] where the first String is an error message.
If you're ready to adopt some scalaz. disjunction from scalaz is very useful and natural in handling erronous scenarios. It's like scala Either but scalaz disjunction \/ is right biased. you'll have success value on the right and exception on left. wrapping your code block with \/.fromTryCatch returns exception on the left side of disjunction. right will always have success value. mapping over disjunction is easiser than Scala Either since disjunction is right biased and easily gives you value from right.
import scalaz.\/
private def findCustomer(userId: Long): Future[\/[Throwable,Option[Customer]] ]= future {
\/.fromTryCatch {
Some(gateway.find(userId))
}
}
I have the following piece of code that I am trying to enhance:
I am using the java.nio.file package to represent a directory or a file as a Path.
So here goes:
import java.nio.file.{Paths,DirectoryStream,Files,
Path,DirectoryIteratorException}
val path: Path = Paths.get(directoryPath)
var directoryStream: Option[DirectoryStream[Path]] = None
// so far so good
try {
directoryStream = Some(Files.newDirectoryStream(pathO))
// this is where i get into trouble
def getMeDirStream: DirectoryStream[Path] =
if (!directoryStream.isEmpty && directoryStream.isDefined)
getMeDirStream.get
else
None
// invoke the iterator() method of dstream here
}
The above piece of code will not compile because I do not know what to return in the else, and right now, for the life of me, I can only come up with None, which the compiler simply does not like and I would like to learn what should be its replacement.
I want this example to be a learning lesson of Option and Some for me.
Okay, this is where I choke. I would like to check if the directoryStream is not empty and is defined, and then if this is the case, I would like to invoke getMeDirStream.get to invoke the iterator() method on it.
The API for Option tells me that invoking the get() method could result in a java.util.NoSuchElementException if the option is empty.
If the directoryStream is empty I want to return something and not None, because IntelliJ is telling me that "Expression of type None.type doesn't conform to expected type DirectoryStream[Path]".
Now, I am being all naive about this.
I would like to know the following:
What should I return in the else other than None?
Should I wrap the getMeDirStream.get in a try-catch with a java.util.NoSuchElementException, even though I am checking if the directoryStream is empty or not.?
What is the purpose of a try-catch in the getMeDirStream.get, if there is indeed such a need?
How can I clean up the above piece of code to incorporate correct checks for being isDefined and for catching appropriate exceptions?
Once I know what to return in the else (and after putting in the appropriate try-catch block if necessary), I would like to invoke the iterator() method on getMeDirStream to do some downstream operations.
Some and None are subtypes of Option, but to be more correct, they are actually two different cases of Option or data constructors. In other words, even though Scala allows you to directly invoke a Some or a None you should still regard their type to be Option. The more important thing to take from this is that you should never under any circumstance invoke Option#get as it is unsafe.
The intention of Option is to indicate the possibility that a value does not exist. If you care about the errors, then you should probably look at using Either instead (or Scalaz's Either called \/).
You can keep the computation within the Option context and then only extract the value later, or provide a default.
def fromTryCatch[A](a: => A): Either[Throwable, A] = try { Right(a) } catch { case e: Throwable => Left(e) }
val getMeDirStream: Option[java.util.Iterator[Path]] =
for {
path <- fromTryCatch(Paths.get(directoryPath)).toOption
directoryStream <- fromTryCatch(Files.newDirectoryStream(pathO)).toOption
} yield directoryStream.iterator()
Later, or right after, you can get the iterator, or provide a default value:
val iterator = getMeDirStream.getOrElse(java.util.Collections.emptyIterator[Path])
Your specific questions are difficult to address because it's unclear exactly what you're trying to achieve. In particular, when you ask what the purpose of the try block is... Well, you wrote it, so only you can answer that.
In general, you never call get on an Option. You either use pattern matching:
option match {
case Some(value) => /* ... */
case None => /* ... */
}
or you use methods like map, flatMap, and foreach (or the equivalent comprehension syntax that gpampara's code uses).
My revision of gpampara's answer:
import scala.collection.convert.wrapAll._
import scala.util.Try
import java.nio.file.{Paths, Files, Path}
val getMeDirStream: Option[Iterator[Path]] =
for {
path <- Try(Paths.get("")).toOption
directoryStream <- Try(Files.newDirectoryStream(path)).toOption
} yield directoryStream.iterator
Changes:
Using Try(...).toOption instead of Either
Using implicits in scala.collection.convert to return the result as a Scala Iterator.
Try is similar to Option. Instead of Some and None, it has Success and Failure subtypes, and the failure case includes a Throwable, whereas None is just a singleton with no additional information.
I wanted to create a scala compiler plugin that would prevents the call of some functions. For example System.exit.
The idea behind the scene is to let people write Scala scripts that would be interpreted on the fly. But I want to ensure that some dangerous actions are prohibited - the way it is done can definitely be discussed.
I started with the example in http://www.scala-lang.org/node/140 and started to replace the Apply section.
Doing some pattern matching I was able to extract a ClassSymbol for the right part of the compilation unit.
Then I wanted to do something like in the example that would be like:
classSymbol.tpe =:= global.typeOf[System]
unfortunately they don't match I get System.type on one side and System on the other.
Of course I could compare String values but I think that there is perhaps a nicer way to achieve this.
Has anyone any advice ?
Just in case a larger part of the code:
def apply(unit: global.CompilationUnit) {
for (global.Apply(fun, _) <- unit.body) {
fun.symbol match {
case method: global.MethodSymbol =>
val classSymbol = method.owner
println(classSymbol.fullName)
println(classSymbol.tpe =:= global.typeOf[System])
case _ => ()
}
}
Perhaps,
val jls = global.findMemberFromRoot(TermName("java.lang.System"))