I have a function in which I need to combine results from two Either objects.
I need the Right side of "request" if "handlingResult" is a Right, and it is a given fact that if "handlingResult" is a Right, "request" is also a Right.
If "handlingResult" is Left, I need its value to build a response.
Right now this is the implementation (both FailingResponse and SuccessfulResponse extend ValuationResponse):
def getResponse(handlingResult : Either[FailureReason, List[StockValuation]]
,request : Either[Error, ValuationRequest]
): ValuationResponse = {
handlingResult.fold(
failureReason =>
FailingResponse(failureReason.message
,failureReason.statusCode),
listOfValuations =>
SuccessfulResponse(listOfValuations
,request.right.get.symbol
,request.right.get.function
,StatusCodes.SUCCESS))
}
But I suspect that accessing an either directly is not a good practice, such as in
request.right.get.symbol
What would be a good way to achieve the same behavior but doing it in a recommendable way?
Either is right-biased in Scala 2.12 and up, so you can use a for-comprehension
def getResponse(handlingResult : Either[FailureReason, List[StockValuation]]
,request : Either[Error, ValuationRequest]
): ValuationResponse = {
val result = for {
result <- handlingResult
req <- request
} yield {
SuccessfulResponse(result, req.symbol, req.function, SUCCESS)
}
result match {
case Right(resp) => resp
case Left(FailureReason(msg, code)) => FailingResponse(msg, code)
case Left(Error) => FailingResponse("failed for unknown reasons", SOME_NEW_CODE)
}
}
Note that although you don't expect the last case statement to ever match, it should be there for completeness, and a new code, SOME_NEW_CODE can be made to indicate that something unexpected occurred.
Related
I have two functions: one returns a Future[Thing Or Exception] and another that returns Future[Boolean. I want a function that calls both and returns Future[Thing Or Exception]. If the boolean function returns false I want to return an exception, else the return of the other function.
I have code like this but a) I hate the cast and b) when run on the "boolean gets true" path I get this error when I eventually Await.result on the return in my test code: "Promise$DefaultPromise cannot be cast to org.scalatic.Or".
def thingFuture: Future[Thing Or Exception]
def boolFuture: Future[Boolean]
def combineFutures: Future[Thing Or Exception] = {
val result = boolFuture.map {x =>
x match {
case true => thingFuture
case false => Exception
}
}
// without the cast compiler says result is of type Future[Object]
result.asInstanceOf[Future[Thing Or Exception]]
}
I've also tried this but it gets the same Promise error on the success path
def combineFutures: Future[Thing Or Exception] = {
val result = boolFuture.map {x =>
x match {
case true => thingFuture.map { y =>
y match {
case Good(thing) => thing
case Bad(exception) => exception
}
case false => Exception
}
}
}
Can anyone tell me how to compose two futures with different return types? Thanks!
Every future can be completed with failed state in case exception has occurred, so you can simply return thingFuture in the "happy path" and throw an exception in case boolean is false. This will return a Future.failed with the underlying exception.
val result = boolFuture.flatMap {x =>
x match {
case true => thingFuture
case false => throw new Exception("whatever")
}
}
Note the flatMap instead of map. Because we map the underlying value of one future into a yet another future, by using simple map we would wind up with Future[Future[Thing]].
Also note that instead of throwing an exception, you could also return a Future.failed(throw new Exception("whatever")) and the result would be the same - in both case you get a failed future.
EDIT: I just realized Or comes from scalactic, which I never used, but the philosophy remains the same. You need to flatMap your boolean future and your ThingOrException future in order to wind up with Future[ThingOrException]. If you ever find yourself in a situation where you need to flatMap a Future, but one of the case clauses returns an ordinary value (e.g. in case of true return Future[Thing], in case of false return just Exception) then you can wrap the ordinary value into a future. This way all branches return a future and flatMap will work correctly. For example:
val someOtherFuture = Future(43)
val someOrdinaryValue = 44
Future(someInteger).flatMap {
case 42 => someOtherFuture
case _ => Future(someOrdinaryValue)
}
In order to simplify things for the runtime machinery a bit, you can also write Future.successful(someOrdinaryValue) in which case no background computation is started.
As far as I can tell from Scalatic documentation, you can get an instance of Right Or Left by either Good(Right) or Bad(Left).
That means the composition can potentially look like this:
boolFuture.flatMap(b => if (b) thingFuture else Future.successful(Bad(new Exception())))
The types should unify to Future[Or[Thing, Exception]]
Say I have three database access functions foo, bar, and baz that can each return Option[A] where A is some model class, and the calls depend on each other.
I would like to call the functions sequentially and in each case, return an appropriate error message if the value is not found (None).
My current code looks like this:
Input is a URL: /x/:xID/y/:yID/z/:zID
foo(xID) match {
case None => Left(s"$xID is not a valid id")
case Some(x) =>
bar(yID) match {
case None => Left(s"$yID is not a valid id")
case Some(y) =>
baz(zID) match {
case None => Left(s"$zID is not a valid id")
case Some(z) => Right(process(x, y, z))
}
}
}
As can be seen, the code is badly nested.
If instead, I use a for comprehension, I cannot give specific error messages, because I do not know which step failed:
(for {
x <- foo(xID)
y <- bar(yID)
z <- baz(zID)
} yield {
Right(process(x, y, z))
}).getOrElse(Left("One of the IDs was invalid, but we do not know which one"))
If I use map and getOrElse, I end up with code almost as nested as the first example.
Is these some better way to structure this to avoid the nesting while allowing specific error messages?
You can get your for loop working by using right projections.
def ckErr[A](id: String, f: String => Option[A]) = (f(id) match {
case None => Left(s"$id is not a valid id")
case Some(a) => Right(a)
}).right
for {
x <- ckErr(xID, foo)
y <- ckErr(yID, bar)
z <- ckErr(zID, baz)
} yield process(x,y,z)
This is still a little clumsy, but it has the advantage of being part of the standard library.
Exceptions are another way to go, but they slow things down a lot if the failure cases are common. I'd only use that if failure was truly exceptional.
It's also possible to use non-local returns, but it's kind of awkward for this particular setup. I think right projections of Either are the way to go. If you really like working this way but dislike putting .right all over the place, there are various places you can find a "right-biased Either" which will act like the right projection by default (e.g. ScalaUtils, Scalaz, etc.).
Instead of using an Option I would instead use a Try. That way you have the Monadic composition that you'd like mixed with the ability to retain the error.
def myDBAccess(..args..) =
thingThatDoesStuff(args) match{
case Some(x) => Success(x)
case None => Failure(new IdError(args))
}
I'm assuming in the above that you don't actually control the functions and can't refactor them to give you a non-Option. If you did, then simply substitute Try.
I know this question was answered some time back, but I wanted to give an alternative to the accepted answer.
Given that, in your example, the three Options are independent, you can treat them as Applicative Functors and use ValidatedNel from Cats to simplify and aggregate the handling of the unhappy path.
Given the code:
import cats.data.Validated.{invalidNel, valid}
def checkOption[B, T](t : Option[T])(ifNone : => B) : ValidatedNel[B, T] = t match {
case None => invalidNel(ifNone)
case Some(x) => valid(x)
def processUnwrappedData(a : Int, b : String, c : Boolean) : String = ???
val o1 : Option[Int] = ???
val o2 : Option[String] = ???
val o3 : Option[Boolean] = ???
You can then replicate obtain what you want with:
//import cats.syntax.cartesian._
(
checkOption(o1)(s"First option is not None") |#|
checkOption(o2)(s"Second option is not None") |#|
checkOption(o3)(s"Third option is not None")
) map (processUnwrappedData)
This approach will allow you to aggregate failures, which was not possible in your solution (as using for-comprehensions enforces sequential evaluation). More examples and documentation can be found here and here.
Finally this solution uses Cats Validated but could easily be translated to Scalaz Validation
I came up with this solution (based on #Rex's solution and his comments):
def ifTrue[A](boolean: Boolean)(isFalse: => A): RightProjection[A, Unit.type] =
Either.cond(boolean, Unit, isFalse).right
def none[A](option: Option[_])(isSome: => A): RightProjection[A, Unit.type] =
Either.cond(option.isEmpty, Unit, isSome).right
def some[A, B](option: Option[A])(ifNone: => B): RightProjection[B, A] =
option.toRight(ifNone).right
They do the following:
ifTrue is used when a function returns a Boolean, with true being the "success" case (e.g.: isAllowed(userId)). It actually returns Unit so should be used as _ <- ifTrue(...) { error } in a for comprehension.
none is used when a function returns an Option with None being the "success" case (e.g.: findUser(email) for creating accounts with unique email addresses). It actually returns Unit so should be used as _ <- none(...) { error } in a for comprehension.
some is used when a function returns an Option with Some() being the "success" case (e.g.: findUser(userId) for a GET /users/userId). It returns the contents of the Some: user <- some(findUser(userId)) { s"user $userId not found" }.
They are used in a for comprehension:
for {
x <- some(foo(xID)) { s"$xID is not a valid id" }
y <- some(bar(yID)) { s"$yID is not a valid id" }
z <- some(baz(zID)) { s"$zID is not a valid id" }
} yield {
process(x, y, z)
}
This returns an Either[String, X] where the String is an error message and the X is the result of calling process.
What is the most idiomatic and concise way to perform the following: One future that may or may not execute depending or a result of a previous future execution, like so:
def getItFromHere : Future[Option[Something]] = ...
def getItFromThere : Future[Option[Something]] = ...
def getIt : Future[Option[Something]] = {
for {
maybeSomething <- getItFromHere
probablySomething <- maybeSomething.getOrElse(getItFromThere) // Obviously can't be done!!!
}
yield probablySomething
}
A specific use-case example:
Get an item from cache and only if it wasn't found in cache, get it from the database.
I'm taking an assumption here that getItFromHere and getItFromThere will not fail with an exception. They will either return Some[Something] or None. Feel free to take this assumption into account, or giving a better without it.
Note: I understand the internal mechanics of for-comprehension which is actually translated to map/flatMap/filter internally.
You could fail the future if no item is found in the cache and then recover it with the retrieval.
// assuming getItFromHere() fails if no item is found
getItFromHere() recoverWith { case _ => getItFromThere() }
Or with the Option you could do it like this:
getItFromHere() flatMap {
case Some(x) => Future.successful(Some(x))
case None => getItFromThere()
}
Actually your example is almost there. You only need to match the Future type expected by the monadic composition:
for {
fut <- someFuture
something <- fut.map(x=>someFuture).getOrElse(someBetterFuture)
} yield something
For the sake of simplicity, here someFuture is a val. If you use a def, avoid re-calculating the operation by packaging your result back into a future. Folding that into the question's code:
for {
maybeSomething <- getItFromHere
probablySomething <- maybeSomething.map(x=> Future.succcessful(maybeSomething)).getOrElse(getItFromThere)
} yield probablySomething
if you use the OptionT transformer from scalaz:
def getItFromHere : OptionT[Future,Something] = OptionT(...)
def getItFromThere : OptionT[Future,Something] = OptionT(...)
def getIt : Future[Option[Something]] = (getItFromHere orElse getItFromThere).run
I'm developing a method that is supposed to persist an object, if it passes a list of conditions.
If any (or many) condition fail (or any other kind of error appears), a list with the errors should be returned, if everything goes well, a saved entity should be returned.
I was thinking about something like this (it's pseudocode, of course):
request.body.asJson.map { json =>
json.asOpt[Wine].map { wine =>
wine.save.map { wine =>
Ok(toJson(wine.update).toString)
}.getOrElse { errors => BadRequest(toJson(errors))}
}.getOrElse { BadRequest(toJson(Error("Invalid Wine entity")))}
}.getOrElse { BadRequest(toJson(Error("Expecting JSON data")))}
That is, I'd like to treat it like an Option[T], that if any validation fails, instead of returning None it gives me the list of errors...
The idea is to return an array of JSON errors...
So the question would be, is this the right way to handle these kind of situation? And what would be the way to accomplish it in Scala?
--
Oops, just posted the question and discovered Either
http://www.scala-lang.org/api/current/scala/Either.html
Anyway, I'd like to know what you think about the chosen approach, and if there's any other better alternative to handle it.
Using scalaz you have Validation[E, A], which is like Either[E, A] but has the property that if E is a semigroup (meaning things that can be concatenated, like lists) than multiple validated results can be combined in a way that keeps all the errors that occured.
Using Scala 2.10-M6 and Scalaz 7.0.0-M2 for example, where Scalaz has a custom Either[L, R] named \/[L, R] which is right-biased by default:
import scalaz._, Scalaz._
implicit class EitherPimp[E, A](val e: E \/ A) extends AnyVal {
def vnel: ValidationNEL[E, A] = e.validation.toValidationNEL
}
def parseInt(userInput: String): Throwable \/ Int = ???
def fetchTemperature: Throwable \/ Int = ???
def fetchTweets(count: Int): Throwable \/ List[String] = ???
val res = (fetchTemperature.vnel |#| fetchTweets(5).vnel) { case (temp, tweets) =>
s"In $temp degrees people tweet ${tweets.size}"
}
Here result is a Validation[NonEmptyList[Throwable], String], either containing all the errors occured (temp sensor error and/or twitter error or none) or the successful message. You can then switch back to \/ for convenience.
Note: The difference between Either and Validation is mainly that with Validation you can accumulate errors, but cannot flatMap to lose the accumulated errors, while with Either you can't (easily) accumulate but can flatMap (or in a for-comprehension) and possibly lose all but the first error message.
About error hierarchies
I think this might be of interest for you. Regardless of using scalaz/Either/\//Validation, I experienced that getting started was easy but going forward needs some additional work. The problem is, how do you collect errors from multiple erring functions in a meaningful way? Sure, you can just use Throwable or List[String] everywhere and have an easy time, but doesn't sound too much usable or interpretable. Imagine getting a list of errors like "child age missing" :: "IO error reading file" :: "division by zero".
So my choice is to create error hierarchies (using ADT-s), just like as one would wrap checked exceptions of Java into hierarchies. For example:
object errors {
object gamestart {
sealed trait Error
case class ResourceError(e: errors.resource.Error) extends Error
case class WordSourceError(e: errors.wordsource.Error) extends Error
}
object resource {
case class Error(e: GdxRuntimeException)
}
object wordsource {
case class Error(e: /*Ugly*/ Any)
}
}
Then when using result of erring functions with different error types, I join them under a relevant parent error type.
for {
wordSource <-
errors.gamestart.WordSourceError <-:
errors.wordsource.Error <-:
wordSourceCreator.doCreateWordSource(mtRandom).catchLeft.unsafePerformIO.toEither
resources <-
errors.gamestart.ResourceError <-:
GameViewResources(layout)
} yield ...
Here f <-: e maps the function f on the left of e: \/ since \/ is a Bifunctor. For se: scala.Either you might have se.left.map(f).
This may be further improved by providing shapeless HListIsos to be able to draw nice error trees.
Revisions
Updated: (e: \/).vnel lifts the failure side into a NonEmptyList so if we have a failure we have at least one error (was: or none).
If you have Option values, and you want to turn them into success/failure values, you can turn an Option into an Either using the toLeft or toRight method.
Usually a Right represents success, so use o.toRight("error message") to turn Some(value) into Right(value) and None into Left("error message").
Unfortunately Scala doesn't recognise this right-bias by default, so you have to jump through a hoop (by calling the .right method) in order to neatly compose your Eithers in a for-comprehension.
def requestBodyAsJson: Option[String] = Some("""{"foo":"bar"}""")
def jsonToWine(json: String): Option[Wine] = sys.error("TODO")
val wineOrError: Either[String, Wine] = for {
body <- requestBodyAsJson.toRight("Expecting JSON Data").right
wine <- jsonToWine(body).toRight("Invalid Wine entity").right
} yield wine
If you need an empty value, instead of using Either[A,Option[B]] you can use lift Box, which can have three values:
Full (there is a valid result)
Empty (no result, but no error either)
Failure (an error happened)
Box are more flexible than Either thanks to a rich API. Of course, although they were created for Lift, you can use them in any other framework.
well, this is my attemp using Either
def save() = CORSAction { request =>
request.body.asJson.map { json =>
json.asOpt[Wine].map { wine =>
wine.save.fold(
errors => JsonBadRequest(errors),
wine => Ok(toJson(wine).toString)
)
}.getOrElse (JsonBadRequest("Invalid Wine entity"))
}.getOrElse (JsonBadRequest("Expecting JSON data"))
}
And wine.save is like the following:
def save(wine: Wine): Either[List[Error],Wine] = {
val errors = validate(wine)
if (errors.length > 0) {
Left(errors)
} else {
DB.withConnection { implicit connection =>
val newId = SQL("""
insert into wine (
name, year, grapes, country, region, description, picture
) values (
{name}, {year}, {grapes}, {country}, {region}, {description}, {picture}
)"""
).on(
'name -> wine.name, 'year -> wine.year, 'grapes -> wine.grapes,
'country -> wine.country, 'region -> wine.region, 'description -> wine.description,
'picture -> wine.picture
).executeInsert()
val newWine = for {
id <- newId;
wine <- findById(id)
} yield wine
newWine.map { wine =>
Right(wine)
}.getOrElse {
Left(List(ValidationError("Could not create wine")))
}
}
}
}
Validate checks several preconditions. I still have to add a try/catch to catch any db error
I'm still looking for a way to improve the whole thing, it feels much to verbose to my taste...
Option monad is a great expressive way to deal with something-or-nothing things in Scala. But what if one needs to log a message when "nothing" occurs? According to the Scala API documentation,
The Either type is often used as an
alternative to scala.Option where Left
represents failure (by convention) and
Right is akin to Some.
However, I had no luck to find best practices using Either or good real-world examples involving Either for processing failures. Finally I've come up with the following code for my own project:
def logs: Array[String] = {
def props: Option[Map[String, Any]] = configAdmin.map{ ca =>
val config = ca.getConfiguration(PID, null)
config.properties getOrElse immutable.Map.empty
}
def checkType(any: Any): Option[Array[String]] = any match {
case a: Array[String] => Some(a)
case _ => None
}
def lookup: Either[(Symbol, String), Array[String]] =
for {val properties <- props.toRight('warning -> "ConfigurationAdmin service not bound").right
val logsParam <- properties.get("logs").toRight('debug -> "'logs' not defined in the configuration").right
val array <- checkType(logsParam).toRight('warning -> "unknown type of 'logs' confguration parameter").right}
yield array
lookup.fold(failure => { failure match {
case ('warning, msg) => log(LogService.WARNING, msg)
case ('debug, msg) => log(LogService.DEBUG, msg)
case _ =>
}; new Array[String](0) }, success => success)
}
(Please note this is a snippet from a real project, so it will not compile on its own)
I'd be grateful to know how you are using Either in your code and/or better ideas on refactoring the above code.
Either is used to return one of possible two meaningful results, unlike Option which is used to return a single meaningful result or nothing.
An easy to understand example is given below (circulated on the Scala mailing list a while back):
def throwableToLeft[T](block: => T): Either[java.lang.Throwable, T] =
try {
Right(block)
} catch {
case ex => Left(ex)
}
As the function name implies, if the execution of "block" is successful, it will return "Right(<result>)". Otherwise, if a Throwable is thrown, it will return "Left(<throwable>)". Use pattern matching to process the result:
var s = "hello"
throwableToLeft { s.toUpperCase } match {
case Right(s) => println(s)
case Left(e) => e.printStackTrace
}
// prints "HELLO"
s = null
throwableToLeft { s.toUpperCase } match {
case Right(s) => println(s)
case Left(e) => e.printStackTrace
}
// prints NullPointerException stack trace
Hope that helps.
Scalaz library has something alike Either named Validation. It is more idiomatic than Either for use as "get either a valid result or a failure".
Validation also allows to accumulate errors.
Edit: "alike" Either is complettly false, because Validation is an applicative functor, and scalaz Either, named \/ (pronounced "disjonction" or "either"), is a monad.
The fact that Validation can accumalate errors is because of that nature. On the other hand, / has a "stop early" nature, stopping at the first -\/ (read it "left", or "error") it encounters. There is a perfect explanation here: http://typelevel.org/blog/2014/02/21/error-handling.html
See: http://scalaz.googlecode.com/svn/continuous/latest/browse.sxr/scalaz/example/ExampleValidation.scala.html
As requested by the comment, copy/paste of the above link (some lines removed):
// Extracting success or failure values
val s: Validation[String, Int] = 1.success
val f: Validation[String, Int] = "error".fail
// It is recommended to use fold rather than pattern matching:
val result: String = s.fold(e => "got error: " + e, s => "got success: " + s.toString)
s match {
case Success(a) => "success"
case Failure(e) => "fail"
}
// Validation is a Monad, and can be used in for comprehensions.
val k1 = for {
i <- s
j <- s
} yield i + j
k1.toOption assert_≟ Some(2)
// The first failing sub-computation fails the entire computation.
val k2 = for {
i <- f
j <- f
} yield i + j
k2.fail.toOption assert_≟ Some("error")
// Validation is also an Applicative Functor, if the type of the error side of the validation is a Semigroup.
// A number of computations are tried. If the all success, a function can combine them into a Success. If any
// of them fails, the individual errors are accumulated.
// Use the NonEmptyList semigroup to accumulate errors using the Validation Applicative Functor.
val k4 = (fNel <**> fNel){ _ + _ }
k4.fail.toOption assert_≟ some(nel1("error", "error"))
The snippet you posted seems very contrived. You use Either in a situation where:
It's not enough to just know the data isn't available.
You need to return one of two distinct types.
Turning an exception into a Left is, indeed, a common use case. Over try/catch, it has the advantage of keeping the code together, which makes sense if the exception is an expected result. The most common way of handling Either is pattern matching:
result match {
case Right(res) => ...
case Left(res) => ...
}
Another interesting way of handling Either is when it appears in a collection. When doing a map over a collection, throwing an exception might not be viable, and you may want to return some information other than "not possible". Using an Either enables you to do that without overburdening the algorithm:
val list = (
library
\\ "books"
map (book =>
if (book \ "author" isEmpty)
Left(book)
else
Right((book \ "author" toList) map (_ text))
)
)
Here we get a list of all authors in the library, plus a list of books without an author. So we can then further process it accordingly:
val authorCount = (
(Map[String,Int]() /: (list filter (_ isRight) map (_.right.get)))
((map, author) => map + (author -> (map.getOrElse(author, 0) + 1)))
toList
)
val problemBooks = list flatMap (_.left.toSeq) // thanks to Azarov for this variation
So, basic Either usage goes like that. It's not a particularly useful class, but if it were you'd have seen it before. On the other hand, it's not useless either.
Cats has a nice way to create an Either from exception-throwing code:
val either: Either[NumberFormatException, Int] =
Either.catchOnly[NumberFormatException]("abc".toInt)
// either: Either[NumberFormatException,Int] = Left(java.lang.NumberFormatException: For input string: "abc")
in https://typelevel.org/cats/datatypes/either.html#working-with-exception-y-code