What is the most 'scala-ic' way to capture a value (possibly one that is not idempotent) for logging and returning the same value.
I can think of 'return' statement the only way to do it, but apparently using 'return' should be avoided in scala .
Use case:
def myfunc(argument) : ReturnType{
val response:ReturnType = dependency()
// dependency() is not idemptotent
// so calling more than once will have side-effects
logger.debug(response.member1 , response.member2)
return response
}
Is there a way to achieve this without using a 'return' keyword.
I am a newbie to scala so some (or most) of what I said could be wrong, and would be happy to be corrected.
Just reifying #Shadowlands answer.
def myfunc(argument: ArgType): ReturnType {
val response = dependency()
logger.debug(response.member1, response.member2)
response
}
Related
I'm new rather new to Scala so I think this might be a very small problem.
I'm currently trying to change the method chat from using the deprecated WebSocket.async to WebSocket.tryAccept. The application uses the sample chat found at PlayFramework websocket-chat
I'm having trouble creating the complex Future type that the method requires.
This is the old method:
def chat() = WebSocket.async[JsValue] {
request =>
ChatRoom.join("User: 1")
}
New method:
def chat2() = WebSocket.tryAccept[JsValue] {
request =>
try {
// ChatRoom.join returns (iteratee,enumerator)
ChatRoom.join("User: 1").map(e => Right(e))
} catch {
case e: Exception =>
Left(Ok("Failed")) // Error here
}
}
My error message:
found : Left[Result,Nothing]
required: Future[Either[Result,(Iteratee[JsValue, _], Enumerator[JsValue])]]
I have no idea how I am supposed to create such a complex result for such a simple message.
Although ChatRoom.join("User: 1").map(e => Right(e)) doesn't show any errors now, I'm unsure if this is the correct implementation.
I'm not in front of an IDE at the moment, so I can't answer fully, but the return type it's asking for isn't as complex as it seems. An "Either" is a "Left" or a "Right" in the same way that an "Option" is a "Some" or a "None". So what it's asking for is a Future (which Websocket.async should also have required) that contains either a Left[Result] -- the fail-to-connect case, or a Right[(Iteratee, Enumerator)] -- the success case. Assuming that Chatroom.join returns a Future[(Iteratee, Enumerator)], the map operation is simply wrapping that in a "Right". The first thing I'd try is wrapping Left(Ok("Failed")) in a Future and see what happens.
In the Circumflex framework, you can map an URL to a block like this:
get("/foo") = {
"hello, world!"
}
which, when browsing to /foo, will show the given string as expected. Now, to write a complete web application, you almost always need some form of authentication and authorisation. I'm trying to write some kind of wrapper for the above construct, so I can write this:
get("/foo") = requireLogin {
"hello, world!"
}
The requireLogin method would then check if the user is logged in, and if yes, execute the given block. If not, however, it should do a redirect to the login page.
Now I somehow can't get the syntax right (i'm still a Scala newbie). How would you do this in a generic fashion?
Try something like this:
def executeMaybe[A](work: => A): Option[A] =
if (util.Random.nextBoolean)
Some(work)
else
None
This executes the passed code with probability 0.5, returning Some(<result delivered by work>), or returns None is the other cases. You can call it either like this:
val v = executeMaybe(42)
or with block notation:
val v = executeMaybe {
// do some work
// provide return value
}
The trick is to use a by-name parameter, signalled by the => symbol. Read more e.g. here: http://daily-scala.blogspot.com/2009/12/by-name-parameter-to-function.html
The way I asked it, Jean-Philippe's answer is correct.
But here's some information specific to Circumflex:
In the Circumflex RequestRouter, the following can be used to implement the required method:
def requireLogin (f: => RouteResponse ): RouteResponse = {
if(loggedIn) {
return f
}
else {
return sendRedirect("/login")
}
}
The reason behind this was getting clear with the hint from Jean-Philippe's answer, and once I remembered that the following call isn't an assignment of a block to some internal data, but is mapped to another method call instead.
So, the call
get("/") = {...}
is actually mapped to this:
get.update("/", {...})
The block is passed in as a By-Name parameter, so the return value of requireLogin must be the same - which, for Circumflex, is RouteResponse, and not a function.
You also can use j2ee container authentication with <login-config> and <security-constraint> stuff inside web.xml
In my scala code:
QueueManager.add(getObject)
where getObject is a method that returns an object of type QueueObject.
def getObject : QueuObject = {
val response = //some response
return response
}
Is there a way I can check for the response being null, while adding the QueueObject? I know I can do this:
if (getObject != null)
QueueManager.add(getObject)
But I do not wish to add a level of indentation. Is there an operator that does that inline?
Thanks.
Try to avoid using null in Scala. It's really there only for interoperability with Java. In Scala, use Option for things that might be empty. If you're calling a Java API method that might return null, wrap it in an Option immediately.
def getObject : Option[QueueObject] = {
// Wrap the Java result in an Option (this will become a Some or a None)
Option(someJavaObject.getResponse)
}
Note: You don't need to put it in a val or use an explicit
return statement in Scala; the result will be the value of
the last expression in the block (in fact, since there's only one statement, you don't even need a block).
def getObject : Option[QueueObject] = Option(someJavaObject.getResponse)
Besides what the others have already shown (for example calling foreach on the Option, which might be slightly confusing), you could also call map on it (and ignore the result of the map operation if you don't need it):
getObject map QueueManager.add
This will do nothing if the Option is a None, and call QueueManager.add if it is a Some.
I find using a regular if however clearer and simpler than using any of these "tricks" just to avoid an indentation level. You could also just write it on one line:
if (getObject.isDefined) QueueManager.add(getObject.get)
or, if you want to deal with null instead of using Option:
if (getObject != null) QueueManager.add(getObject)
edit - Ben is right, be careful to not call getObject more than once if it has side-effects; better write it like this:
val result = getObject
if (result.isDefined) QueueManager.add(result.get)
or:
val result = getObject
if (result != null) QueueManager.add(result)
Option(getObject) foreach (QueueManager add)
If it instead returned Option[QueueObject] you could use a construct like getObject.foreach { QueueManager.add }. You can wrap it right inline with Option(getObject).foreach ... because Option[QueueObject](null) is None.
Although I'm sure #Ben Jackson's asnwer with Option(getObject).foreach is the preferred way of doing it, I like to use an AnyRef pimp that allows me to write:
getObject ifNotNull ( QueueManager.add(_) )
I find it reads better.
And, in a more general way, I sometimes write
val returnVal = getObject ifNotNull { obj =>
returnSomethingFrom(obj)
} otherwise {
returnSomethingElse
}
... replacing ifNotNull with ifSome if I'm dealing with an Option. I find it clearer than first wrapping in an option and then pattern-matching it.
(For the implementation, see Implementing ifTrue, ifFalse, ifSome, ifNone, etc. in Scala to avoid if(...) and simple pattern matching and the Otherwise0/Otherwise1 classes.)
I'm trying to write a class for a scala project and I get this error in multiple places with keywords such as class, def, while.
It happens in places like this:
var continue = true
while (continue) {
[..]
}
And I'm sure the error is not there since when I isolate that code in another class it doesn't give me any error.
Could you please give me a rule of thumb for such errors? Where should I find them? are there some common syntactic errors elsewhere when this happens?
It sounds like you're using reserved keywords as variable names. "Continue", for instance, is a Java keyword.
You probably don't have parentheses or braces matched somewhere, and the compiler can't tell until it hits a structure that looks like the one you showed.
The other possibility is that Scala sometimes has trouble distinguishing between the end of a statement with a new one on the next line, and a multi-line statement. In that case, just drop the ; at the end of the first line and see if the compiler's happy. (This doesn't seem like it fits your case, as Scala should be able to tell that nothing should come after true, and that you're done assigning a variable.)
Can you let us know what this code is inside? Scala expects "expressions" i.e. things that resolve to a particular value/type. In the case of "var continue = true", this does not evaluate to a value, so it cannot be at the end of an expression (i.e. inside an if-expression or match-expression or function block).
i.e.
def foo() = {
var continue = true
while (continue) {
[..]
}
}
This is a problem, as the function block is an expression and needs to have an (ignored?) return value, i.e.
def foo() = {
var continue = true
while (continue) {
[..]
}
()
}
() => a value representing the "Unit" type.
I get this error when I forget to put an = sign after a function definition:
def function(val: String):Boolean {
// Some stuff
}
Solved. IntelliJ didn't highlight the fact that my imports were incomplete.
Hi,
I have a simple Scala program that I'm trying to develop using jMock. Setting basic expectations works nicely but for some reason Scala does not understand my attempt to return a value from a mock object. My maven build spews out the following error
TestLocalCollector.scala:45: error: not found: value returnValue
one (nodeCtx).getParameter("FilenameRegex"); will( returnValue(regex))
^
And the respective code snippets are
#Before def setUp() : Unit = { nodeCtx = context.mock(classOf[NodeContext]) }
...
// the value to be returned
val regex = ".*\\.data"
...
// setting the expectations
one (nodeCtx).getParameter("FilenameRegex"); will( returnValue(regex))
To me it sounds that Scala is expecting that the static jMock method returnValue would be a val? What am I missing here?
Are you sure about the ';'?
one (nodeCtx).getParameter("FilenameRegex") will( returnValue(regex))
might work better.
In this example you see a line like:
expect {
one(blogger).todayPosts will returnValue(List(Post("...")))
}
with the following comment:
Specify what the return value should be in the same expression by defining "will" as Scala infix operator.
In the Java equivalent we would have to make a separate method call (which our favorite IDE may insist on putting on the next line!)
one(blogger).todayPosts; will(returnValue(List(Post("..."))))
^
|
-- semicolon only in the *Java* version
The OP explains it himself:
the returnValue static method was not visible, thus the errors.
And the will method just records an action on the latest mock operation, that's why it can be on the next line or after the semicolon :)
import org.jmock.Expectations
import org.jmock.Expectations._
...
context.checking(
new Expectations {
{ oneOf (nodeCtx).getParameter("FilenameRegex") will( returnValue(".*\\.data") ) }
}
)