scala-option | 易学教程

Why doesn't Option have a fold method?

阅读更多关于 Why doesn't Option have a fold method?

问题 I wonder why scala.Option doesn't have a method fold like this defined: fold(ifSome: A => B , ifNone: => B) equivalent to map(ifSome).getOrElse(ifNone) Is there no better than using map + getOrElse ? 回答1: You can do: opt foldLeft (els) ((x, y) => fun(x)) or (els /: opt) ((x,y) => fun(x)) (Both solutions will evaluate els by value, which might not be what you want. Thanks to Rex Kerr for pointing at it.) Edit: But what you really want is Scalaz’s catamorphism cata (basically a fold which not

Reading multiple variables from an object wrapped in Option[]

阅读更多关于 Reading multiple variables from an object wrapped in Option[]

问题 I have a variable obj: Option[MyObject] and want to extract multiple variables from it - if the object is not set, default values should be used. Currently I do it like this: val var1 = obj match { case Some(o) => e.var1 case _ => "default1" } val var2 = obj match { case Some(o) => e.var2 case _ => "default2" } ... which is extremely verbose. I know I could do it like this: val var1 = if (obj.isDefined) obj.get.var1 else "default1" val var2 = if (obj.isDefined) obj.get.var2 else "default2"

make play-json read the empty string as None for a type of Option[T]

阅读更多关于 make play-json read the empty string as None for a type of Option[T]

问题 I'm attempting to parse json from the GitHub API with play-json, and encountering a problem with the merge_commit_sha field on Pull Requests (incidentally, I know this field is deprecated, but don't want to discuss that in this parsing problem!). Unfortunately merge_commit_sha field comes back as the empty string in some cases: "merge_commit_sha": "" This is how the field is declared in my case class: merge_commit_sha: Option[ObjectId], I have an implicit Format[ObjectId], which does not

In Scala, is there a pre-existing library function for converting exceptions to Options?

阅读更多关于 In Scala, is there a pre-existing library function for converting exceptions to Options?

问题 This is basically to wrap java factory methods which throw exceptions if the item can't be created based on the inputs. I'm looking for something in the base library like: def exceptionToOption[A](f: => A):Option[A] ={ try{ Some(f)} catch{ case e:Exception => None} } Usage: val id:Option[UUID] = exceptionToOption(UUID.fromString("this will produce None")) I know I can write my own but I want to check I am not re-inventing the wheel. 回答1: Use scala.util.control.Exception: import scala.util

Coalescing options in Scala

阅读更多关于 Coalescing options in Scala

问题 Most SQL implementations (this question has nothing to do with SQL, it is just an example) offer the function COALESCE(x1,x2,...,xn) which returns x1 if it is not NULL , x2 otherwise only if x2 is not NULL neither and so on. If all xi values are NULL then the result is NULL . I wanted to get something like SQL's COALESCE in Scala for Option values being NULL replaced by None . I'll give you some examples: > coalesce(None,Some(3),Some(4)) res0: Some(3) > coalesce(Some(1),None,Some(3),Some(4))

How can I (best) convert an Option into a Try?

阅读更多关于 How can I (best) convert an Option into a Try?

问题 How can I (best) convert an Option returned by a method call into a Try (by preference, although an Either or a scalaz \/ or even a Validation might be OK) including specifying a Failure value if appropriate? For example, I have the following code, which feels kludgy, but does at least do (most of) the job: import scala.util._ case class ARef(value: String) case class BRef(value: String) case class A(ref: ARef, bRef: BRef) class MismatchException(msg: String) extends RuntimeException(msg)

Calling Java API from Scala with null argument

阅读更多关于 Calling Java API from Scala with null argument

问题 I have some Scala code that needs to call a Java API The Java API takes arguments that may be null. My Scala, of course, uses Option . For example, let's say I have a Java object constructor Foo(Integer) where the Integer may be null . I want to call it given a Scala bar: Option[Int] . I tried this import scala.collection.JavaConversions._ import scala.collection.JavaConverters._ val foo = Foo( bar.getOrElse(null) ) But got this compile error Error:(335, 44) type mismatch; found : Any

How to convert X => Option[R] to PartialFunction[X,R]

阅读更多关于 How to convert X => Option[R] to PartialFunction[X,R]

问题 As long as we have a PartialFunction[X,R] it's very easy to convert it to a function returning Option[R] , e.g. def pfToOptf[X, R](f: PartialFunction[X,R])(x: X) = if (f.isDefinedAt(x)) Some(f(x)) else None However, what if the task is opposite: suppose I have a function f getting X as an argument and returning Option[R] as a result. And I want to make a PartialFunction[X,R] out of it. What is the best way? What I've come up with looks pretty ugly to my taste: def optfToPf[X,R](f: X => Option

Is there a scala identity function?

阅读更多关于 Is there a scala identity function?

问题 If I have something like a List[Option[A]] and I want to convert this into a List[A] , the standard way is to use flatMap : scala> val l = List(Some("Hello"), None, Some("World")) l: List[Option[java.lang.String]] = List(Some(Hello), None, Some(World)) scala> l.flatMap( o => o) res0: List[java.lang.String] = List(Hello, World) Now o => o is just an identity function. I would have thought there'd be some way to do: l.flatMap(Identity) //return a List[String] However, I can't get this to work

scala return on first Some in list

阅读更多关于 scala return on first Some in list

I have a list l:List[T1] and currently im doing the following: myfun : T1 -> Option[T2] val x: Option[T2] = l.map{ myfun(l) }.flatten.find(_=>true) The myfun function returns None or Some, flatten throws away all the None's and find returns the first element of the list if any. This seems a bit hacky to me. Im thinking that there might exist some for-comprehension or similar that will do this a bit less wasteful or more clever. For example: I dont need any subsequent answers if myfun returns any Some during the map of the list l . How about: l.toStream flatMap (myfun andThen (_.toList))