CODEWITHSUNDEEP
listscalapattern-matchingscala-collections
Freewind
Freewind

Reputation: 198398

How to fix the pattern-matching exhaustive warning?

Some scala code:

val list = List(Some("aaa"), Some("bbb"), None, ...)

list.filter(_!=None).map {
  case Some(x) => x + "!!!"
  // I don't want to handle `None` case since they are not possible
  // case None
}

When I run it, the compiler complains:

<console>:9: warning: match may not be exhaustive.
It would fail on the following input: None
                  list.filter(_!=None).map {
                                   ^
res0: List[String] = List(aaa!!!, bbb!!!)

How to fix that warning without providing the case None line?

Upvotes: 6

Views: 1367

Answers (5)

Govind Singh
Govind Singh

Reputation: 15490

some other way to solve this issue, without filter and pattern matching

scala> list.flatten map (_ + "!!!")

or

scala> list.flatMap (_ map (_ + "!!!"))

Upvotes: 0

user3821614
user3821614

Reputation:

You can use get method instead of pattern matching.
Here is example code: 

scala> val list = List(Some("aaa"), Some("bbb"), None)
list: List[Option[String]] = List(Some(aaa), Some(bbb), None)
scala> list.filter(_ != None).map(_.get + "!!!")
res0: List[String] = List(aaa!!!, bbb!!!)

Upvotes: 0

Govind Singh
Govind Singh

Reputation: 15490

you can use flatten

scala> val list = List(Some("aaa"), Some("bbb"), None).flatten
list: List[String] = List(aaa, bbb)

scala> list.map {
     |   x => x + "!!!" 
     | }
res1: List[String] = List(aaa!!!, bbb!!!)

Upvotes: 3

ebo
ebo

Reputation: 2747

You could use the @unchecked annotation, although that requires some additional code:

list.filter(_!=None).map { x => ( x : @unchecked) match {
  case Some(x) => x + "!!!"
 }
}

Upvotes: 2

Eastsun
Eastsun

Reputation: 18869

If you are using map after filter, you may to use collect.

list collect { case Some(x) => x + "!!!" }

Upvotes: 12

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