CODEWITHSUNDEEP
scala
axiopisty
axiopisty

Reputation: 5146

error: type mismatch; found: Unit.type required: Unit

Code snippets are from the Scala REPL.

This is perfectly legal Scala even though it is not very useful:

scala> class Test {
  private val xs = scala.collection.mutable.ArrayBuffer[Int]()
  private val func = Some((x: Int)=>{
  })
  private var opt: Option[(Int)=>Unit] = None
  opt = func
}
defined class Test

However, if I try to do something inside func the code will not compile:

scala> class Test {
  private val xs = scala.collection.mutable.ArrayBuffer[Int]()
  private val func = Some((x: Int)=>{
    xs += x
  })
  private var opt: Option[(Int)=>Unit] = None
  opt = func
}
<console>:13: error: type mismatch;
 found   : Some[Int => Test.this.xs.type]
 required: Option[Int => Unit]
         opt = func
               ^

Instead it produces the type mismatch error and lists the types found and required. While this error message is useful, I don't know how to fix it. My next attempt was this:

scala> class Test {
  private val xs = scala.collection.mutable.ArrayBuffer[Int]()
  private val func = Some((x: Int)=>{
    xs += x
    Unit
  })
  private var opt: Option[(Int)=>Unit] = None
  opt = func
}
<console>:14: error: type mismatch;
 found   : Some[Int => Unit.type]
 required: Option[Int => Unit]
         opt = func
               ^

I understand that there is a type mismatch occurring, but I don't understand why there is a type mismatch or how to fix it in such a way that I can do something useful inside func.

Why is there a type mismatch and how can it be fixed?

Upvotes: 0

Views: 3248

Answers (1)

Eugene Zhulenev
Eugene Zhulenev

Reputation: 9734

Function inside options uses mutable array buffer and returns it as a result, but you need Unit, you can do it this way:

private val func = Some((x: Int)=> {xs += x; ()})

or use append which return type is Unit

private val func = Some((x: Int) => xs.append(x))

Upvotes: 3

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