found Unit: required [Something Else]

Question

My code is:

   var storage: Storage =
    if (config.storageType == StorageType.MAP)
      MapStorage()
    else if (config.storageType == StorageType.LIST)
      ListStorage()
    else ()

The error I am receiving is

[E007] Type Mismatch Error
Found:    Unit                                                                                                                                                     
Required: ****.utils.Storage

*** is just placed to omit the actual value. I know that there is no stand-alone if statement in scala and hence the last else() statement is present in the code.

Dmytro Mitin · Accepted Answer · 2022-12-16T13:40:06.230

You can't return () having type Unit if you promised to return Storage (storage: Storage).

Either return a Storage

val storage: Storage =
  if (config.storageType == StorageType.MAP)
    MapStorage()
  else if (config.storageType == StorageType.LIST)
    ListStorage()
  else MapStorage() // or ListStorage() or some other storage

or make one of the cases default

val storage: Storage =
  if (config.storageType == StorageType.MAP)
    MapStorage()
  else // default case
    ListStorage()

or if you really can't return any of storages in the default case, return null (it's worse since this can lead to NullPointerException in the code for unknown reason)

val storage: Storage =
  if (config.storageType == StorageType.MAP)
    MapStorage()
  else if (config.storageType == StorageType.LIST)
    ListStorage()
  else null 

// null has type `Null`, which is a subtype of any reference type, e.g. Storage
// Null <: Storage
// Null | Storage =:= Storage

or throw exception with a proper message (it's better than returning null)

val storage: Storage =
  if (config.storageType == StorageType.MAP)
    MapStorage()
  else if (config.storageType == StorageType.LIST)
    ListStorage()
  else throw new RuntimeException("unexpected storage type")

// throwing exception has type Nothing, which is a subtype of any type, e.g. Storage
// Nothing <: Null <: Storage
// Nothing | Storage =:= Storage

or change signature to Option (it's even better than throwing but you'll probably have to fix also places where storage is used)

val storage: Option[Storage] =
  if (config.storageType == StorageType.MAP)
    Some(MapStorage())
  else if (config.storageType == StorageType.LIST)
    Some(ListStorage())
  else None

Technically, you can return union type Unit | Storage in Scala 3 (or its emulation in Scala 2) but such signature would be weird.

found Unit: required [Something Else]

1 Answers1