Scala Right.left用法及代碼示例

將此 Either 投影為 Left.

這允許 for-comprehensions 在 Either 實例的左側，顛倒 Either 的通常 right-bias。

例如

for (s <- Left("flower").left) yield s.length // Left(6)

繼續與 scala.Option 類比，LeftProjection 聲明 Left 在某些代碼中應該類似於 Some。

// using Option
def interactWithDB(x: Query): Option[Result] =
  try Some(getResultFromDatabase(x))
  catch {
    case _: SQLException => None
  }

// this will only be executed if interactWithDB returns a Some
val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
report match {
  case Some(r) => send(r)
  case None    => log("report not generated, not sure why...")
}

// using Either
def interactWithDB(x: Query): Either[Exception, Result] =
  try Right(getResultFromDatabase(x))
  catch {
    case e: SQLException => Left(e)
  }

 // run a report only if interactWithDB returns a Right
 val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
 report match {
   case Right(r) => send(r)
   case Left(e)  => log(s"report not generated, reason was $e")
 }
 // only report errors
 for (e <- interactWithDB(someQuery).left) log(s"query failed, reason was $e")