如何在Scala中返回None

Question

For my first Scala program, I am trying to write a small utility which will convert an XML file from one schema to another.

I started writing a method which will give me the file contents:

  def loadFile(filename: String, encoding: String = "utf-8"):Option[String] = {
    try
    {
      val source = scala.io.Source.fromFile(filename, encoding)
      val contents = source.mkString
      source.close()
      return Some(contents)
    }
    catch 
    {
      return None
    }

  }

But it does not compile. I get back "value apply is not a member of Nothing" and "value isDefinedAt is not a member of Nothing" as error messages from the line return None .

All the examples I can find returning an Option use matching, but this does not make sense here. I just want to not fail if I can't read the file for some reason.

What should I do in this case? Is there a pattern for doing this kind of thing in Scala?

Answer 1

In this specific case (exception handling), I'd suggest using a Try instead.

def loadFile(filename: String, encoding: String = "utf-8"):Option[String] = {
    Try {
      val source = scala.io.Source.fromFile(filename, encoding)
      val contents = source.mkString
      source.close()
      return Some(contents)
    }.toOption
}

I would, however, recommend against catching the exception at all. You're swallowing the cause for the error by returning None : is it a FileNotFoundException ? a standard IOException ? was there a message with the error ( Unsupported encoding comes to mind…)?

My rule of thumb is to let the caller deal with exceptions. If he doesn't care about the error itself, handling things is as easy as:

Try {loadFile("test.txt")}.toOption

Better yet, since Try has all the required methods, it can be used in a for-comprehension in a rather neat way:

for(s <- Try {loadFile("test.txt")};
    i <- Try {s.toInt}) yield i

This will result in either a Success[Int] or a Failure that contains an exception describing exactly what went wrong.

Answer 2

There is all about "catch" .

In scala it's supposed to be like this to make it compile:

  def loadFile(filename: String, encoding: String = "utf-8"):Option[String] = {
    try {
      val source = scala.io.Source.fromFile(filename, encoding)
      val contents = source.mkString
      source.close()
      Some(contents)
    } catch {
      case x: IOException =>  None
      case x => errorHandler(x) // if any other exception
    }
  }

  def errorHandler(e: Any) = None // put some logic here..

So use:

catch { case: x:ExceptionType ={ .. handling .. }}

In Scala catch is a function that accepts another function as argument. So having what you have would complain about apply function. case provides that function (PartialFunction) that catch wants. (in nutshell)

Note: All exceptions are unchecked in Scala even IOException .

Answer 3

Try this:

 def loadFile(filename: String, encoding: String = "utf-8"):Option[String] = {
try
{
  val source = scala.io.Source.fromFile(filename, encoding)
  val contents = source.mkString
  source.close()
  return Some(contents)
}
catch
  {
    case e:Exception=>{
      return None
    }

  }

}