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Scala’s simple strategy to reduce debugging costs: “Some” and “None”

Null Pointer Exceptions (NPEs) are most common runtime errors in Java (more than 500.000 results in Google). Most Java libraries are interspersed with methods returning null if the computation could not be finished. Invoking method calls on null raises Null Pointer Exceptions.

The best practice in Java to avoid such errors is a test against null.

if(aObject == null) {...

or pass the null test to another function.

For example, Instead of writing:

if(aResultString.equals(“literal”)) {
...

you write:

if(“literal”.equals(aResultString)) {
...

More than eleven years ago (beginning with the java era) a data type existed, avoiding the NPE problem:

data  Maybe a =  Nothing | Just a deriving (Eq, Ord, Read, Show)

A Haskell one-liner ported to Java could have prevented thousands of errors.

Why?

You cannot forget to test against None (unlike the null check) because this would raise a compile time error. Based on Haskells Maybe-Type, Scala defines the Some and None Type with:

final case class Some[+A](x: A) extends Option[A]
...

and

case object None extends Option[Nothing]
...

To test an Option Type against None you write:

val result =  a getOrElse b..

If a is Some(_) than result is _. If result is None, than result is b. The value of result is therefore always defined!

Runtime Errors on my News Page Kungle.de so far

To Keep it short:
None

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This entry was posted on Sunday, August 9th, 2009 at 8:31 pm and is filed under Kungle News, Lift, Scala, english. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

2 Responses to “Scala’s simple strategy to reduce debugging costs: “Some” and “None””

  1. James Iry Says:
    August 10th, 2009 at 2:45 pm

    If you’re curious, Scala’s “Option, Some, None” comes from the ML family which uses “option, SOME, NONE” (http://www.standardml.org/Basis/option.html). Haskell is actually the odd one out by using Maybe, Just, Nothing.

  2. Andrew Binstock Says:
    August 11th, 2009 at 8:46 pm

    There are multiple interesting ways to attack this problem. Scala’s is elegant. Groovy handles this in part with an interesting solution: the ?. operator for dereferencing. It performs the dereference only if the element is not null.

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