never-throw-an-exception.md

Never throw an exception

Do not throw exceptions in your code, avoid using a function that throws when you can. If not possible catch the exception an return a relevant result.

In Scala, exceptions are not documented and very hard to track (just like null), so you can't be sure everything is correctly handled and given a big enough program, you can be sure something is NOT correctly handled.

Exceptions, like every side-effect, are breaking referential transparency which makes understanding code really hard and refactoring very fragile.

Instead of throwing exceptions, use a specific type to represent the function semantics:

• Option[A] when something can be present or not
• Either[E, A] when possible errors are limited
• Try[A] when you don't know what may fail

Common throwing functions and how to replace them:

• Seq[A].head => Seq[A].headOption
• Seq[A].tail => Seq[A].drop(1)
• Seq[A].last => Seq[A].lastOption
• Seq[A].reduce => Seq[A].reduceOption or Seq[A].fold
• Option[A].get => Option[A].getOrElse
• Try[A].get => Try[A].getOrElse

Other references

This rule is also present in other best practice guides:

• Nicolas Rinaudo best practices
• Alexandru Nedelcu best practices