Best Practices

Remarks#

Prefer vals, immutable objects, and methods without side effects. Reach for them first. Use vars, mutable objects, and methods with side effects when you have a specific need and justification for them.

— Programming in Scala, by Odersky, Spoon, and Venners

There are more example and guideline in this presentation by Odersky.

Keep it simple

Do not overcomplicate simple tasks. Most of the time you will need only:

algebraic datatypes
structural recursion
monad-like api (map, flatMap, fold)

There is plenty of complicated stuff in Scala, such as:

Cake pattern or Reader Monad for Dependency Injection.
Passing arbitrary values as implicit arguments.

These things are not clear for newcomers: avoid using them before you understand them. Using advanced concepts without a real need obfuscates the code, making it less maintainable.

Don’t pack too much in one expression.

Find meaningful names for computation units.
Use for comprehensions or map to combine computations together.

Let’s say you have something like this:

if (userAuthorized.nonEmtpy) {
  makeRequest().map {
    case Success(respone) =>
      someProcessing(..)
      if (resendToUser) {
        sendToUser(...)
      }
    ...
  }
}

If all your functions return Either or another Validation-like type, you can write:

for {
  user     <- authorizeUser
  response <- requestToThirdParty(user)
  _        <- someProcessing(...)
} {
  sendToUser
}

Prefer a Functional Style, Reasonably

By default:

Use val, not var, wherever possible. This allows you to take seamless advantage of a number of functional utilities, including work distribution.
Use recursion and comprehensionss, not loops.
Use immutable collections. This is a corrolary to using val whenever possible.
Focus on data transformations, CQRS-style logic, and not CRUD.

There are good reasons to choose non-functional style:

var can be used for local state (for example, inside an actor).
mutable gives better performance in certain situations.