use-types-to-define-constraints.md

Use types to define constraints

As types are checked by to compiler which guarantee them to be correct. It's a good idea to encode our constraints in types so we can be sure that if it compiles, it works.

In Scala we already have some types doing that:

• Option represents possibly empty values and replace null
• Try represents a result of computation that may fail
• Future represents an async result that may fail

But we can use other interesting ones such as NonEmptyList (see cats implementation for example).

And we can create custom ones as needed:

import scala.util.{Try, Success, Failure}

class Length(val value: Int) extends AnyVal

object Length {
  def apply[A](in: Seq[A]): Length = new Length(in.length)
  
  def from(in: Int): Try[Length] =
    if(in >= 0) Success(new Length(in))
    else Failure(new Exception(s"Unable to build a Length with a negative Int ($in)"))
}

This implementation guarantees that if you take a Length parameter, its value will be positive, not need to check for it anymore \o/