September 2018
Intermediate to advanced
398 pages
9h 43m
English
Content preview from Scala Programming Projects
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Introducing Validated
The cats.data.Validated[E, A]type is very similar to Either[E, A]. It is an ADT that represents a value of either an Invalid type or a Valid type. A simplified definition would be the following:
sealed trait Validated[+E, +A]case class Valid[+A](a: A) extends Validated[Nothing, A]case class Invalid[+E](e: E) extends Validated[E, Nothing]
We will see what the + sign in front of a type parameter means in the section on covariance and contravariance in Chapter 4, Advanced Features. For now, though, do not worry about it.
Similarly to the definition of Option, the definitions use contravariance and Nothing. This way, Valid[A] is a subtype of Validated[E, A] for any E; and Invalid[E] is a subtype of Validated[E, A] for any ...
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