Validation example

validation.scala

import scalaz._
import Scalaz._

import shapeless._
import shapeless.record._
import shapeless.ops.record._
import shapeless.syntax.singleton._

object console extends App {

  case class FieldError(message: String)

  trait Validable extends Product {
    implicit val v:Validable = this
    implicit class RichValidationNel(vn: ValidationNel[FieldError, Validable]) {
      def and(r: ValidationNel[FieldError, Validable]) = (vn |@| r)((a, _) => a)
      def +(r: ValidationNel[FieldError, Validable]) = and(r)
    }

    def validate: ValidationNel[FieldError, Validable]
  }

  case class Constrain[T](errorMessage: String)(f: T => Boolean) {
    def apply(fieldName: String, fieldValue: T)(implicit obj: Validable): ValidationNel[FieldError, Validable] =
      if (f(fieldValue)) {
        obj.successNel[FieldError]
      } else {
        FieldError(s"field: $fieldName $errorMessage").failureNel[Validable]
      }
  }

  object Constrain {
    def stringLength(length: Int): Constrain[String] =
      Constrain[String](s"should not be longer than `$length`")(_.length < length)

    def intBetween(min: Int, max: Int): Constrain[Int] =
      Constrain[Int](s"should be between min:`$min` and max:`$max`")(f => min < f && f < max)
  }

  case class Foo(i: Int, s: String) extends Validable {
    def validate =
      Constrain.stringLength(0)("s", s) and
      Constrain.intBetween(1, 5)("i", i)
  }
}