abuse.scala

// improper use of a pattern match IMO
val negative = Constraint[Int] {
  case i if i < 0 => Valid
  _ => Invalid("Must be a negative number.")
}

// why not use if-else ?
val negative2 = Constraint[Int](i =>
  if (i < 0) Valid
  else       Invalid("Must be a negative number."))

// or in what ever style you prefer
val negative3 = Constraint[Int] { i =>
  if (i < 0)
    Valid
  else
    Invalid("Must be a negative number.")
}

I agree that the use of pattern matching isn't necessary. Although, it's possible that using a partial function gives other advantages in this example. Not sure TBH