Monoid

• Every monoid is a semiGroup but not the other way around. It has all characteristics of a semigroup + more
• Laws:
  • Left Identity append(a, 0) == a
  • Right Identity append(0, a) == a
• NotEmpty is not a monoid since it has no zero value.
• Permission systems should be implemented as monoidal systems.

trait Monoid[A] extends SemiGroup[A] {
  def zero: A // example: Zero value for a string is ""
  // Note that we have inherited append from SemiGroup
}

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Resources

1. Functional Scala by John A. De Goes (Toronto Edition)
2. Scalaz Monoid