Mastering Functional Programming

The Type Class pattern

Sometimes, the effect type that we are going to use is not known in advance. Consider the problem of logging. Logging can be performed to a list or a file. Logging to a list can be implemented using the Writer effect type.

A Writer is an abstraction of a pair of a result and a log generated by a computation. In its simplest form, a Writer can be understood as a pair of a list of strings and an arbitrary result. We can define the Writer effect type as follows:

case class SimpleWriter[A](log: List[String], value: A) {  def flatMap[B](f: A => SimpleWriter[B]): SimpleWriter[B] = {    val wb: SimpleWriter[B] = f(value)    SimpleWriter(log ++ wb.log, wb.value)  }  def map[B](f: A => B): SimpleWriter[B] = SimpleWriter(log, f(value) ...