Iterate a list of functions A => Option[B] returning the first Some

FirstSome.scala

object play {
  import scalaz._
  import Scalaz._

  def foo(i: Int) =  if (i > 0) Some(i) else None
 
  def boo(i: Int) =  if (i <= 0) Some(0) else None

  val l = foo _ :: boo _ :: Nil
 
  l.foldMap(_.map(_.first))  apply  -1
  //> res0: scalaz.@@[Option[Int],scalaz.Tags.First] = Some(0)
  
}

May I attempt explaining what's happening? :-) (Might also be useful for those who see the gist in future.)

If you have a Monoid[A] instance you can combine two values of type As and get a value of type A.

Turns out there are three ways to combine Options. See lines 114-123 here - https://github.com/scalaz/scalaz/blob/master/core/src/main/scala/scalaz/Semigroup.scala#L114.

We want the second one - FirstOption. Hence the necessity of conversions.

Now, as Chris Marshall says, "monoids beget monoids". You can trivially derive Monoid[Option[A]] instance from a Monoid[A] instance, and you can trivially derive a Monoid[B => A] instance from a Monoid[A] instance. Combining these two, you can trivially derive a Monoid[B => Option[A] instance from a Monoid[A] instance. Which is what happens here.

You combine all the values in your list using the monoidal combination, and get a function which behaves as expected i.e. picks the result of first successful computation.

Nicely done. Purely to demonstrate that a new function is being produced (using the derived monoid as explained by Rahul), I might assign the result of the foldMap to a value and then apply it with normal parens syntax.