sample code for Cats MTL ApplicativeHandle

applicativeHandle.sc

import cats.syntax.applicative._
import cats.Applicative
import cats.data.Validated
import cats.instances.string._
import cats.mtl.FunctorRaise
import cats.mtl.syntax.handle._
import cats.mtl.instances.handle._
import cats.syntax.traverse._
import cats.instances.list._

def parseNumber[F[_]: Applicative](in: String)(implicit F: FunctorRaise[F, String]): F[Double] =
  if (in.matches("-?[0-9]+")) in.toDouble.pure[F]
  else                        F.raise(in) // raise 構文

List("100", "abc", "200", "def") traverse parseNumber[Validated[String, ?]]
// Invalid(abcdef)

List("100", "abc", "200", "def") traverse { s =>
  parseNumber[Validated[String, ?]](s).handle[String](_ => Double.NaN) // handle 構文。
}
// Valid(List(100.0, NaN, 200.0, NaN))