Notes from dailylife.

Notes.md

Variance:

• Covariant ( return type )
• Contravariant ( argument type )

Covariant type on contravariant position:

case class Box[+A](value: A) {
  def set(a: A): Box[A] = Box(a)
}

Error:(4, 11) covariant type A occurs in contravariant position in type A of value a
  def set(a: A): Box[A] = Box(a)

Why this should not compile ?

val catBox = new Box[Cat]
val animalBox: Box[Animal] = catBox

val dog = new Dog
animalBox.set(dog) // runtime-error: a box of cats contains a dog!

Benefits of using FT

Principle of least power (you know what your code can and cannot do)

Constraints on FT

def foo[F[_]: Sync] = F.delay(db.read)
// Sync appears everywhere
def bar[F[_]: Sync] = foo.flatMap ...

vs

trait Db[F[_]] {
  def read[A]: F[A]
}
object Db {
  def apply[F[_]](implicit Db: Db[F]): Db[F] = Db
  def create[F[_]: Sync] = new Db[F] { ... }
}

// Sync only appears close to the top level
def bar[F[_]: Db: Monad] = Db[F].read.flatMap ...

String as Seq

scala.String is just an alias for java.lang.String. There's implicit def wrapString(s: String): WrappedString at scala.Predef object that is automatically imported. WrappedString allows treating String as an "efficient" IndexedSeq (subtype of Seq)