Scala Applicatives?

Question

In Scala, how can I append an Option[A] to Option[A]:

Meaning:

Some("foo") ??? Some("bar") == Some("foobar")

In Haskell I'd use an applicative:

ghci>import Control.Applicative
ghci> (++) <$> (Just "foo") <*> (Just "bar")
Just "foobar"

Are there Applicatives in Scala's standard Typesafe library?

Note that in this specific case if you're using Scalaz the semigroup instance for `Option[A: Semigroup]` is a little clearer—you just write `a |+| b`. — Travis Brown, Sep 19 '14 at 14:23

score 3 · Accepted Answer · answered Sep 19 '14 at 02:19

3

With scalaz you can do it this way:

  import scalaz.syntax.applicative._
  import scalaz.std.option._

  val l: Option[String] = Some("foo")
  val r: Option[String] = Some("bar")
  val result = ^(l, r)(_ + _)
  println(result) // Some("foobar")

answered Sep 19 '14 at 02:19

Eugene Zhulenev

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score 2 · Answer 2 · answered Sep 19 '14 at 05:55

2

Applicatives aren't in the standard library, but you can use for-comprehensions like monads in Haskell:

for {
  l <- Some("foo")
  r <- Some("bar")
} yield (l + r)

(or rewrite it with flatMap, of course). Otherwise, go with scalaz, as in Eugene's answer.

answered Sep 19 '14 at 05:55

Alexey Romanov

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score 0 · Answer 3 · answered Sep 19 '14 at 08:25

If you just want to achieve this particular syntax, you can combine an implicit class with a monad for-comprehension:

implicit class RichOption(a: Option[String]) {
  def ???(b: Option[String]) = for (sa <- a; sb <- b) yield sa+sb
}

scala> Some("foo") ??? Some("bar") == Some("foobar")
res4: Boolean = true

Scala Applicatives?

3 Answers3