Add attemptOption to Alternative#4862
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Implements the standard `optional` parser-combinator (Haskell's `Control.Applicative.optional`): lift a possibly-empty F[A] into an always-non-empty F[Option[A]] by combining `map(_.some)` with `pure(None)` via `combineK`. Added to the `Alternative` trait with a default implementation and to the simulacrum-style `Ops` trait so it is reachable via `import cats.syntax.alternative._`. Closes typelevel#2936. Signed-off-by: Onyeka Obi <softwareengineerasaservant@isurvivable.cv>
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@MavenRain , thank you for the PR! I've got a few comments:
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satorg
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May 16, 2026
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| test("attemptOption") { | ||
| assert(Alternative[Option].attemptOption(Option(5)) === Some(Some(5))) | ||
| assert(Alternative[Option].attemptOption(Option.empty[Int]) === Some(None)) | ||
| assert(Alternative[List].attemptOption(List(1, 2, 3)) === List(Some(1), Some(2), Some(3), None)) | ||
| assert(Alternative[List].attemptOption(List.empty[Int]) === List(None)) | ||
| } |
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This test doesn't seem providing any value because the following two property based tests should cover all these cases already.
johnynek
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May 16, 2026
| * }}} | ||
| */ | ||
| def attemptOption[A](fa: F[A]): F[Option[A]] = | ||
| combineK(map(fa)((a: A) => Some(a): Option[A]), pure(Option.empty[A])) |
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nit: I'd rather add:
private val fempty: F[Option[Nothing]] = pure(Option.empty[Nothing])
def attemptOption[A](fa: F[A]): F[Option[A]] =
combineK(map(fa)((a: A) => Some(a): Option[A]), widen[Option[A]](fempty))Since widen by default is a cast (and generally 0 cost), this prevents allocating the fempty on every call.
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Closes #2936.
Adds
attemptOption[A](fa: F[A]): F[Option[A]]toAlternative,implementing the standard
optionalparser-combinator (Haskell'sControl.Applicative.optional): tryfa, surface its result asSome, and combine withpure(None)so the result always succeeds atleast once.