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Add Applicative for Arrow #2078

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merged 4 commits into from
Dec 10, 2017
Merged

Add Applicative for Arrow #2078

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559d561
Add Apply for Arrow
stephen-lazaro Dec 7, 2017
55ac269
Strengthen to Applicative, move to Applicative
stephen-lazaro Dec 8, 2017
5d91b01
More tests, explicit wiring for laws check
stephen-lazaro Dec 8, 2017
8cfc67e
Drop doctests
stephen-lazaro Dec 8, 2017
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22 changes: 22 additions & 0 deletions core/src/main/scala/cats/arrow/Arrow.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -48,3 +48,25 @@ import simulacrum.typeclass
def split[A, B, C, D](f: F[A, B], g: F[C, D]): F[(A, C), (B, D)] =
andThen(first(f), second(g))
}
object Arrow {
/**
* Creates a semigroupal functor for `F`, holding domain fixed and combining
* over the codomain.
*
* Example:
* {{{
* scala> import cats.implicits._
* scala> import cats.arrow.Arrow.catsSemigroupalForArrow
* scala> val toLong: Int => Long = _.toLong
* scala> val double: Int => Int = 2*_
* scala> val f: Int => (Long, Int) = catsSemigroupalForArrow.product(toLong, double)
* scala> f(3)
* res0: (Long, Int) = (3,6)
* }}}
*/
implicit def catsSemigroupalForArrow[F[_, _], A](implicit F: Arrow[F]): Semigroupal[F[A, ?]] = new Apply[F[A, ?]] {
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it's probably worthwhile to override product.

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Good catch, will do.

def map[B, C](fb: F[A, B])(f: B => C): F[A, C] = F.rmap(fb)(f)
def ap[B, C](ff: F[A, B => C])(fb: F[A, B]): F[A, C] =
F.rmap(F.andThen(F.lift((x: A) => (x, x)), F.split(ff, fb)))(tup => tup._1(tup._2))
}
}