Haskell風のApply

README.md

scalazのapply2は全部適用してF[C]を返すけど、1つだけ適用してF[B => C]返す方がいいのではないかと思った。

あと、scalazのap3 ~ ap8いらなくない？

*(A => B => C) => F[B => C] => F[C]だと(A => B => C) => F[C]より余計にオブジェクト生成される可能性があるかもしれない。

Apply.scala

import scala.language.higherKinds

trait Functor[F[_]]{
	def fmap[A, B](fa: F[A])(f: A => B): F[B]
}

trait Apply[F[_]] extends Functor[F]{
	def apply1[A, B](fa: F[A])(f: A => B): F[B] = fmap(fa)(f)
	def apply2[A, B, C](fa: F[A])(f: (A, B) => C): F[B => C] = apply1(fa)(a => (b => f(a, b)))
	def apply3[A, B, C, D](fa: F[A])(f: (A, B, C) => D): F[B => C => D] = apply1(fa)(a => b => c => f(a, b, c))
	def fapply[A, B](fa: F[A])(f: F[A => B]): F[B]
}

object Apply{
	 implicit val optionApply = new Apply[Option]{
		override def fmap[A, B](oa: Option[A])(f: A => B): Option[B] = oa.map(f)
		override def fapply[A, B](fa: Option[A])(f: Option[A => B]): Option[B] =
			for{
				a <- fa
				g <- f
			} yield g(a)
	 }

	implicit class ApplyOps[F[_], A](self: F[A])(implicit val af: Apply[F]){
		//Haskellの<$>と同じにしたかったけど、使えなかったから<@>にした
		def <@>[B](f: A => B): F[B] = af.apply1(self)(f)
		def <@>[B, C](f: (A, B) => C): F[B => C] = af.apply2(self)(f)
		def <@>[B, C, D](f: (A, B, C) => D): F[B => C => D] = af.apply3(self)(f)
		def <*>[B](f: F[A => B]): F[B] = af.fapply(self)(f)
	}
}

object Main{
	import Apply._

	def main(args: Array[String]): Unit = {
		val f: (Int, Int, Int) => Int = _+_+_
		val f2: Option[Int => Int => Int] = Option(1) <@> f
		val f3: Option[Int => Int] = Option(2) <*> f2
		val r: Option[Int] = Option(3) <*> f3

		val r2: Option[Int] = Option(3) <*> (Option(2) <*> (Option(1) <@> f))

		println(r) //Some(6)
		println(r2) //Some(6)
	}
}