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FreeMonoid
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object FreeMonoidComponent { | |
import MonoidComponent._ | |
// FreeMonoidの定義 | |
type FreeMonoid[A] = List[A] | |
object FreeMonoid { | |
def apply[A](xs: A*): FreeMonoid[A] = xs.toList | |
val empty = Nil | |
} | |
// インスタンス定義 | |
implicit def monoidFreeMonoidInstance[A] = new Monoid[FreeMonoid[A]] { | |
override def <>(l1: FreeMonoid[A], l2: FreeMonoid[A]): FreeMonoid[A] = l1 ++ l2 | |
override val mempty = FreeMonoid.empty | |
} | |
// インタプリタの定義 | |
def interpretMonoid[A, B: Monoid](f: (A => B), a: FreeMonoid[A]): B = (f, a) match { | |
case (f, FreeMonoid.empty) => implicitly[Monoid[B]].mempty | |
case (f, a :: as) => <>(f(a), interpretMonoid(f, as)) | |
} | |
} | |
object FreeMonoidTest { | |
import FreeMonoidComponent._ | |
sealed trait BankOp | |
case class Deposit(d: Int) extends BankOp | |
case class Withdraw(w: Int) extends BankOp | |
def interpretOp(b: BankOp): Int = b match { | |
case Deposit(d) => d | |
case Withdraw(w) => -w | |
} | |
def interpret(bs: FreeMonoid[BankOp]): Int = interpretMonoid(interpretOp, bs) | |
def main(args: Array[String]) = { | |
val program = FreeMonoid(Deposit(10), Withdraw(5), Withdraw(2), Deposit(3)) | |
// FreeMagma[String]で、String => Intな関数fを適用しながらMagma[Int]を利用したのと同じ原理。 | |
// BankOpはMonoidではないが(=Monoidのインスタンスを具体的に定義していないが)、 | |
// FreeMonoid[BankOp]の中身を取り出しながら、BankOp => Intな関数fを適用していくと | |
// Monoid[Int]を利用することが出来る。 | |
// BankOpでもMonoidっぽいことができるようになった! | |
val result = interpret(program) | |
println(result) // 6 | |
} | |
} |
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