Created
October 18, 2012 12:27
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Writer exercise
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import scalaz._, Scalaz._ | |
case class Writer[W: Monoid, +A](run: (A, W)) { | |
def map[B](f: A ⇒ B): Writer[W, B] = | |
Writer { | |
val (a, w) = run | |
(f(a), w) | |
} | |
def flatMap[B](f: A ⇒ Writer[W, B]): Writer[W, B] = | |
Writer { | |
val (a, w1) = run | |
val (b, w2) = f(a).run | |
(b, w1 |+| w2) | |
} | |
} | |
object Writer { | |
def point[W: Monoid, A](a: A): Writer[W, A] = Writer((a, Monoid[W].zero)) | |
def sequence[W: Monoid, A](t: List[Writer[W, A]]): Writer[W, List[A]] = | |
t.foldLeft(point[W, List[A]](Nil: List[A])) { (acc, v) ⇒ | |
for { | |
x ← v | |
xs ← acc | |
} yield x::xs | |
} | |
def filterM[W: Monoid, A](p: A ⇒ Writer[W, Boolean], a: List[A]): Writer[W, List[A]] = | |
a match { | |
case x::xs ⇒ for { | |
flg ← p(x) | |
ys ← filterM(p, xs) | |
} yield if(flg) x::ys else ys | |
case Nil ⇒ point(Nil) | |
} | |
def tell[W: Monoid](w: W): Writer[W, Unit] = Writer(((), w)) | |
} |
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