챕터 7 문제 풀이 6번까지 추가

homework/7장/한윤석.kt

@@ -0,0 +1,56 @@
+// ex1
+fun <T> map2(a: Par<T>, b: Par<T>, block: (lx: T, rx: T) -> T): Par<T> =
+    Par(block(a.get, b.get))
+
+// ex2
+// ex3
+fun <A, B, C> map2(
+    a: Par<A>,
+    b: Par<B>,
+    f: (A, B) -> C
+): Par<C> = { es: ExecutorService ->
+    val af: Future<A> = a(es)
+    val bf: Future<B> = b(es)
+    TimedMap2Future(af, bf, f)
+}
+
+data class TimedMap2Future<A, B, C>(
+  val pa: Future<A>,
+  val pb: Future<B>,
+  val f: (A, B) -> C
+): Future<C> {
+  override fun get(to: Long, tu: Timeunit): C {
+    val timeoutMills = TimeUnit.MILLISECONDS.convert(to, tu)
+
+    val start = System.currentTimeMillis()
+    val a = pa.get(to, tu)
+    val duration = System.currentTimeMillis() - start
+
+    val remainder = timeoutMills - duration
+    val b = pb.get(remainder, TimeUnit.MILLISECONDS)
+    return f(a, b)
+  }
+}
+
+// ex4
+fun <A. B> asyncF(f: (A) -> B): (A) -> Par<B> = { a: A -> lazyUnit(f(a)) }
+
+// ex5
+fun <A> sequence(ps: List<Par<A>>): Par<List<A>> = when {
+    ps.isEmpty() -> unit(emptyList())
+    ps.size == 1 -> map(ps.head()) { listOf(it) }
+    else -> {
+        val (l, r) = ps.splitAt(ps.size / 2)
+        map2(sequence(l), sequence(r)) { lx, rx -> lx + rx }
+    }
+}
+
+// ex6 
+fun <A> parFilter(list: List<A>, f: (A) -> Boolean): Par<List<A>> {
+  val pars: List<Par<List<A>>> = list.map(lazyUnit { it })
+  return map(sequence(pars)) {
+    it.flatMap { if (f(it)) listOf(it) else emptyList() }
+  }
+}
+
+// ex7