Add Handlers inspired by Effekt and corresponding tests

Author: kyay10

library/src/commonMain/kotlin/effekt/handler.kt

@@ -0,0 +1,32 @@
+package effekt
+
+import Prompt
+import abortS0
+import reset
+import shift0
+import kotlin.jvm.JvmInline
+
+public interface Handler<R, E> {
+  public fun prompt(): ObscurePrompt<E>
+  public suspend fun unit(value: R): E
+  public suspend fun <A> use(body: suspend (Cont<A, E>) -> E): A = prompt().prompt.shift0(body)
+  public fun <A> useAbort(body: suspend () -> E): A = prompt().prompt.abortS0(body)
+}
+
+public suspend fun <R, E, H : Handler<R, E>> handle(
+  handler: ((() -> ObscurePrompt<E>) -> H), body: suspend H.() -> R
+): E {
+  val prompt = ObscurePrompt(Prompt<E>())
+  val handler = handler { prompt }
+  return handler.handle(body)
+}
+
+public suspend fun <R, E, H : Handler<R, E>> H.handle(body: suspend H.() -> R): E {
+  return prompt().prompt.reset {
+    val res = body()
+    prompt().prompt.abortS0 { unit(res) }
+  }
+}
+
+@JvmInline
+public value class ObscurePrompt<E> internal constructor(internal val prompt: Prompt<E>)

library/src/commonMain/kotlin/effekt/state.kt

@@ -0,0 +1,38 @@
+package effekt
+
+import get
+import newReset
+import pushState
+import set
+import shift0
+
+public interface StateScope {
+  public suspend fun <T> field(init: T): Field<T>
+  public interface Field<T> {
+    public suspend fun get(): T
+    public suspend fun set(value: T)
+  }
+}
+
+public suspend inline fun <T> StateScope.Field<T>.update(f: (T) -> T) {
+  set(f(get()))
+}
+
+public suspend fun <R> region(body: suspend StateScope.() -> R): R = newReset {
+  body(StateScopeImpl(this))
+}
+
+private class StateScopeImpl<R>(val prompt: Prompt<R>) : StateScope {
+  override suspend fun <T> field(init: T): StateScope.Field<T> = FieldImpl<T>().apply {
+    prompt.shift0 {
+      pushState(init) {
+        it(Unit)
+      }
+    }
+  }
+
+  private class FieldImpl<T> : StateScope.Field<T>, State<T> {
+    override suspend fun get(): T = (this as State<T>).get()
+    override suspend fun set(value: T) = (this as State<T>).set(value)
+  }
+}

library/src/commonMain/kotlin/handle.kt

@@ -356,6 +356,22 @@ class ContinuationTest {
       }
     }
   }
+
+  @Test
+  fun ex4dot2dot1() = runTest {
+    topReset<Int> {
+      shift0 { k -> k(k(100)) } + 10
+    } + 1 shouldBe 121
+  }
+
+  @Test
+  fun ex4dot2dot2() = runTest {
+    topReset<Int> p1@{
+      if(newReset p2@{
+        shift<Boolean, Int> { 21 }
+      }) 1 else 2
+    } * 2 shouldBe 42
+  }
 }
 
 val stackSafeIterations: Int = when (platform) {