Experimental control-flow operator (#368)

* AST and syntax for delimited control * stashing for later * a little more progress * progress on delimited continuations * delimit, suspend, and resume implemented (draft) * example that generates the natural numbers * fixes * tinkering * changed demo to experimental * comments and name changes * describe delimited continuations in the README * renamed Snapshot to Continuation, edits to comments * Update executable_semantics/ast/statement.h improve comment for MakeDelimitStmt Co-authored-by: Dave Abrahams <dabrahams@google.com> * Update executable_semantics/interpreter/interpreter.cpp remove snake_case Co-authored-by: Dave Abrahams <dabrahams@google.com> * edits to comments, change name of variable * updates to handle review edits * trailing whitespace * fixes to delimited continuations, added more tests, also fixed assignment to do a copy * improvements from Geoffrey * new test from Geoffrey, fix for empty blocks * more suggestions from Geoffrey * more tests for delimited continuations, renaming some of them * renamed test files * improve a comment * sketch of creating continuation * initial implementation of shift/reset style continuations * more documentation * fix some camel case * implemented deep copy of continuations, added a test case for it * fixed a bug and got the recursive test case working * removed __delimit, polished up __continuation * back to shallow copy for continuations * suggestions from Geoffrey * removed structured binding (for now) * Update executable_semantics/ast/expression.cpp Co-authored-by: Geoff Romer <gromer@google.com> * responses to Geoffrey Co-authored-by: Dave Abrahams <dabrahams@google.com> Co-authored-by: Geoff Romer <gromer@google.com>
2026-10-04 22:02:52 +01:00 · 2021-03-26 11:22:48 -04:00
parent 6df0d51516
commit 3fa72d2984
35 changed files with 674 additions and 31 deletions
@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test that assignment performs a copy and does not create an alias.
+
+fn main() -> Int {
+  var Int: x = -1;
+  {
+     var Int: y = 0;
+     x = y;
+     // y dies here
+  }
+  return x;
+}
@@ -0,0 +1 @@
+result: 0
@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test that assignment performs a copy and does not create an alias.
+
+fn main() -> Int {
+  var Int: x = 0;
+  var Int: y = x;
+  x = 1;
+  return y;
+}
@@ -0,0 +1 @@
+result: 0
@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  var Int: x = 0;
+  {
+    // empty block
+  }
+  return x;
+}
@@ -0,0 +1 @@
+result: 0
@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test that creating a continuation doesn't do anything.
+
+fn main() -> Int {
+  var Int: x = 0;
+  __continuation k {
+    x = x + 1;
+  }
+  return x;
+}
@@ -0,0 +1 @@
+result: 0
@@ -0,0 +1,14 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test creating and running a continuation.
+
+fn main() -> Int {
+  var Int: x = 0;
+  __continuation k {
+    x = x + 1;
+  }
+  __run k;
+  return x;
+}
@@ -0,0 +1 @@
+result: 1
@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test pausing a continuation with `__await` and restarting it with
+// `__run`.
+
+fn main() -> Int {
+  var Int: x = 0;
+  __continuation k {
+    x = x + 1;
+    __await;
+    x = x + 2;
+  }
+  __run k;
+  __run k;
+  return x;
+}
@@ -0,0 +1 @@
+result: 3
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Assignment for continuations is shallow, so `k2` refers to the same
+// continuation as `k1`.
+
+fn main() -> Int {
+  var Int: x = 0;
+  __continuation k1 {
+    x = x + 1;
+    __await;
+    x = x + 2;
+  }
+  var __Continuation: k2 = k1;
+  __run k1;
+  __run k2;
+  return x;
+}
@@ -0,0 +1 @@
+result: 3
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test access to block-scoped variables upon resuming a continuation.
+
+fn main() -> Int {
+  var Int: y = 0;
+  __continuation k {
+    var Int: x = 0;
+    x = x + 1;
+    __await;
+    x = x + 2;
+    y = x;
+  }
+  __run k;
+  __run k;
+  return y;
+}
@@ -0,0 +1 @@
+result: 3
@@ -0,0 +1,33 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test recursive functions inside continuations.
+
+var Int: current = 0;
+
+fn CountUpTo(Int: x) -> Int {
+  if (x == 0) {
+    current = 0;
+    __await;
+    return 0;
+  } else {
+    current = 1 + CountUpTo(x - 1);
+    __await;
+    return current;
+  }
+}
+
+fn main() -> Int {
+  __continuation k {
+    CountUpTo(5);
+  }
+  var Int: sum = 0;
+  var Int: count = 5;
+  while (not (count == 0)) {
+    __run k;
+    sum = sum + current;
+    count = count - 1;
+  }
+  return sum;
+}
@@ -0,0 +1 @@
+result: 10
@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test the way in which copying of continuations interacts with data
+// on the stack such as the variable `x`. In this example the copy
+// happens after the variable `x` is created.
+
+var Int: y = 0;
+
+fn main() -> Int {
+  __continuation k1 {
+    var Int: x = 0;
+    x = x + 1;
+    __await;
+    x = x + 2;
+    y = x;
+  }
+  __run k1;
+  var __Continuation: k2 = k1;
+  __run k2;
+  return y;
+}
@@ -0,0 +1 @@
+result: 3
@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test the way in which copying of continuations interacts with data
+// on the stack such as the variable `x`. In this example the copy
+// happens before the variable `x` is created, so each continuation
+// creates a different `x`.
+
+var Int: y = 0;
+
+fn main() -> Int {
+  __continuation k1 {
+    var Int: x = 0;
+    x = x + 1;
+    __await;
+    y = x;
+  }
+  var __Continuation: k2 = k1;
+  __run k1;
+  __run k2;
+  __run k1;
+  __run k2;
+  return y;
+}
@@ -0,0 +1 @@
+result: 1