feat: implement symmetric coroutines with fiber.call and fiber.run distinction

Add new `fiber.call` method that remembers the calling fiber for symmetric transfer, while `fiber.run` now performs tail-call style switching without preserving the caller. Introduce re-entrancy guards preventing a fiber from being called multiple times, update all test fixtures to use the new API, and add comprehensive test coverage for call semantics including direct/indirect re-entrancy, value passing, and finished fiber errors.

test/fiber/call.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before") // expect: before
+fiber.call         // expect: fiber
+IO.print("after")  // expect: after

test/fiber/call_direct_reenter.wren

@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.call // expect runtime error: Fiber has already been called.
+}
+
+fiber.call

test/fiber/call_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.call // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call

test/fiber/call_return_implicit_null.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+var result = fiber.call // expect: fiber
+IO.print(result)        // expect: null

test/fiber/call_return_value.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+  return "result"
+}
+
+var result = fiber.call // expect: fiber
+IO.print(result)        // expect: result

test/fiber/call_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("call")
+}
+
+fiber.call // expect: call
+fiber.call // expect runtime error: Cannot call a finished fiber.

test/fiber/call_with_value.wren

@@ -0,0 +1,9 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")    // expect: before
+fiber.call("ignored") // expect: fiber
+IO.print("after")     // expect: after

test/fiber/call_with_value_direct_reenter.wren

@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.call(2) // expect runtime error: Fiber has already been called.
+}
+
+fiber.call(1)

test/fiber/call_with_value_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.call(3) // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call(2)
+}
+
+b.call(1)

test/fiber/call_with_value_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("call")
+}
+
+fiber.call(1) // expect: call
+fiber.call(2) // expect runtime error: Cannot call a finished fiber.

test/fiber/closure.wren

@@ -17,9 +17,9 @@ var closure
   }
 }
 
-fiber.run    // expect: before
+fiber.call   // expect: before
 closure.call // expect: before
-fiber.run
+fiber.call
 closure.call // expect: after
-fiber.run    // expect: after
+fiber.call   // expect: after
 closure.call // expect: final

test/fiber/is_done.wren

@@ -5,7 +5,7 @@ var fiber = new Fiber {
 }
 
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 1
+fiber.call              // expect: 1
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 2
+fiber.call              // expect: 2
 IO.print(fiber.isDone)  // expect: true

test/fiber/resume_caller.wren

@@ -4,12 +4,12 @@ var b = new Fiber {
 
 var a = new Fiber {
   IO.print("begin fiber a")
-  b.run
+  b.call
   IO.print("end fiber a")
 }
 
 IO.print("begin main")
-a.run
+a.call
 IO.print("end main")
 
 // expect: begin main

test/fiber/run.wren

@@ -4,6 +4,6 @@ var fiber = new Fiber {
 
 IO.print("before") // expect: before
 fiber.run          // expect: fiber
-IO.print("after")  // expect: after
 
-// TODO: Test handles error if fiber tries to run itself.
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")

test/fiber/run_direct_reenter.wren

@@ -0,0 +1,10 @@
+var fiber
+
+fiber = new Fiber {
+  IO.print(1) // expect: 1
+  fiber.run
+  IO.print(2) // expect: 2
+}
+
+fiber.call
+IO.print(3) // expect: 3

test/fiber/run_indirect_reenter.wren

@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4

test/fiber/run_return_implicit_null.wren

@@ -1,6 +0,0 @@
-var fiber = new Fiber {
-  IO.print("fiber")
-}
-
-var result = fiber.run // expect: fiber
-IO.print(result)       // expect: null

test/fiber/run_return_value.wren

@@ -1,7 +0,0 @@
-var fiber = new Fiber {
-  IO.print("fiber")
-  return "result"
-}
-
-var result = fiber.run // expect: fiber
-IO.print(result)       // expect: result

test/fiber/run_when_done.wren

@@ -1,6 +1,13 @@
-var fiber = new Fiber {
+var a = new Fiber {
   IO.print("run")
 }
 
-fiber.run // expect: run
-fiber.run // expect runtime error: Cannot run a finished fiber.
+// Run a through an intermediate fiber since it will get discarded and we need
+// to return to the main one after a completes.
+var b = new Fiber {
+  a.run
+  IO.print("nope")
+}
+
+b.call // expect: run
+a.run // expect runtime error: Cannot run a finished fiber.

test/fiber/run_with_value.wren

@@ -4,6 +4,8 @@ var fiber = new Fiber {
 
 // The first value passed to the fiber is ignored, since there's no yield call
 // to return it.
-IO.print("before")   // expect: before
+IO.print("before")    // expect: before
 fiber.run("ignored") // expect: fiber
-IO.print("after")    // expect: after
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")

test/fiber/run_with_value_direct_reenter.wren

@@ -0,0 +1,10 @@
+var fiber
+
+fiber = new Fiber {
+  IO.print(1) // expect: 1
+  fiber.run("ignored")
+  IO.print(2) // expect: 2
+}
+
+fiber.call
+IO.print(3) // expect: 3

test/fiber/run_with_value_indirect_reenter.wren

@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run("ignored")
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run("ignored")
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4

test/fiber/run_with_value_when_done.wren

@@ -1,6 +0,0 @@
-var fiber = new Fiber {
-  IO.print("run")
-}
-
-fiber.run(1) // expect: run
-fiber.run(2) // expect runtime error: Cannot run a finished fiber.

test/fiber/yield.wren

@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.run // expect: fiber 1
-IO.print("main 1") // expect: main 1
-result = fiber.run // expect: fiber 2
-IO.print("main 2") // expect: main 2
-result = fiber.run // expect: fiber 3
-IO.print("main 3") // expect: main 3
+var result = fiber.call // expect: fiber 1
+IO.print("main 1")  // expect: main 1
+result = fiber.call // expect: fiber 2
+IO.print("main 2")  // expect: main 2
+result = fiber.call // expect: fiber 3
+IO.print("main 3")  // expect: main 3

test/fiber/yield_return_call_value.wren

@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print(result)
 }
 
-fiber.run // expect: fiber 1
+fiber.call // expect: fiber 1
 IO.print("main 1") // expect: main 1
-fiber.run("run 1") // expect: run 1
+fiber.call("call 1") // expect: call 1
 IO.print("main 2") // expect: main 2
-fiber.run // expect: null
+fiber.call // expect: null
 IO.print("main 3") // expect: main 3

test/fiber/yield_return_run_value.wren

@@ -0,0 +1,12 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+  var result = Fiber.yield
+  IO.print(result)
+}
+
+fiber.call // expect: fiber
+IO.print("main") // expect: main
+fiber.run("run") // expect: run
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")

test/fiber/yield_with_no_caller.wren

@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run

test/fiber/yield_with_value.wren

@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.run // expect: fiber 1
+var result = fiber.call // expect: fiber 1
 IO.print(result) // expect: yield 1
-result = fiber.run // expect: fiber 2
+result = fiber.call // expect: fiber 2
 IO.print(result) // expect: yield 2
-result = fiber.run // expect: fiber 3
+result = fiber.call // expect: fiber 3
 IO.print(result) // expect: null

test/fiber/yield_with_value_with_no_caller.wren

@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield(1) // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run