1 //: Continuations are a powerful primitive for constructing advanced kinds of
  2 //: control *policies* like back-tracking.
  3 //:
  4 //: In Mu, continuations are first-class and delimited, and are constructed
  5 //: out of two primitives:
  6 //:
  7 //:  * 'call-with-continuation-mark' marks the top of the call stack and then
  8 //:    calls the provided recipe.
  9 //:  * 'return-continuation-until-mark' copies the top of the stack
 10 //:    until the mark, and returns it as the result of
 11 //:    'call-with-continuation-mark' (which might be a distant ancestor on the
 12 //:    call stack; intervening calls don't return)
 13 //:
 14 //: The resulting slice of the stack can now be called just like a regular
 15 //: recipe.
 16 //:
 17 //: See the example programs continuation*.mu to get a sense for the
 18 //: possibilities.
 19 //:
 20 //: Refinements:
 21 //:  * You can call a single continuation multiple times, and it will preserve
 22 //:    the state of its local variables at each stack frame between calls.
 23 //:    The stack frames of a continuation are not destroyed until the
 24 //:    continuation goes out of scope. See continuation2.mu.
 25 //:  * 'return-continuation-until-mark' doesn't consume the mark, so you can
 26 //:    return multiple continuations based on a single mark. In combination
 27 //:    with the fact that 'return-continuation-until-mark' can return from
 28 //:    regular calls, just as long as there was an earlier call to
 29 //:    'call-with-continuation-mark', this gives us a way to create resumable
 30 //:    recipes. See continuation3.mu.
 31 //:  * 'return-continuation-until-mark' can take ingredients to return just
 32 //:    like other 'return' instructions. It just implicitly also returns a
 33 //:    continuation as the first result. See continuation4.mu.
 34 //:  * Conversely, you can pass ingredients to a continuation when calling it,
 35 //:    to make it available to products of 'return-continuation-until-mark'.
 36 //:    See continuation5.mu.
 37 //:
 38 //: Inspired by James and Sabry, "Yield: Mainstream delimited continuations",
 39 //: Workshop on the Theory and Practice of Delimited Continuations, 2011.
 40 //: https://www.cs.indiana.edu/~sabry/papers/yield.pdf
 41 //:
 42 //: Caveats:
 43 //:  * At the moment we can't statically type-check continuations. So we raise
 44 //:    runtime errors for a call that doesn't return a continuation when the
 45 //:    caller expects, or one that returns a continuation when the caller
 46 //:    doesn't expect it. This shouldn't cause memory corruption, though.
 47 //:    There should still be no way to lookup addresses that aren't allocated.
 48 
 49 :(before "End Mu Types Initialization")
 50 type_ordinal continuation = Type_ordinal["continuation"] = Next_type_ordinal++;
 51 Type[continuation].name = "continuation";
 52 
 53 //: A continuation can be called like a recipe.
 54 :(before "End is_mu_recipe Atom Cases(r)")
 55 if (r.type->name == "continuation") return true;
 56 
 57 //: However, it can't be type-checked like most recipes. Pretend it's like a
 58 //: header-less recipe.
 59 :(after "Begin Reagent->Recipe(r, recipe_header)")
 60 if (r.type->atom && r.type->name == "continuation") {
 61   result_header.has_header = false;
 62   return result_header;
 63 }
 64 
 65 :(scenario delimited_continuation)
 66 recipe main [
 67   1:continuation <- call-with-continuation-mark 233/mark, f, 77  # 77 is an argument to f
 68   2:number <- copy 5
 69   {
 70   ¦ 2:number <- call 1:continuation, 2:number  # jump to 'return-continuation-until-mark' below
 71   ¦ 3:boolean <- greater-or-equal 2:number, 8
 72   ¦ break-if 3:boolean
 73   ¦ loop
 74   }
 75 ]
 76 recipe f [
 77   11:number <- next-ingredient
 78   12:number <- g 11:number
 79   return 12:number
 80 ]
 81 recipe g [
 82   21:number <- next-ingredient
 83   22:number <- return-continuation-until-mark 233/mark
 84   23:number <- add 22:number, 1
 85   return 23:number
 86 ]
 87 # first call of 'g' executes the part before return-continuation-until-mark
 88 +mem: storing 77 in location 21
 89 +run: {2: "number"} <- copy {5: "literal"}
 90 +mem: storing 5 in location 2
 91 # calls of the continuation execute the part after return-continuation-until-mark
 92 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
 93 +mem: storing 5 in location 22
 94 +mem: storing 6 in location 2
 95 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
 96 +mem: storing 6 in location 22
 97 +mem: storing 7 in location 2
 98 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
 99 +mem: storing 7 in location 22
100 +mem: storing 8 in location 2
101 # first call of 'g' does not execute the part after return-continuation-until-mark
102 -mem: storing 77 in location 22
103 # calls of the continuation don't execute the part before return-continuation-until-mark
104 -mem: storing 5 in location 21
105 -mem: storing 6 in location 21
106 -mem: storing 7 in location 21
107 # termination
108 -mem: storing 9 in location 2
109 
110 :(before "End call Fields")
111 int is_base_of_continuation;
112 :(before "End call Constructor")
113 is_base_of_continuation = 0;
114 
115 :(before "End Primitive Recipe Declarations")
116 CALL_WITH_CONTINUATION_MARK,
117 :(before "End Primitive Recipe Numbers")
118 Recipe_ordinal["call-with-continuation-mark"] = CALL_WITH_CONTINUATION_MARK;
119 :(before "End Primitive Recipe Checks")
120 case CALL_WITH_CONTINUATION_MARK: {
121   break;
122 }
123 :(before "End Primitive Recipe Implementations")
124 case CALL_WITH_CONTINUATION_MARK: {
125   // like call, but mark the current call as a 'base of continuation' call
126   // before pushing the next one on it
127   if (Trace_stream) {
128   ¦ ++Trace_stream->callstack_depth;
129   ¦ trace("trace") << "delimited continuation; incrementing callstack depth to " << Trace_stream->callstack_depth << end();
130   ¦ assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
131   }
132   const instruction& caller_instruction = current_instruction();
133   Current_routine->calls.front().is_base_of_continuation = current_instruction().ingredients.at(0).value;
134   Current_routine->calls.push_front(call(Recipe_ordinal[current_instruction().ingredients.at(1).name]));
135   ingredients.erase(ingredients.begin());  // drop the mark
136   ingredients.erase(ingredients.begin());  // drop the callee
137   finish_call_housekeeping(caller_instruction, ingredients);
138   continue;
139 }
140 
141 //: save the slice of current call stack until the 'call-with-continuation-mark'
142 //: call, and return it as the result.
143 //: todo: implement delimited continuations in Mu's memory
144 :(before "End Types")
145 struct delimited_continuation {
146   call_stack frames;
147   int nrefs;
148   delimited_continuation(call_stack::iterator begin, call_stack::iterator end) :frames(call_stack(begin, end)), nrefs(0) {}
149 };
150 :(before "End Globals")
151 map<long long int, delimited_continuation> Delimited_continuation;
152 long long int Next_delimited_continuation_id = 1;  // 0 is null just like an address
153 :(before "End Reset")
154 Delimited_continuation.clear();
155 Next_delimited_continuation_id = 1;
156 
157 :(before "End Primitive Recipe Declarations")
158 RETURN_CONTINUATION_UNTIL_MARK,
159 :(before "End Primitive Recipe Numbers")
160 Recipe_ordinal["return-continuation-until-mark"] = RETURN_CONTINUATION_UNTIL_MARK;
161 :(before "End Primitive Recipe Checks")
162 case RETURN_CONTINUATION_UNTIL_MARK: {
163   break;
164 }
165 :(before "End Primitive Recipe Implementations")
166 case RETURN_CONTINUATION_UNTIL_MARK: {
167   // I don't know how to think about next-ingredient in combination with
168   // continuations, so seems cleaner to just kill it. Functions have to read
169   // their inputs before ever returning a continuation.
170   Current_routine->calls.front().ingredient_atoms.clear();
171   Current_routine->calls.front().next_ingredient_to_process = 0;
172   // copy the current call stack until the most recent marked call
173   call_stack::iterator find_base_of_continuation(call_stack&, int);  // manual prototype containing '::'
174   call_stack::iterator base = find_base_of_continuation(Current_routine->calls, /*mark identifier*/current_instruction().ingredients.at(0).value);
175   if (base == Current_routine->calls.end()) {
176   ¦ raise << maybe(current_recipe_name()) << "couldn't find a 'call-with-continuation-mark' to return to\n" << end();
177   ¦ raise << maybe(current_recipe_name()) << "call stack:\n" << end();
178   ¦ for (call_stack::iterator p = Current_routine->calls.begin();  p != Current_routine->calls.end();  ++p)
179   ¦ ¦ raise << maybe(current_recipe_name()) << "  " << get(Recipe, p->running_recipe).name << '\n' << end();
180   ¦ break;
181   }
182   trace("run") << "creating continuation " << Next_delimited_continuation_id << end();
183   put(Delimited_continuation, Next_delimited_continuation_id, delimited_continuation(Current_routine->calls.begin(), base));
184   while (Current_routine->calls.begin() != base) {
185   ¦ if (Trace_stream) {
186   ¦ ¦ --Trace_stream->callstack_depth;
187   ¦ ¦ assert(Trace_stream->callstack_depth >= 0);
188   ¦ }
189   ¦ Current_routine->calls.pop_front();
190   }
191   // return it as the result of the marked call
192   products.resize(1);
193   products.at(0).push_back(Next_delimited_continuation_id);
194   // return any other ingredients passed in
195   copy(/*skip mark identifier*/++ingredients.begin(), ingredients.end(), inserter(products, products.end()));
196   ++Next_delimited_continuation_id;
197   break;  // continue to process rest of marked call
198 }
199 
200 :(code)
201 call_stack::iterator find_base_of_continuation(call_stack& c, int mark) {
202   for (call_stack::iterator p = c.begin(); p != c.end(); ++p)
203   ¦ if (p->is_base_of_continuation == mark) return p;
204   return c.end();
205 }
206 
207 //: overload 'call' for continuations
208 :(after "Begin Call")
209 if (is_mu_continuation(current_instruction().ingredients.at(0))) {
210   // copy multiple calls on to current call stack
211   assert(scalar(ingredients.at(0)));
212   trace("run") << "calling continuation " << ingredients.at(0).at(0) << end();
213   if (!contains_key(Delimited_continuation, ingredients.at(0).at(0)))
214   ¦ raise << maybe(current_recipe_name()) << "no such delimited continuation " << current_instruction().ingredients.at(0).original_string << '\n' << end();
215   const call_stack& new_frames = get(Delimited_continuation, ingredients.at(0).at(0)).frames;
216   for (call_stack::const_reverse_iterator p = new_frames.rbegin(); p != new_frames.rend(); ++p) {
217   ¦ Current_routine->calls.push_front(*p);
218   ¦ // ensure that the presence of a continuation keeps its stack frames from being reclaimed
219   ¦ increment_refcount(Current_routine->calls.front().default_space);
220   }
221   if (Trace_stream) {
222   ¦ Trace_stream->callstack_depth += SIZE(new_frames);
223   ¦ trace("trace") << "calling delimited continuation; growing callstack depth to " << Trace_stream->callstack_depth << end();
224   ¦ assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
225   }
226   // no call housekeeping; continuations don't support next-ingredient
227   copy(/*drop continuation*/++ingredients.begin(), ingredients.end(), inserter(products, products.begin()));
228   break;  // record results of resuming 'return-continuation-until-mark' instruction
229 }
230 
231 :(scenario continuations_can_return_values)
232 def main [
233   local-scope
234   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
235 ]
236 def f [
237   local-scope
238   g
239 ]
240 def g [
241   local-scope
242   return-continuation-until-mark 233/mark, 34
243   stash [continuation called]
244 ]
245 # entering main
246 +mem: new alloc: 1000
247 +run: {k: "continuation"}, {1: "number", "raw": ()} <- call-with-continuation-mark {233: "literal", "mark": ()}, {f: "recipe-literal"}
248 # entering f
249 +mem: new alloc: 1004
250 # entering g
251 +mem: new alloc: 1007
252 # return control to main
253 +run: return-continuation-until-mark {233: "literal", "mark": ()}, {34: "literal"}
254 # no allocs abandoned yet
255 +mem: storing 34 in location 1
256 # end of main
257 # make sure no memory leaks..
258 +mem: trying to reclaim local k:continuation
259 +mem: automatically abandoning 1007
260 +mem: automatically abandoning 1004
261 +mem: automatically abandoning 1000
262 # ..even though we never called the continuation
263 -app: continuation called
264 
265 :(scenario continuations_continue_to_matching_mark)
266 def main [
267   local-scope
268   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
269   add 1, 1
270 ]
271 def f [
272   local-scope
273   k2:continuation <- call-with-continuation-mark 234/mark, g
274   add 2, 2
275 ]
276 def g [
277   local-scope
278   return-continuation-until-mark 233/mark, 34
279   stash [continuation called]
280 ]
281 +run: add {1: "literal"}, {1: "literal"}
282 -run: add {2: "literal"}, {2: "literal"}
283 
284 //: Allow shape-shifting recipes to return continuations.
285 
286 :(scenario call_shape_shifting_recipe_with_continuation_mark)
287 def main [
288   1:num <- call-with-continuation-mark 233/mark, f, 34
289 ]
290 def f x:_elem -> y:_elem [
291   local-scope
292   load-ingredients
293   y <- copy x
294 ]
295 +mem: storing 34 in location 1
296 
297 :(before "End resolve_ambiguous_call(r, index, inst, caller_recipe) Special-cases")
298 if (inst.name == "call-with-continuation-mark") {
299   if (SIZE(inst.ingredients) > 1 && is_recipe_literal(inst.ingredients.at(/*skip mark*/1))) {
300   resolve_indirect_continuation_call(r, index, inst, caller_recipe);
301   return;
302   }
303 }
304 :(code)
305 void resolve_indirect_continuation_call(const recipe_ordinal r, int index, instruction& inst, const recipe& caller_recipe) {
306   instruction inst2;
307   inst2.name = inst.ingredients.at(/*skip mark*/1).name;
308   for (int i = /*skip mark and recipe*/2;  i < SIZE(inst.ingredients);  ++i)
309   ¦ inst2.ingredients.push_back(inst.ingredients.at(i));
310   for (int i = /*skip continuation*/1;  i < SIZE(inst.products);  ++i)
311   ¦ inst2.products.push_back(inst.products.at(i));
312   resolve_ambiguous_call(r, index, inst2, caller_recipe);
313   inst.ingredients.at(/*skip mark*/1).name = inst2.name;
314   inst.ingredients.at(/*skip mark*/1).set_value(get(Recipe_ordinal, inst2.name));
315 }
316 
317 :(scenario call_shape_shifting_recipe_with_continuation_mark_and_no_outputs)
318 def main [
319   1:continuation <- call-with-continuation-mark 233/mark, f, 34
320 ]
321 def f x:_elem [
322   local-scope
323   load-ingredients
324   return-continuation-until-mark 233/mark
325 ]
326 $error: 0
327 
328 :(scenario continuation1)
329 def main [
330   local-scope
331   k:continuation <- call-with-continuation-mark 233/mark, create-yielder
332   10:num/raw <- call k
333 ]
334 def create-yielder -> n:num [
335   local-scope
336   load-inputs
337   return-continuation-until-mark 233/mark
338   return 1
339 ]
340 +mem: storing 1 in location 10
341 $error: 0
342 
343 //: Ensure that the presence of a continuation keeps its stack frames from being reclaimed.
344 
345 :(scenario continuations_preserve_local_scopes)
346 def main [
347   local-scope
348   k:continuation <- call-with-continuation-mark 233/mark, f
349   call k
350   return 34
351 ]
352 def f [
353   local-scope
354   g
355 ]
356 def g [
357   local-scope
358   return-continuation-until-mark 233/mark
359   add 1, 1
360 ]
361 # entering main
362 +mem: new alloc: 1000
363 +run: {k: "continuation"} <- call-with-continuation-mark {233: "literal", "mark": ()}, {f: "recipe-literal"}
364 # entering f
365 +mem: new alloc: 1004
366 # entering g
367 +mem: new alloc: 1007
368 # return control to main
369 +run: return-continuation-until-mark {233: "literal", "mark": ()}
370 # no allocs abandoned yet
371 # finish running main
372 +run: call {k: "continuation"}
373 +run: add {1: "literal"}, {1: "literal"}
374 +run: return {34: "literal"}
375 # now k is reclaimed
376 +mem: trying to reclaim local k:continuation
377 # at this point all allocs in the continuation are abandoned
378 +mem: automatically abandoning 1007
379 +mem: automatically abandoning 1004
380 # finally the alloc for main is abandoned
381 +mem: automatically abandoning 1000
382 
383 :(before "End Increment Refcounts(canonized_x)")
384 if (is_mu_continuation(canonized_x)) {
385   int continuation_id = data.at(0);
386   if (continuation_id == 0) return;
387   if (!contains_key(Delimited_continuation, continuation_id)) {
388   ¦ raise << maybe(current_recipe_name()) << "missing delimited continuation: " << canonized_x.name << '\n';
389   ¦ return;
390   }
391   delimited_continuation& curr = get(Delimited_continuation, continuation_id);
392   trace("run") << "incrementing refcount of continuation " << continuation_id << ": " << curr.nrefs << " -> " << 1+curr.nrefs << end();
393   ++curr.nrefs;
394   return;
395 }
396 
397 :(before "End Decrement Refcounts(canonized_x)")
398 if (is_mu_continuation(canonized_x)) {
399   int continuation_id = get_or_insert(Memory, canonized_x.value);
400   if (continuation_id == 0) return;
401   delimited_continuation& curr = get(Delimited_continuation, continuation_id);
402   assert(curr.nrefs > 0);
403   --curr.nrefs;
404   trace("run") << "decrementing refcount of continuation " << continuation_id << ": " << 1+curr.nrefs << " -> " << curr.nrefs << end();
405   if (curr.nrefs > 0) return;
406   trace("run") << "reclaiming continuation " << continuation_id << end();
407   // temporarily push the stack frames for the continuation to the call stack before reclaiming their spaces
408   // (because reclaim_default_space() relies on the default-space being reclaimed being at the top of the stack)
409   for (call_stack::const_iterator p = curr.frames.begin(); p != curr.frames.end(); ++p) {
410   ¦ Current_routine->calls.push_front(*p);
411   ¦ reclaim_default_space();
412   ¦ Current_routine->calls.pop_front();
413   }
414   Delimited_continuation.erase(continuation_id);
415   return;
416 }
417 
418 :(scenario continuations_can_be_copied)
419 def main [
420   local-scope
421   k:continuation <- call-with-continuation-mark 233/mark, f
422   k2:continuation <- copy k
423   # reclaiming k and k2 shouldn't delete f's local scope twice
424 ]
425 def f [
426   local-scope
427   load-ingredients
428   return-continuation-until-mark 233/mark
429   return 0
430 ]
431 $error: 0
432 
433 :(code)
434 bool is_mu_continuation(reagent/*copy*/ x) {
435   canonize_type(x);
436   return x.type && x.type->atom && x.type->value == get(Type_ordinal, "continuation");
437 }
438 
439 // helper for debugging
440 void dump(const int continuation_id) {
441   if (!contains_key(Delimited_continuation, continuation_id)) {
442   ¦ raise << "missing delimited continuation: " << continuation_id << '\n' << end();
443   ¦ return;
444   }
445   delimited_continuation& curr = get(Delimited_continuation, continuation_id);
446   dump(curr.frames);
447 }