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 //:  * There can be multiple continuation marks on the stack at once;
 38 //:    'call-with-continuation-mark' and 'return-continuation-until-mark' both
 39 //:    need to pass in a tag to coordinate on the correct mark. This allows us
 40 //:    to save multiple continuations for different purposes (say if one is
 41 //:    for exceptions) with overlapping stack frames. See exception.mu.
 42 //:
 43 //: Inspired by James and Sabry, "Yield: Mainstream delimited continuations",
 44 //: Workshop on the Theory and Practice of Delimited Continuations, 2011.
 45 //: https://www.cs.indiana.edu/~sabry/papers/yield.pdf
 46 //:
 47 //: Caveats:
 48 //:  * At the moment we can't statically type-check continuations. So we raise
 49 //:    runtime errors for a call that doesn't return a continuation when the
 50 //:    caller expects, or one that returns a continuation when the caller
 51 //:    doesn't expect it. This shouldn't cause memory corruption, though.
 52 //:    There should still be no way to lookup addresses that aren't allocated.
 53 
 54 :(before "End Mu Types Initialization")
 55 type_ordinal continuation = Type_ordinal["continuation"] = Next_type_ordinal++;
 56 Type[continuation].name = "continuation";
 57 
 58 //: A continuation can be called like a recipe.
 59 :(before "End is_mu_recipe Atom Cases(r)")
 60 if (r.type->name == "continuation") return true;
 61 
 62 //: However, it can't be type-checked like most recipes. Pretend it's like a
 63 //: header-less recipe.
 64 :(after "Begin Reagent->Recipe(r, recipe_header)")
 65 if (r.type->atom && r.type->name == "continuation") {
 66   result_header.has_header = false;
 67   return result_header;
 68 }
 69 
 70 :(scenario delimited_continuation)
 71 recipe main [
 72   1:continuation <- call-with-continuation-mark 233/mark, f, 77  # 77 is an argument to f
 73   2:number <- copy 5
 74   {
 75     2:number <- call 1:continuation, 2:number  # jump to 'return-continuation-until-mark' below
 76     3:boolean <- greater-or-equal 2:number, 8
 77     break-if 3:boolean
 78     loop
 79   }
 80 ]
 81 recipe f [
 82   11:number <- next-ingredient
 83   12:number <- g 11:number
 84   return 12:number
 85 ]
 86 recipe g [
 87   21:number <- next-ingredient
 88   22:number <- return-continuation-until-mark 233/mark
 89   23:number <- add 22:number, 1
 90   return 23:number
 91 ]
 92 # first call of 'g' executes the part before return-continuation-until-mark
 93 +mem: storing 77 in location 21
 94 +run: {2: "number"} <- copy {5: "literal"}
 95 +mem: storing 5 in location 2
 96 # calls of the continuation execute the part after return-continuation-until-mark
 97 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
 98 +mem: storing 5 in location 22
 99 +mem: storing 6 in location 2
100 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
101 +mem: storing 6 in location 22
102 +mem: storing 7 in location 2
103 +run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
104 +mem: storing 7 in location 22
105 +mem: storing 8 in location 2
106 # first call of 'g' does not execute the part after return-continuation-until-mark
107 -mem: storing 77 in location 22
108 # calls of the continuation don't execute the part before return-continuation-until-mark
109 -mem: storing 5 in location 21
110 -mem: storing 6 in location 21
111 -mem: storing 7 in location 21
112 # termination
113 -mem: storing 9 in location 2
114 
115 :(before "End call Fields")
116 int continuation_mark_tag;
117 :(before "End call Constructor")
118 continuation_mark_tag = 0;
119 
120 :(before "End Primitive Recipe Declarations")
121 CALL_WITH_CONTINUATION_MARK,
122 :(before "End Primitive Recipe Numbers")
123 Recipe_ordinal["call-with-continuation-mark"] = CALL_WITH_CONTINUATION_MARK;
124 :(before "End Primitive Recipe Checks")
125 case CALL_WITH_CONTINUATION_MARK: {
126   if (SIZE(inst.ingredients) < 2) {
127     raise << maybe(get(Recipe, r).name) << "'" << to_original_string(inst) << "' requires at least two ingredients: a mark number and a recipe to call\n" << end();
128   }
129   break;
130 }
131 :(before "End Primitive Recipe Implementations")
132 case CALL_WITH_CONTINUATION_MARK: {
133   // like call, but mark the current call as a 'base of continuation' call
134   // before pushing the next one on it
135   if (Trace_stream) {
136     ++Trace_stream->callstack_depth;
137     trace("trace") << "delimited continuation; incrementing callstack depth to " << Trace_stream->callstack_depth << end();
138     assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
139   }
140   instruction/*copy*/ caller_instruction = current_instruction();
141   Current_routine->calls.front().continuation_mark_tag = current_instruction().ingredients.at(0).value;
142   Current_routine->calls.push_front(call(ingredients.at(1).at(0)));
143   // drop the mark
144   caller_instruction.ingredients.erase(caller_instruction.ingredients.begin());
145   ingredients.erase(ingredients.begin());
146   // drop the callee
147   caller_instruction.ingredients.erase(caller_instruction.ingredients.begin());
148   ingredients.erase(ingredients.begin());
149   finish_call_housekeeping(caller_instruction, ingredients);
150   continue;
151 }
152 
153 :(scenario next_ingredient_inside_continuation)
154 recipe main [
155   call-with-continuation-mark 233/mark, f, 1/true
156 ]
157 recipe f [
158   10:bool <- next-input
159 ]
160 +mem: storing 1 in location 10
161 
162 :(scenario delimited_continuation_out_of_recipe_variable)
163 recipe main [
164   x:recipe <- copy f
165   call-with-continuation-mark 233/mark, x, 1/true
166 ]
167 recipe f [
168   10:bool <- next-input
169 ]
170 +mem: storing 1 in location 10
171 
172 //: save the slice of current call stack until the 'call-with-continuation-mark'
173 //: call, and return it as the result.
174 //: todo: implement delimited continuations in Mu's memory
175 :(before "End Types")
176 struct delimited_continuation {
177   call_stack frames;
178   int nrefs;
179   delimited_continuation(call_stack::iterator begin, call_stack::iterator end) :frames(call_stack(begin, end)), nrefs(0) {}
180 };
181 :(before "End Globals")
182 map<long long int, delimited_continuation> Delimited_continuation;
183 long long int Next_delimited_continuation_id = 1;  // 0 is null just like an address
184 :(before "End Reset")
185 Delimited_continuation.clear();
186 Next_delimited_continuation_id = 1;
187 
188 :(before "End Primitive Recipe Declarations")
189 RETURN_CONTINUATION_UNTIL_MARK,
190 :(before "End Primitive Recipe Numbers")
191 Recipe_ordinal["return-continuation-until-mark"] = RETURN_CONTINUATION_UNTIL_MARK;
192 :(before "End Primitive Recipe Checks")
193 case RETURN_CONTINUATION_UNTIL_MARK: {
194   if (inst.ingredients.empty()) {
195     raise << maybe(get(Recipe, r).name) << "'" << to_original_string(inst) << "' requires at least one ingredient: a mark tag (number)\n" << end();
196   }
197   break;
198 }
199 :(before "End Primitive Recipe Implementations")
200 case RETURN_CONTINUATION_UNTIL_MARK: {
201   // I don't know how to think about next-ingredient in combination with
202   // continuations, so seems cleaner to just kill it. Functions have to read
203   // their inputs before ever returning a continuation.
204   Current_routine->calls.front().ingredient_atoms.clear();
205   Current_routine->calls.front().next_ingredient_to_process = 0;
206   // copy the current call stack until the most recent marked call
207   call_stack::iterator find_base_of_continuation(call_stack&, int);  // manual prototype containing '::'
208   call_stack::iterator base = find_base_of_continuation(Current_routine->calls, /*mark tag*/current_instruction().ingredients.at(0).value);
209   if (base == Current_routine->calls.end()) {
210     raise << maybe(current_recipe_name()) << "couldn't find a 'call-with-continuation-mark' to return to with tag " << current_instruction().ingredients.at(0).original_string << '\n' << end();
211     raise << maybe(current_recipe_name()) << "call stack:\n" << end();
212     for (call_stack::iterator p = Current_routine->calls.begin();  p != Current_routine->calls.end();  ++p)
213       raise << maybe(current_recipe_name()) << "  " << get(Recipe, p->running_recipe).name << '\n' << end();
214     break;
215   }
216   trace("run") << "creating continuation " << Next_delimited_continuation_id << end();
217   put(Delimited_continuation, Next_delimited_continuation_id, delimited_continuation(Current_routine->calls.begin(), base));
218   while (Current_routine->calls.begin() != base) {
219     if (Trace_stream) {
220       --Trace_stream->callstack_depth;
221       assert(Trace_stream->callstack_depth >= 0);
222     }
223     Current_routine->calls.pop_front();
224   }
225   // return it as the result of the marked call
226   products.resize(1);
227   products.at(0).push_back(Next_delimited_continuation_id);
228   // return any other ingredients passed in
229   copy(/*skip mark tag*/++ingredients.begin(), ingredients.end(), inserter(products, products.end()));
230   ++Next_delimited_continuation_id;
231   break;  // continue to process rest of marked call
232 }
233 
234 :(code)
235 call_stack::iterator find_base_of_continuation(call_stack& c, int mark_tag) {
236   for (call_stack::iterator p = c.begin(); p != c.end(); ++p)
237     if (p->continuation_mark_tag == mark_tag) return p;
238   return c.end();
239 }
240 
241 //: overload 'call' for continuations
242 :(after "Begin Call")
243 if (is_mu_continuation(current_instruction().ingredients.at(0))) {
244   // copy multiple calls on to current call stack
245   assert(scalar(ingredients.at(0)));
246   trace("run") << "calling continuation " << ingredients.at(0).at(0) << end();
247   if (!contains_key(Delimited_continuation, ingredients.at(0).at(0)))
248     raise << maybe(current_recipe_name()) << "no such delimited continuation " << current_instruction().ingredients.at(0).original_string << '\n' << end();
249   const call_stack& new_frames = get(Delimited_continuation, ingredients.at(0).at(0)).frames;
250   for (call_stack::const_reverse_iterator p = new_frames.rbegin(); p != new_frames.rend(); ++p)
251     Current_routine->calls.push_front(*p);
252   if (Trace_stream) {
253     Trace_stream->callstack_depth += SIZE(new_frames);
254     trace("trace") << "calling delimited continuation; growing callstack depth to " << Trace_stream->callstack_depth << end();
255     assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
256   }
257   // no call housekeeping; continuations don't support next-ingredient
258   copy(/*drop continuation*/++ingredients.begin(), ingredients.end(), inserter(products, products.begin()));
259   break;  // record results of resuming 'return-continuation-until-mark' instruction
260 }
261 
262 :(scenario continuations_can_return_values)
263 def main [
264   local-scope
265   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
266 ]
267 def f [
268   local-scope
269   g
270 ]
271 def g [
272   local-scope
273   return-continuation-until-mark 233/mark, 34
274   stash [continuation called]
275 ]
276 +mem: storing 34 in location 1
277 
278 :(scenario continuations_continue_to_matching_mark)
279 def main [
280   local-scope
281   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
282   add 1, 1
283 ]
284 def f [
285   local-scope
286   k2:continuation <- call-with-continuation-mark 234/mark, g
287   add 2, 2
288 ]
289 def g [
290   local-scope
291   return-continuation-until-mark 233/mark, 34
292   stash [continuation called]
293 ]
294 +run: add {1: "literal"}, {1: "literal"}
295 -run: add {2: "literal"}, {2: "literal"}
296 
297 //: Allow shape-shifting recipes to return continuations.
298 
299 :(scenario call_shape_shifting_recipe_with_continuation_mark)
300 def main [
301   1:num <- call-with-continuation-mark 233/mark, f, 34
302 ]
303 def f x:_elem -> y:_elem [
304   local-scope
305   load-ingredients
306   y <- copy x
307 ]
308 +mem: storing 34 in location 1
309 
310 :(before "End resolve_ambiguous_call(r, index, inst, caller_recipe) Special-cases")
311 if (inst.name == "call-with-continuation-mark") {
312   if (SIZE(inst.ingredients) > 1 && is_recipe_literal(inst.ingredients.at(/*skip mark*/1))) {
313   resolve_indirect_continuation_call(r, index, inst, caller_recipe);
314   return;
315   }
316 }
317 :(code)
318 void resolve_indirect_continuation_call(const recipe_ordinal r, int index, instruction& inst, const recipe& caller_recipe) {
319   instruction inst2;
320   inst2.name = inst.ingredients.at(/*skip mark*/1).name;
321   for (int i = /*skip mark and recipe*/2;  i < SIZE(inst.ingredients);  ++i)
322     inst2.ingredients.push_back(inst.ingredients.at(i));
323   for (int i = /*skip continuation*/1;  i < SIZE(inst.products);  ++i)
324     inst2.products.push_back(inst.products.at(i));
325   resolve_ambiguous_call(r, index, inst2, caller_recipe);
326   inst.ingredients.at(/*skip mark*/1).name = inst2.name;
327   inst.ingredients.at(/*skip mark*/1).set_value(get(Recipe_ordinal, inst2.name));
328 }
329 
330 :(scenario call_shape_shifting_recipe_with_continuation_mark_and_no_outputs)
331 def main [
332   1:continuation <- call-with-continuation-mark 233/mark, f, 34
333 ]
334 def f x:_elem [
335   local-scope
336   load-ingredients
337   return-continuation-until-mark 233/mark
338 ]
339 $error: 0
340 
341 :(scenario continuation1)
342 def main [
343   local-scope
344   k:continuation <- call-with-continuation-mark 233/mark, create-yielder
345   10:num/raw <- call k
346 ]
347 def create-yielder -> n:num [
348   local-scope
349   load-inputs
350   return-continuation-until-mark 233/mark
351   return 1
352 ]
353 +mem: storing 1 in location 10
354 $error: 0
355 
356 :(code)
357 bool is_mu_continuation(reagent/*copy*/ x) {
358   canonize_type(x);
359   return x.type && x.type->atom && x.type->value == get(Type_ordinal, "continuation");
360 }
361 
362 // helper for debugging
363 void dump(const int continuation_id) {
364   if (!contains_key(Delimited_continuation, continuation_id)) {
365     raise << "missing delimited continuation: " << continuation_id << '\n' << end();
366     return;
367   }
368   delimited_continuation& curr = get(Delimited_continuation, continuation_id);
369   dump(curr.frames);
370 }