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   if (Trace_stream) {
219     Trace_stream->callstack_depth += SIZE(new_frames);
220     trace("trace") << "calling delimited continuation; growing callstack depth to " << Trace_stream->callstack_depth << end();
221     assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
222   }
223   // no call housekeeping; continuations don't support next-ingredient
224   copy(/*drop continuation*/++ingredients.begin(), ingredients.end(), inserter(products, products.begin()));
225   break;  // record results of resuming 'return-continuation-until-mark' instruction
226 }
227 
228 :(scenario continuations_can_return_values)
229 def main [
230   local-scope
231   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
232 ]
233 def f [
234   local-scope
235   g
236 ]
237 def g [
238   local-scope
239   return-continuation-until-mark 233/mark, 34
240   stash [continuation called]
241 ]
242 +mem: storing 34 in location 1
243 
244 :(scenario continuations_continue_to_matching_mark)
245 def main [
246   local-scope
247   k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f
248   add 1, 1
249 ]
250 def f [
251   local-scope
252   k2:continuation <- call-with-continuation-mark 234/mark, g
253   add 2, 2
254 ]
255 def g [
256   local-scope
257   return-continuation-until-mark 233/mark, 34
258   stash [continuation called]
259 ]
260 +run: add {1: "literal"}, {1: "literal"}
261 -run: add {2: "literal"}, {2: "literal"}
262 
263 //: Allow shape-shifting recipes to return continuations.
264 
265 :(scenario call_shape_shifting_recipe_with_continuation_mark)
266 def main [
267   1:num <- call-with-continuation-mark 233/mark, f, 34
268 ]
269 def f x:_elem -> y:_elem [
270   local-scope
271   load-ingredients
272   y <- copy x
273 ]
274 +mem: storing 34 in location 1
275 
276 :(before "End resolve_ambiguous_call(r, index, inst, caller_recipe) Special-cases")
277 if (inst.name == "call-with-continuation-mark") {
278   if (SIZE(inst.ingredients) > 1 && is_recipe_literal(inst.ingredients.at(/*skip mark*/1))) {
279   resolve_indirect_continuation_call(r, index, inst, caller_recipe);
280   return;
281   }
282 }
283 :(code)
284 void resolve_indirect_continuation_call(const recipe_ordinal r, int index, instruction& inst, const recipe& caller_recipe) {
285   instruction inst2;
286   inst2.name = inst.ingredients.at(/*skip mark*/1).name;
287   for (int i = /*skip mark and recipe*/2;  i < SIZE(inst.ingredients);  ++i)
288     inst2.ingredients.push_back(inst.ingredients.at(i));
289   for (int i = /*skip continuation*/1;  i < SIZE(inst.products);  ++i)
290     inst2.products.push_back(inst.products.at(i));
291   resolve_ambiguous_call(r, index, inst2, caller_recipe);
292   inst.ingredients.at(/*skip mark*/1).name = inst2.name;
293   inst.ingredients.at(/*skip mark*/1).set_value(get(Recipe_ordinal, inst2.name));
294 }
295 
296 :(scenario call_shape_shifting_recipe_with_continuation_mark_and_no_outputs)
297 def main [
298   1:continuation <- call-with-continuation-mark 233/mark, f, 34
299 ]
300 def f x:_elem [
301   local-scope
302   load-ingredients
303   return-continuation-until-mark 233/mark
304 ]
305 $error: 0
306 
307 :(scenario continuation1)
308 def main [
309   local-scope
310   k:continuation <- call-with-continuation-mark 233/mark, create-yielder
311   10:num/raw <- call k
312 ]
313 def create-yielder -> n:num [
314   local-scope
315   load-inputs
316   return-continuation-until-mark 233/mark
317   return 1
318 ]
319 +mem: storing 1 in location 10
320 $error: 0
321 
322 :(code)
323 bool is_mu_continuation(reagent/*copy*/ x) {
324   canonize_type(x);
325   return x.type && x.type->atom && x.type->value == get(Type_ordinal, "continuation");
326 }
327 
328 // helper for debugging
329 void dump(const int continuation_id) {
330   if (!contains_key(Delimited_continuation, continuation_id)) {
331     raise << "missing delimited continuation: " << continuation_id << '\n' << end();
332     return;
333   }
334   delimited_continuation& curr = get(Delimited_continuation, continuation_id);
335   dump(curr.frames);
336 }