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 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
 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 bool is_base_of_continuation;
112 :(before "End call Constructor")
113 is_base_of_continuation = false;
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 = true;
134   Current_routine->calls.push_front(call(Recipe_ordinal[current_instruction().ingredients.at(0).name]));
135   ingredients.erase(ingredients.begin());  // drop the callee
136   finish_call_housekeeping(caller_instruction, ingredients);
137   continue;
138 }
139 
140 //: save the slice of current call stack until the 'call-with-continuation-mark'
141 //: call, and return it as the result.
142 //: todo: implement delimited continuations in Mu's memory
143 :(before "End Globals")
144 map<long long int, call_stack> Delimited_continuation;
145 long long int Next_delimited_continuation_id = 1;  // 0 is null just like an address
146 :(before "End Reset")
147 Delimited_continuation.clear();
148 Next_delimited_continuation_id = 1;
149 
150 :(before "End Primitive Recipe Declarations")
151 RETURN_CONTINUATION_UNTIL_MARK,
152 :(before "End Primitive Recipe Numbers")
153 Recipe_ordinal["return-continuation-until-mark"] = RETURN_CONTINUATION_UNTIL_MARK;
154 :(before "End Primitive Recipe Checks")
155 case RETURN_CONTINUATION_UNTIL_MARK: {
156   break;
157 }
158 :(before "End Primitive Recipe Implementations")
159 case RETURN_CONTINUATION_UNTIL_MARK: {
160   // first clear any existing ingredients, to isolate the creation of the
161   // continuation from its calls
162   Current_routine->calls.front().ingredient_atoms.clear();
163   Current_routine->calls.front().next_ingredient_to_process = 0;
164   // copy the current call stack until the most recent marked call
165   call_stack::iterator find_base_of_continuation(call_stack& c);  // manual prototype containing '::'
166   call_stack::iterator base = find_base_of_continuation(Current_routine->calls);
167   if (base == Current_routine->calls.end()) {
168   ¦ raise << maybe(current_recipe_name()) << "couldn't find a 'call-with-continuation-mark' to return to\n" << end();
169   ¦ raise << maybe(current_recipe_name()) << "call stack:\n" << end();
170   ¦ for (call_stack::iterator p = Current_routine->calls.begin();  p != Current_routine->calls.end();  ++p)
171   ¦ ¦ raise << maybe(current_recipe_name()) << "  " << get(Recipe, p->running_recipe).name << '\n' << end();
172   ¦ break;
173   }
174   trace("run") << "creating continuation " << Next_delimited_continuation_id << end();
175   Delimited_continuation[Next_delimited_continuation_id] = call_stack(Current_routine->calls.begin(), base);
176   while (Current_routine->calls.begin() != base) {
177   ¦ if (Trace_stream) {
178   ¦ ¦ --Trace_stream->callstack_depth;
179   ¦ ¦ assert(Trace_stream->callstack_depth >= 0);
180   ¦ }
181   ¦ Current_routine->calls.pop_front();
182   }
183   // return it as the result of the marked call
184   products.resize(1);
185   products.at(0).push_back(Next_delimited_continuation_id);
186   // return any other ingredients passed in
187   copy(ingredients.begin(), ingredients.end(), inserter(products, products.end()));
188   ++Next_delimited_continuation_id;
189   break;  // continue to process rest of marked call
190 }
191 
192 :(code)
193 call_stack::iterator find_base_of_continuation(call_stack& c) {
194   for (call_stack::iterator p = c.begin(); p != c.end(); ++p)
195   ¦ if (p->is_base_of_continuation) return p;
196   return c.end();
197 }
198 
199 //: overload 'call' for continuations
200 :(after "Begin Call")
201 if (current_instruction().ingredients.at(0).type->atom
202   ¦ && current_instruction().ingredients.at(0).type->name == "continuation") {
203   // copy multiple calls on to current call stack
204   assert(scalar(ingredients.at(0)));
205   trace("run") << "calling continuation " << ingredients.at(0).at(0) << end();
206   if (Delimited_continuation.find(ingredients.at(0).at(0)) == Delimited_continuation.end())
207   ¦ raise << maybe(current_recipe_name()) << "no such delimited continuation " << current_instruction().ingredients.at(0).original_string << '\n' << end();
208   const call_stack& new_calls = Delimited_continuation[ingredients.at(0).at(0)];
209   const call& caller = (SIZE(new_calls) > 1) ? *++new_calls.begin() : Current_routine->calls.front();
210   for (call_stack::const_reverse_iterator p = new_calls.rbegin(); p != new_calls.rend(); ++p) {
211   ¦ Current_routine->calls.push_front(*p);
212   ¦ // ensure that the presence of a continuation keeps its stack frames from being reclaimed
213   ¦ int space_address = Current_routine->calls.front().default_space;
214   ¦ if (space_address != 0) {
215   ¦ ¦ int refcount = get_or_insert(Memory, space_address);
216   ¦ ¦ trace("mem") << "incrementing refcount of " << space_address << ": " << refcount << " -> " << refcount+1 << end();
217   ¦ ¦ put(Memory, space_address, refcount+1);
218   ¦ }
219   }
220   if (Trace_stream) {
221   ¦ Trace_stream->callstack_depth += SIZE(new_calls);
222   ¦ trace("trace") << "calling delimited continuation; growing callstack depth to " << Trace_stream->callstack_depth << end();
223   ¦ assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
224   }
225   ingredients.erase(ingredients.begin());  // drop the callee
226   finish_call_housekeeping(to_instruction(caller), ingredients);
227   copy(ingredients.begin(), ingredients.end(), inserter(products, products.begin()));
228   break;  // record results of resuming 'return-continuation-until-mark' instruction
229 }
230 
231 //: Ensure that the presence of a continuation keeps its stack frames from being reclaimed.
232 
233 :(scenario continuations_preserve_local_scopes)
234 def main [
235   local-scope
236   k:continuation <- call-with-continuation-mark f
237   call k
238   return 34
239 ]
240 def f [
241   local-scope
242   g
243 ]
244 def g [
245   local-scope
246   return-continuation-until-mark
247   add 1, 1
248 ]
249 # entering main
250 +mem: new alloc: 1000
251 +run: {k: "continuation"} <- call-with-continuation-mark {f: "recipe-literal"}
252 # entering f
253 +mem: new alloc: 1004
254 # entering g
255 +mem: new alloc: 1007
256 # return control to main
257 +run: return-continuation-until-mark
258 # no allocs abandoned yet
259 # finish running main
260 +run: call {k: "continuation"}
261 +run: add {1: "literal"}, {1: "literal"}
262 +run: return {34: "literal"}
263 # now k is reclaimed
264 +mem: trying to reclaim local k:continuation
265 # at this point all allocs in the continuation are abandoned
266 +mem: automatically abandoning 1007
267 +mem: automatically abandoning 1004
268 # finally the alloc for main is abandoned
269 +mem: automatically abandoning 1000
270 
271 :(after "Begin Decrement Refcounts(canonized_x)")
272 if (is_mu_continuation(canonized_x)) {
273   int continuation_id = get_or_insert(Memory, canonized_x.value);
274   trace("run") << "reclaiming continuation " << continuation_id << end();
275   if (continuation_id == 0) return;
276   const call_stack& continuation_calls = get(Delimited_continuation, continuation_id);
277   // temporarily push the stack frames for the continuation one last time on to the call stack
278   for (call_stack::const_reverse_iterator p = continuation_calls.rbegin(); p != continuation_calls.rend(); ++p)
279   ¦ Current_routine->calls.push_front(*p);
280   // reclaim their spaces while popping them
281   // (because reclaim_default_space() relies on the default-space being reclaimed being at the top of the stack)
282   for (call_stack::const_iterator p = continuation_calls.begin(); p != continuation_calls.end(); ++p) {
283   ¦ reclaim_default_space();
284   ¦ Current_routine->calls.pop_front();
285   }
286   return;
287 }
288 
289 :(code)
290 bool is_mu_continuation(reagent/*copy*/ x) {
291   canonize_type(x);
292   return x.type && x.type->atom && x.type->value == get(Type_ordinal, "continuation");
293 }
294 
295 :(scenario continuations_can_return_values)
296 def main [
297   local-scope
298   k:continuation, 1:num/raw <- call-with-continuation-mark f
299 ]
300 def f [
301   local-scope
302   g
303 ]
304 def g [
305   local-scope
306   return-continuation-until-mark 34
307   stash [continuation called]
308 ]
309 # entering main
310 +mem: new alloc: 1000
311 +run: {k: "continuation"}, {1: "number", "raw": ()} <- call-with-continuation-mark {f: "recipe-literal"}
312 # entering f
313 +mem: new alloc: 1004
314 # entering g
315 +mem: new alloc: 1007
316 # return control to main
317 +run: return-continuation-until-mark {34: "literal"}
318 # no allocs abandoned yet
319 +mem: storing 34 in location 1
320 # end of main
321 # make sure no memory leaks..
322 +mem: trying to reclaim local k:continuation
323 +mem: automatically abandoning 1007
324 +mem: automatically abandoning 1004
325 +mem: automatically abandoning 1000
326 # ..even though we never called the continuation
327 -app: continuation called