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//: Continuations are a powerful primitive for constructing advanced kinds of //: control *policies* like back-tracking. //: //: In Mu, continuations are first-class and delimited, and are constructed //: out of two primitives: //: //: * 'call-with-continuation-mark' marks the top of the call stack and then //: calls the provided recipe. //: * 'return-continuation-until-mark' copies the top of the stack //: until the mark, and returns it as the result of //: 'call-with-continuation-mark' (which might be a distant ancestor on the //: call stack; intervening calls don't return) //: //: The resulting slice of the stack can now be called just like a regular //: recipe. //: //: See the example programs continuation*.mu to get a sense for the //: possibilities. //: //: Refinements: //: * You can call a single continuation multiple times, and it will preserve //: the state of its local variables at each stack frame between calls. //: The stack frames of a continuation are not destroyed until the //: continuation goes out of scope. See continuation2.mu. //: * 'return-continuation-until-mark' doesn't consume the mark, so you can //: return multiple continuations based on a single mark. In combination //: with the fact that 'return-continuation-until-mark' can return from //: regular calls, just as long as there was an earlier call to //: 'call-with-continuation-mark', this gives us a way to create resumable //: recipes. See continuation3.mu. //: * 'return-continuation-until-mark' can take ingredients to return just //: like other 'return' instructions. It just implicitly also returns a //: continuation as the first result. See continuation4.mu. //: * Conversely, you can pass ingredients to a continuation when calling it, //: to make it available to products of 'return-continuation-until-mark'. //: See continuation5.mu. //: * There can be multiple continuation marks on the stack at once; //: 'call-with-continuation-mark' and 'return-continuation-until-mark' both //: need to pass in a tag to coordinate on the correct mark. This allows us //: to save multiple continuations for different purposes (say if one is //: for exceptions) with overlapping stack frames. See exception.mu. //: //: Inspired by James and Sabry, "Yield: Mainstream delimited continuations", //: Workshop on the Theory and Practice of Delimited Continuations, 2011. //: https://www.cs.indiana.edu/~sabry/papers/yield.pdf //: //: Caveats: //: * At the moment we can't statically type-check continuations. So we raise //: runtime errors for a call that doesn't return a continuation when the //: caller expects, or one that returns a continuation when the caller //: doesn't expect it. This shouldn't cause memory corruption, though. //: There should still be no way to lookup addresses that aren't allocated. :(before "End Mu Types Initialization") type_ordinal continuation = Type_ordinal["continuation"] = Next_type_ordinal++; Type[continuation].name = "continuation"; //: A continuation can be called like a recipe. :(before "End is_mu_recipe Atom Cases(r)") if (r.type->name == "continuation") return true; //: However, it can't be type-checked like most recipes. Pretend it's like a //: header-less recipe. :(after "Begin Reagent->Recipe(r, recipe_header)") if (r.type->atom && r.type->name == "continuation") { result_header.has_header = false; return result_header; } :(code) void test_delimited_continuation() { run( "recipe main [\n" " 1:continuation <- call-with-continuation-mark 233/mark, f, 77\n" // 77 is an argument to f " 2:num <- copy 5\n" " {\n" " 2:num <- call 1:continuation, 2:num\n" // jump to 'return-continuation-until-mark' below " 3:bool <- greater-or-equal 2:num, 8\n" " break-if 3:bool\n" " loop\n" " }\n" "]\n" "recipe f [\n" " 11:num <- next-ingredient\n" " 12:num <- g 11:num\n" " return 12:num\n" "]\n" "recipe g [\n" " 21:num <- next-ingredient\n" " 22:num <- return-continuation-until-mark 233/mark\n" " 23:num <- add 22:num, 1\n" " return 23:num\n" "]\n" ); // first call of 'g' executes the part before return-continuation-until-mark CHECK_TRACE_CONTENTS( // first call of 'g' executes the part before return-continuation-until-mark "mem: storing 77 in location 21\n" "run: {2: \"number\"} <- copy {5: \"literal\"}\n" "mem: storing 5 in location 2\n" // calls of the continuation execute the part after return-continuation-until-mark "run: {2: \"number\"} <- call {1: \"continuation\"}, {2: \"number\"}\n" "mem: storing 5 in location 22\n" "mem: storing 6 in location 2\n" "run: {2: \"number\"} <- call {1: \"continuation\"}, {2: \"number\"}\n" "mem: storing 6 in location 22\n" "mem: storing 7 in location 2\n" "run: {2: \"number\"} <- call {1: \"continuation\"}, {2: \"number\"}\n" "mem: storing 7 in location 22\n" "mem: storing 8 in location 2\n" ); // first call of 'g' does not execute the part after return-continuation-until-mark CHECK_TRACE_DOESNT_CONTAIN("mem: storing 77 in location 22"); // calls of the continuation don't execute the part before return-continuation-until-mark CHECK_TRACE_DOESNT_CONTAIN("mem: storing 5 in location 21"); CHECK_TRACE_DOESNT_CONTAIN("mem: storing 6 in location 21"); CHECK_TRACE_DOESNT_CONTAIN("mem: storing 7 in location 21"); // termination CHECK_TRACE_DOESNT_CONTAIN("mem: storing 9 in location 2"); } :(before "End call Fields") int continuation_mark_tag; :(before "End call Constructor") continuation_mark_tag = 0; :(before "End Primitive Recipe Declarations") CALL_WITH_CONTINUATION_MARK, :(before "End Primitive Recipe Numbers") Recipe_ordinal["call-with-continuation-mark"] = CALL_WITH_CONTINUATION_MARK; :(before "End Primitive Recipe Checks") case CALL_WITH_CONTINUATION_MARK: { if (SIZE(inst.ingredients) < 2) { 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(); } break; } :(before "End Primitive Recipe Implementations") case CALL_WITH_CONTINUATION_MARK: { // like call, but mark the current call as a 'base of continuation' call // before pushing the next one on it trace(Callstack_depth+1, "trace") << "delimited continuation; incrementing callstack depth to " << Callstack_depth << end(); ++Callstack_depth; assert(Callstack_depth < Max_depth); instruction/*copy*/ caller_instruction = current_instruction(); Current_routine->calls.front().continuation_mark_tag = current_instruction().ingredients.at(0).value; Current_routine->calls.push_front(call(ingredients.at(1).at(0))); // drop the mark caller_instruction.ingredients.erase(caller_instruction.ingredients.begin()); ingredients.erase(ingredients.begin()); // drop the callee caller_instruction.ingredients.erase(caller_instruction.ingredients.begin()); ingredients.erase(ingredients.begin()); finish_call_housekeeping(caller_instruction, ingredients); continue; } :(code) void test_next_ingredient_inside_continuation() { run( "recipe main [\n" " call-with-continuation-mark 233/mark, f, true\n" "]\n" "recipe f [\n" " 10:bool <- next-input\n" "]\n" ); CHECK_TRACE_CONTENTS( "mem: storing 1 in location 10\n" ); } void test_delimited_continuation_out_of_recipe_variable() { run( "recipe main [\n" " x:recipe <- copy f\n" " call-with-continuation-mark 233/mark, x, true\n" "]\n" "recipe f [\n" " 10:bool <- next-input\n" "]\n" ); CHECK_TRACE_CONTENTS( "mem: storing 1 in location 10\n" ); } //: save the slice of current call stack until the 'call-with-continuation-mark' //: call, and return it as the result. //: todo: implement delimited continuations in Mu's memory :(before "End Types") struct delimited_continuation { call_stack frames; int nrefs; delimited_continuation(call_stack::iterator begin, call_stack::iterator end) :frames(call_stack(begin, end)), nrefs(0) {} }; :(before "End Globals") map<long long int, delimited_continuation> Delimited_continuation; long long int Next_delimited_continuation_id = 1; // 0 is null just like an address :(before "End Reset") Delimited_continuation.clear(); Next_delimited_continuation_id = 1; :(before "End Primitive Recipe Declarations") RETURN_CONTINUATION_UNTIL_MARK, :(before "End Primitive Recipe Numbers") Recipe_ordinal["return-continuation-until-mark"] = RETURN_CONTINUATION_UNTIL_MARK; :(before "End Primitive Recipe Checks") case RETURN_CONTINUATION_UNTIL_MARK: { if (inst.ingredients.empty()) { raise << maybe(get(Recipe, r).name) << "'" << to_original_string(inst) << "' requires at least one ingredient: a mark tag (number)\n" << end(); } break; } :(before "End Primitive Recipe Implementations") case RETURN_CONTINUATION_UNTIL_MARK: { // I don't know how to think about next-ingredient in combination with // continuations, so seems cleaner to just kill it. Functions have to read // their inputs before ever returning a continuation. Current_routine->calls.front().ingredient_atoms.clear(); Current_routine->calls.front().next_ingredient_to_process = 0; // copy the current call stack until the most recent marked call call_stack::iterator find_base_of_continuation(call_stack&, int); // manual prototype containing '::' call_stack::iterator base = find_base_of_continuation(Current_routine->calls, /*mark tag*/current_instruction().ingredients.at(0).value); if (base == Current_routine->calls.end()) { 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(); raise << maybe(current_recipe_name()) << "call stack:\n" << end(); for (call_stack::iterator p = Current_routine->calls.begin(); p != Current_routine->calls.end(); ++p) raise << maybe(current_recipe_name()) << " " << get(Recipe, p->running_recipe).name << '\n' << end(); break; } trace(Callstack_depth+1, "run") << "creating continuation " << Next_delimited_continuation_id << end(); put(Delimited_continuation, Next_delimited_continuation_id, delimited_continuation(Current_routine->calls.begin(), base)); while (Current_routine->calls.begin() != base) { --Callstack_depth; assert(Callstack_depth >= 0); Current_routine->calls.pop_front(); } // return it as the result of the marked call products.resize(1); products.at(0).push_back(Next_delimited_continuation_id); // return any other ingredients passed in copy(/*skip mark tag*/++ingredients.begin(), ingredients.end(), inserter(products, products.end())); ++Next_delimited_continuation_id; break; // continue to process rest of marked call } :(code) call_stack::iterator find_base_of_continuation(call_stack& c, int mark_tag) { for (call_stack::iterator p = c.begin(); p != c.end(); ++p) if (p->continuation_mark_tag == mark_tag) return p; return c.end(); } //: overload 'call' for continuations :(after "Begin Call") if (is_mu_continuation(current_instruction().ingredients.at(0))) { // copy multiple calls on to current call stack assert(scalar(ingredients.at(0))); trace(Callstack_depth+1, "run") << "calling continuation " << ingredients.at(0).at(0) << end(); if (!contains_key(Delimited_continuation, ingredients.at(0).at(0))) raise << maybe(current_recipe_name()) << "no such delimited continuation " << current_instruction().ingredients.at(0).original_string << '\n' << end(); const call_stack& new_frames = get(Delimited_continuation, ingredients.at(0).at(0)).frames; for (call_stack::const_reverse_iterator p = new_frames.rbegin(); p != new_frames.rend(); ++p) Current_routine->calls.push_front(*p); trace(Callstack_depth+1, "trace") << "calling delimited continuation; growing callstack depth to " << Callstack_depth+SIZE(new_frames) << end(); Callstack_depth += SIZE(new_frames); assert(Callstack_depth < Max_depth); // no call housekeeping; continuations don't support next-ingredient copy(/*drop continuation*/++ingredients.begin(), ingredients.end(), inserter(products, products.begin())); break; // record results of resuming 'return-continuation-until-mark' instruction } :(code) void test_continuations_can_return_values() { run( "def main [\n" " local-scope\n" " k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f\n" "]\n" "def f [\n" " local-scope\n" " g\n" "]\n" "def g [\n" " local-scope\n" " return-continuation-until-mark 233/mark, 34\n" " stash [continuation called]\n" "]\n" ); CHECK_TRACE_CONTENTS( "mem: storing 34 in location 1\n" ); } void test_continuations_continue_to_matching_mark() { run( "def main [\n" " local-scope\n" " k:continuation, 1:num/raw <- call-with-continuation-mark 233/mark, f\n" " add 1, 1\n" "]\n" "def f [\n" " local-scope\n" " k2:continuation <- call-with-continuation-mark 234/mark, g\n" " add 2, 2\n" "]\n" "def g [\n" " local-scope\n" " return-continuation-until-mark 233/mark, 34\n" " stash [continuation called]\n" "]\n" ); CHECK_TRACE_CONTENTS( "run: add {1: \"literal\"}, {1: \"literal\"}\n" ); CHECK_TRACE_DOESNT_CONTAIN("run: add {2: \"literal\"}, {2: \"literal\"}"); } //: Allow shape-shifting recipes to return continuations. void test_call_shape_shifting_recipe_with_continuation_mark() { run( "def main [\n" " 1:num <- call-with-continuation-mark 233/mark, f, 34\n" "]\n" "def f x:_elem -> y:_elem [\n" " local-scope\n" " load-ingredients\n" " y <- copy x\n" "]\n" ); CHECK_TRACE_CONTENTS( "mem: storing 34 in location 1\n" ); } :(before "End resolve_ambiguous_call(r, index, inst, caller_recipe) Special-cases") if (inst.name == "call-with-continuation-mark") { if (SIZE(inst.ingredients) > 1 && is_recipe_literal(inst.ingredients.at(/*skip mark*/1))) { resolve_indirect_continuation_call(r, index, inst, caller_recipe); return; } } :(code) void resolve_indirect_continuation_call(const recipe_ordinal r, int index, instruction& inst, const recipe& caller_recipe) { instruction inst2; inst2.name = inst.ingredients.at(/*skip mark*/1).name; for (int i = /*skip mark and recipe*/2; i < SIZE(inst.ingredients); ++i) inst2.ingredients.push_back(inst.ingredients.at(i)); for (int i = /*skip continuation*/1; i < SIZE(inst.products); ++i) inst2.products.push_back(inst.products.at(i)); resolve_ambiguous_call(r, index, inst2, caller_recipe); inst.ingredients.at(/*skip mark*/1).name = inst2.name; inst.ingredients.at(/*skip mark*/1).set_value(get(Recipe_ordinal, inst2.name)); } void test_call_shape_shifting_recipe_with_continuation_mark_and_no_outputs() { run( "def main [\n" " 1:continuation <- call-with-continuation-mark 233/mark, f, 34\n" "]\n" "def f x:_elem [\n" " local-scope\n" " load-ingredients\n" " return-continuation-until-mark 233/mark\n" "]\n" ); CHECK_TRACE_COUNT("error", 0); } void test_continuation1() { run( "def main [\n" " local-scope\n" " k:continuation <- call-with-continuation-mark 233/mark, create-yielder\n" " 10:num/raw <- call k\n" "]\n" "def create-yielder -> n:num [\n" " local-scope\n" " load-inputs\n" " return-continuation-until-mark 233/mark\n" " return 1\n" "]\n" ); CHECK_TRACE_CONTENTS( "mem: storing 1 in location 10\n" ); CHECK_TRACE_COUNT("error", 0); } :(code) bool is_mu_continuation(reagent/*copy*/ x) { canonize_type(x); return x.type && x.type->atom && x.type->value == get(Type_ordinal, "continuation"); } // helper for debugging void dump(const int continuation_id) { if (!contains_key(Delimited_continuation, continuation_id)) { raise << "missing delimited continuation: " << continuation_id << '\n' << end(); return; } delimited_continuation& curr = get(Delimited_continuation, continuation_id); dump(curr.frames); }


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