//: Continuations are a powerful primitive for constructing advanced kinds of
//: control *policies* like back-tracking. They're usually provided using a
//: primitive called 'call-cc': http://en.wikipedia.org/wiki/Call-with-current-continuation)
//: But in mu 'call-cc' is constructed out of a combination of two primitives:
//: 'current-continuation', which returns a continuation, and
//: 'continue-from', which takes a continuation to switch to.
//: todo: implement continuations in mu's memory
:(before "End Globals")
map<long long int, call_stack> Continuation;
long long int Next_continuation_id = 0;
:(before "End Setup")
Continuation.clear();
Next_continuation_id = 0;
:(before "End Mu Types Initialization")
type_ordinal continuation = Type_ordinal["continuation"] = Next_type_ordinal++;
Type[continuation].name = "continuation";
:(before "End Primitive Recipe Declarations")
CURRENT_CONTINUATION,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["current-continuation"] = CURRENT_CONTINUATION;
:(before "End Primitive Recipe Implementations")
case CURRENT_CONTINUATION: {
// copy the current call stack
Continuation[Next_continuation_id] = Current_routine->calls; // deep copy because calls have no pointers
// make sure calling the copy doesn't spawn the same continuation again
++Continuation[Next_continuation_id].front().running_step_index;
products.resize(1);
products.at(0).push_back(Next_continuation_id);
++Next_continuation_id;
trace("current-continuation") << "new continuation " << Next_continuation_id;
break;
}
:(before "End Primitive Recipe Declarations")
CONTINUE_FROM,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["continue-from"] = CONTINUE_FROM;
:(before "End Primitive Recipe Implementations")
case CONTINUE_FROM: {
assert(scalar(ingredients.at(0)));
long long int c = ingredients.at(0).at(0);
Current_routine->calls = Continuation[c]; // deep copy; calls have no pointers
continue; // skip rest of this instruction
}
:(scenario continuation)
# simulate a loop using continuations
recipe main [
1:number <- copy 0:literal
2:continuation <- current-continuation
{
#? $print 1:number
3:boolean <- greater-or-equal 1:number, 3:literal
break-if 3:boolean
1:number <- add 1:number, 1:literal
continue-from 2:continuation # loop
}
]
+mem: storing 1 in location 1
+mem: storing 2 in location 1
+mem: storing 3 in location 1
-mem: storing 4 in location 1
# ensure every iteration doesn't copy the stack over and over
$current-continuation: 1
:(scenario continuation_inside_caller)
#? % Trace_stream->dump_layer = "all"; #? 1
recipe main [
1:number <- copy 0:literal
2:continuation <- loop-body
{
3:boolean <- greater-or-equal 1:number, 3:literal
break-if 3:boolean
continue-from 2:continuation # loop
}
]
recipe loop-body [
4:continuation <- current-continuation
1:number <- add 1:number, 1:literal
]
+mem: storing 1 in location 1
+mem: storing 2 in location 1
+mem: storing 3 in location 1
-mem: storing 4 in location 1
//:: A variant of continuations is the 'delimited' continuation that can be called like any other recipe.
//:
//: In mu, this is constructed out of two primitives:
//:
//: * 'create-delimited-continuation' lays down a mark on the call
//: stack and calls the provided function (it is sometimes called 'prompt')
//: * 'reply-current-continuation' copies the top of the stack until the
//: mark, and returns it as the response of create-delimited-continuation
//: (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
//: function.
:(scenario delimited_continuation)
recipe main [
1:continuation <- create-delimited-continuation f:recipe 12:literal # 12 is an argument to f
2:number <- copy 5:literal
{
2:number <- call 1:continuation, 2:number # 2 is an argument to g, the 'top' of the continuation
3:boolean <- greater-or-equal 2:number, 8:literal
break-if 3:boolean
loop
}
]
recipe f [
11:number <- next-ingredient
12:number <- g 11:number
reply 12:number
]
recipe g [
21:number <- next-ingredient
rewind-ingredients
reply-delimited-continuation
# calls of the continuation start from here
22:number <- next-ingredient
23:number <- add 22:number, 1:literal
reply 23:number
]
#? ?
# first call of 'g' executes the part before reply-delimited-continuation
+mem: storing 12 in location 21
+run: 2:number <- copy 5:literal
+mem: storing 5 in location 2
# calls of the continuation execute the part after reply-delimited-continuation
+run: 2:number <- call 1:continuation, 2:number
+mem: storing 5 in location 22
+mem: storing 6 in location 2
+run: 2:number <- call 1:continuation, 2:number
+mem: storing 6 in location 22
+mem: storing 7 in location 2
+run: 2:number <- call 1:continuation, 2:number
+mem: storing 7 in location 22
+mem: storing 8 in location 2
# first call of 'g' does not execute the part after reply-delimited-continuation
-mem: storing 12 in location 22
# calls of the continuation don't execute the part before reply-delimited-continuation
-mem: storing 5 in location 21
-mem: storing 6 in location 21
-mem: storing 7 in location 21
# termination
-mem: storing 9 in location 2
//: 'create-delimited-continuation' is like 'call' except it adds a 'reset' mark to
//: the call stack
:(before "End call Fields")
bool is_reset;
:(before "End call Constructor")
is_reset = false;
//: like call, but mark the current call as a 'reset' call before pushing the next one on it
:(before "End Primitive Recipe Declarations")
CREATE_DELIMITED_CONTINUATION,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["create-delimited-continuation"] = CREATE_DELIMITED_CONTINUATION;
:(before "End Primitive Recipe Implementations")
case CREATE_DELIMITED_CONTINUATION: {
Current_routine->calls.front().is_reset = true;
Current_routine->calls.push_front(call(Recipe_ordinal[current_instruction().ingredients.at(0).name]));
ingredients.erase(ingredients.begin()); // drop the callee
goto complete_call;
}
//: save the slice of current call stack until the 'create-delimited-continuation'
//: call, and return it as the result.
//: todo: implement delimited continuations in mu's memory
:(before "End Globals")
map<long long int, call_stack> Delimited_continuation;
long long int Next_delimited_continuation_id = 0;
:(before "End Setup")
Delimited_continuation.clear();
Next_delimited_continuation_id = 0;
:(before "End Primitive Recipe Declarations")
REPLY_DELIMITED_CONTINUATION,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["reply-delimited-continuation"] = REPLY_DELIMITED_CONTINUATION;
:(before "End Primitive Recipe Implementations")
case REPLY_DELIMITED_CONTINUATION: {
// first clear any existing ingredients, to isolate the creation of the
// continuation from its calls
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 'reset' call
call_stack::iterator find_reset(call_stack& c); // manual prototype containing '::'
call_stack::iterator reset = find_reset(Current_routine->calls);
assert(reset != Current_routine->calls.end());
Delimited_continuation[Next_delimited_continuation_id] = call_stack(Current_routine->calls.begin(), reset);
while (Current_routine->calls.begin() != reset) {
--Callstack_depth;
Current_routine->calls.pop_front();
}
// return it as the result of the 'reset' call
products.resize(1);
products.at(0).push_back(Next_delimited_continuation_id);
++Next_delimited_continuation_id;
break; // continue to process rest of 'reset' call
}
:(code)
call_stack::iterator find_reset(call_stack& c) {
for (call_stack::iterator p = c.begin(); p != c.end(); ++p)
if (p->is_reset) return p;
return c.end();
}
//: overload 'call' for continuations
:(after "case CALL:")
if (current_instruction().ingredients.at(0).properties.at(0).second.at(0) == "continuation") {
// copy multiple calls on to current call stack
assert(scalar(ingredients.at(0)));
assert(Delimited_continuation.find(ingredients.at(0).at(0)) != Delimited_continuation.end());
const call_stack& new_calls = Delimited_continuation[ingredients.at(0).at(0)];
for (call_stack::const_reverse_iterator p = new_calls.rbegin(); p != new_calls.rend(); ++p)
Current_routine->calls.push_front(*p);
++current_step_index(); // skip past the reply-delimited-continuation
ingredients.erase(ingredients.begin()); // drop the callee
goto complete_call;
}