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