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Experimental control-flow operator (#368)

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* AST and syntax for delimited control

* stashing for later

* a little more progress

* progress on delimited continuations

* delimit, suspend, and resume implemented (draft)

* example that generates the natural numbers

* fixes

* tinkering

* changed demo to experimental

* comments and name changes

* describe delimited continuations in the README

* renamed Snapshot to Continuation, edits to comments

* Update executable_semantics/ast/statement.h

improve comment for MakeDelimitStmt

Co-authored-by: Dave Abrahams <dabrahams@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

remove snake_case

Co-authored-by: Dave Abrahams <dabrahams@google.com>

* edits to comments, change name of variable

* updates to handle review edits

* trailing whitespace

* fixes to delimited continuations, added more tests, also fixed assignment to do a copy

* improvements from Geoffrey

* new test from Geoffrey, fix for empty blocks

* more suggestions from Geoffrey

* more tests for delimited continuations, renaming some of them

* renamed test files

* improve a comment

* sketch of creating continuation

* initial implementation of shift/reset style continuations

* more documentation

* fix some camel case

* implemented deep copy of continuations, added a test case for it

* fixed a bug and got the recursive test case working

* removed __delimit, polished up __continuation

* back to shallow copy for continuations

* suggestions from Geoffrey

* removed structured binding (for now)

* Update executable_semantics/ast/expression.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* responses to Geoffrey

Co-authored-by: Dave Abrahams <dabrahams@google.com>
Co-authored-by: Geoff Romer <gromer@google.com>
This commit is contained in:
Jeremy G. Siek
2021-03-26 11:22:48 -04:00
committed by GitHub
co-authored by Dave Abrahams Geoff Romer
parent 6df0d51516
commit 3fa72d2984
35 changed files with 674 additions and 31 deletions
Download Patch File Download Diff File Expand all files Collapse all files
executable_semantics/interpreter/interpreter.cpp
+98 -8
View File
@@ -7,6 +7,7 @@
#include <cassert>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <optional>
#include <utility>
@@ -83,6 +84,9 @@ auto CopyVal(const Value* val, int line_num) -> const Value* {
return MakeFunVal(*val->u.fun.name, val->u.fun.param, val->u.fun.body);
case ValKind::PtrV:
return MakePtrVal(val->u.ptr);
case ValKind::ContinuationV:
// Copying a continuation is "shallow".
return val;
case ValKind::FunctionTV:
return MakeFunTypeVal(CopyVal(val->u.fun_type.param, line_num),
CopyVal(val->u.fun_type.ret, line_num));
@@ -99,6 +103,8 @@ auto CopyVal(const Value* val, int line_num) -> const Value* {
return MakeVarTypeVal(*val->u.var_type);
case ValKind::AutoTV:
return MakeAutoTypeVal();
case ValKind::ContinuationTV:
return MakeContinuationTypeVal();
case ValKind::TupleTV: {
auto new_fields = new VarValues();
for (auto& field : *val->u.tuple_type.fields) {
@@ -199,8 +205,10 @@ void PrintState(std::ostream& out) {
PrintStack(state->stack, out);
out << std::endl << "heap: ";
PrintHeap(state->heap, out);
out << std::endl << "env: ";
PrintEnv(CurrentEnv(state), out);
if (!state->stack.IsEmpty() && !state->stack.Top()->scopes.IsEmpty()) {
out << std::endl << "env: ";
PrintEnv(CurrentEnv(state), out);
}
out << std::endl << "}" << std::endl;
}
@@ -242,6 +250,20 @@ auto ValToPtr(const Value* v, int line_num) -> Address {
}
}
// Returns *continuation represented as a list of frames.
//
// - Precondition: continuation->tag == ValKind::ContinuationV.
auto ContinuationToVector(const Value* continuation, int sourceLocation)
-> std::vector<Frame*> {
if (continuation->tag == ValKind::ContinuationV) {
return *continuation->u.continuation.stack;
} else {
std::cerr << sourceLocation << ": runtime error: expected an integer"
<< std::endl;
exit(-1);
}
}
auto EvalPrim(Operator op, const std::vector<const Value*>& args, int line_num)
-> const Value* {
switch (op) {
@@ -278,7 +300,7 @@ void InitGlobals(std::list<Declaration>* fs) {
auto ChoiceDeclaration::InitGlobals(Env& globals) const -> void {
auto alts = new VarValues();
for (auto kv : alternatives) {
auto t = ToType(line_num, InterpExp(Env(), kv.second));
auto t = ToType(this->line_num, InterpExp(Env(), kv.second));
alts->push_back(make_pair(kv.first, t));
}
auto ct = MakeChoiceTypeVal(name, alts);
@@ -496,7 +518,7 @@ auto PatternMatch(const Value* p, const Value* v, Env env,
void PatternAssignment(const Value* pat, const Value* val, int line_num) {
switch (pat->tag) {
case ValKind::PtrV:
state->heap[ValToPtr(pat, line_num)] = val;
state->heap[ValToPtr(pat, line_num)] = CopyVal(val, line_num);
break;
case ValKind::TupleV: {
switch (val->tag) {
@@ -618,6 +640,7 @@ void StepLvalue() {
case ExpressionKind::TypeT:
case ExpressionKind::FunctionT:
case ExpressionKind::AutoT:
case ExpressionKind::ContinuationT:
case ExpressionKind::PatternVariable: {
frame->todo.Pop();
frame->todo.Push(MakeExpToLvalAct());
@@ -743,6 +766,12 @@ void StepExp() {
act->pos++;
break;
}
case ExpressionKind::ContinuationT: {
const Value* v = MakeContinuationTypeVal();
frame->todo.Pop(1);
frame->todo.Push(MakeValAct(v));
break;
}
} // switch (exp->tag)
}
@@ -826,10 +855,14 @@ void StepStmt() {
break;
case StatementKind::Block: {
if (act->pos == -1) {
auto* scope = new Scope(CurrentEnv(state), std::list<std::string>());
frame->scopes.Push(scope);
frame->todo.Push(MakeStmtAct(stmt->u.block.stmt));
act->pos++;
if (stmt->u.block.stmt) {
auto* scope = new Scope(CurrentEnv(state), {});
frame->scopes.Push(scope);
frame->todo.Push(MakeStmtAct(stmt->u.block.stmt));
act->pos++;
} else {
frame->todo.Pop();
}
} else {
Scope* scope = frame->scopes.Top();
KillScope(stmt->line_num, scope);
@@ -876,6 +909,43 @@ void StepStmt() {
}
frame->todo.Push(MakeStmtAct(stmt->u.sequence.stmt));
break;
case StatementKind::Continuation: {
// Create a continuation object by creating a frame similar the
// way one is created in a function call.
Scope* scope = new Scope(CurrentEnv(state), std::list<std::string>());
Stack<Scope*> scopes;
scopes.Push(scope);
Stack<Action*> todo;
todo.Push(
MakeStmtAct(MakeReturn(stmt->line_num, MakeUnit(stmt->line_num))));
todo.Push(MakeStmtAct(stmt->u.continuation.body));
Frame* continuation_frame = new Frame("__continuation", scopes, todo);
Address continuation_address =
AllocateValue(MakeContinuation({continuation_frame}));
// Store the continuation's address in the frame.
continuation_frame->continuation = continuation_address;
// Bind the continuation object to the continuation variable
frame->scopes.Top()->env.Set(*stmt->u.continuation.continuation_variable,
continuation_address);
// Pop the continuation statement.
frame->todo.Pop();
break;
}
case StatementKind::Run:
// Evaluate the argument of the run statement.
frame->todo.Push(MakeExpAct(stmt->u.run.argument));
act->pos++;
break;
case StatementKind::Await:
// Pause the current continuation
frame->todo.Pop();
std::vector<Frame*> paused;
do {
paused.push_back(state->stack.Pop());
} while (!paused.back()->IsContinuation());
// Update the continuation with the paused stack.
state->heap[paused.back()->continuation] = MakeContinuation(paused);
break;
}
}
@@ -1156,6 +1226,7 @@ void HandleValue() {
case ExpressionKind::BoolT:
case ExpressionKind::TypeT:
case ExpressionKind::AutoT:
case ExpressionKind::ContinuationT:
std::cerr << "internal error, bad expression context in handle_value"
<< std::endl;
exit(-1);
@@ -1311,6 +1382,25 @@ void HandleValue() {
frame->todo.Push(MakeValAct(ret_val));
break;
}
case StatementKind::Run: {
frame->todo.Pop(2);
// Push an expression statement action to ignore the result
// value from the continuation.
Action* ignore_result = MakeStmtAct(
MakeExpStmt(stmt->line_num, MakeUnit(stmt->line_num)));
ignore_result->pos = 0;
frame->todo.Push(ignore_result);
// Push the continuation onto the current stack.
std::vector<Frame*> continuation_vector =
ContinuationToVector(val_act->u.val, stmt->line_num);
for (auto frame_iter = continuation_vector.rbegin();
frame_iter != continuation_vector.rend(); ++frame_iter) {
state->stack.Push(*frame_iter);
}
break;
}
case StatementKind::Continuation:
case StatementKind::Await:
case StatementKind::Block:
case StatementKind::Sequence:
case StatementKind::Break: