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//! Boa's implementation of ECMAScript's global `Generator` object.
//!
//! A Generator is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.
//!
//! More information:
//! - [ECMAScript reference][spec]
//! - [MDN documentation][mdn]
//!
//! [spec]: https://tc39.es/ecma262/#sec-generator-objects
//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator
use crate::{
builtins::iterable::create_iter_result_object,
context::intrinsics::Intrinsics,
error::JsNativeError,
js_string,
object::{JsObject, CONSTRUCTOR},
property::Attribute,
realm::Realm,
string::common::StaticJsStrings,
symbol::JsSymbol,
value::JsValue,
vm::{CallFrame, CallFrameFlags, CompletionRecord, GeneratorResumeKind},
Context, JsArgs, JsData, JsError, JsResult, JsString,
};
use boa_gc::{custom_trace, Finalize, Trace};
use boa_profiler::Profiler;
use super::{BuiltInBuilder, IntrinsicObject};
/// Indicates the state of a generator.
#[derive(Debug, Clone, Finalize)]
pub(crate) enum GeneratorState {
SuspendedStart {
/// The `[[GeneratorContext]]` internal slot.
context: GeneratorContext,
},
SuspendedYield {
/// The `[[GeneratorContext]]` internal slot.
context: GeneratorContext,
},
Executing,
Completed,
}
// Need to manually implement, since `Trace` adds a `Drop` impl which disallows destructuring.
unsafe impl Trace for GeneratorState {
custom_trace!(this, mark, {
match &this {
Self::SuspendedStart { context } | Self::SuspendedYield { context } => mark(context),
Self::Executing | Self::Completed => {}
}
});
}
/// Holds all information that a generator needs to continue it's execution.
///
/// All of the fields must be changed with those that are currently present in the
/// context/vm before the generator execution starts/resumes and after it has ended/yielded.
#[derive(Debug, Clone, Trace, Finalize)]
pub(crate) struct GeneratorContext {
pub(crate) stack: Vec<JsValue>,
pub(crate) call_frame: Option<CallFrame>,
}
impl GeneratorContext {
/// Creates a new `GeneratorContext` from the current `Context` state.
pub(crate) fn from_current(context: &mut Context) -> Self {
let mut frame = context.vm.frame().clone();
frame.environments = context.vm.environments.clone();
frame.realm = context.realm().clone();
let fp = frame.fp() as usize;
let stack = context.vm.stack.split_off(fp);
frame.rp = CallFrame::FUNCTION_PROLOGUE + frame.argument_count;
// NOTE: Since we get a pre-built call frame with stack, and we reuse them.
// So we don't need to push the registers in subsequent calls.
frame.flags |= CallFrameFlags::REGISTERS_ALREADY_PUSHED;
Self {
call_frame: Some(frame),
stack,
}
}
/// Resumes execution with `GeneratorContext` as the current execution context.
pub(crate) fn resume(
&mut self,
value: Option<JsValue>,
resume_kind: GeneratorResumeKind,
context: &mut Context,
) -> CompletionRecord {
std::mem::swap(&mut context.vm.stack, &mut self.stack);
let frame = self.call_frame.take().expect("should have a call frame");
let rp = frame.rp;
context.vm.push_frame(frame);
context.vm.frame_mut().rp = rp;
context.vm.frame_mut().set_exit_early(true);
if let Some(value) = value {
context.vm.push(value);
}
context.vm.push(resume_kind);
let result = context.run();
std::mem::swap(&mut context.vm.stack, &mut self.stack);
self.call_frame = context.vm.pop_frame();
assert!(self.call_frame.is_some());
result
}
/// Returns the async generator object, if the function that this [`GeneratorContext`] is from an async generator, [`None`] otherwise.
pub(crate) fn async_generator_object(&self) -> Option<JsObject> {
self.call_frame
.as_ref()
.and_then(|frame| frame.async_generator_object(&self.stack))
}
}
/// The internal representation of a `Generator` object.
#[derive(Debug, Finalize, Trace, JsData)]
pub struct Generator {
/// The `[[GeneratorState]]` internal slot.
pub(crate) state: GeneratorState,
}
impl IntrinsicObject for Generator {
fn init(realm: &Realm) {
let _timer = Profiler::global().start_event(std::any::type_name::<Self>(), "init");
BuiltInBuilder::with_intrinsic::<Self>(realm)
.prototype(
realm
.intrinsics()
.objects()
.iterator_prototypes()
.iterator(),
)
.static_method(Self::next, js_string!("next"), 1)
.static_method(Self::r#return, js_string!("return"), 1)
.static_method(Self::throw, js_string!("throw"), 1)
.static_property(
JsSymbol::to_string_tag(),
Self::NAME,
Attribute::CONFIGURABLE,
)
.static_property(
CONSTRUCTOR,
realm
.intrinsics()
.constructors()
.generator_function()
.prototype(),
Attribute::CONFIGURABLE,
)
.build();
}
fn get(intrinsics: &Intrinsics) -> JsObject {
intrinsics.objects().generator()
}
}
impl Generator {
const NAME: JsString = StaticJsStrings::GENERATOR;
/// `Generator.prototype.next ( value )`
///
/// The `next()` method returns an object with two properties done and value.
/// You can also provide a parameter to the next method to send a value to the generator.
///
/// More information:
/// - [ECMAScript reference][spec]
/// - [MDN documentation][mdn]
///
/// [spec]: https://tc39.es/ecma262/#sec-generator.prototype.next
/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator/next
pub(crate) fn next(
this: &JsValue,
args: &[JsValue],
context: &mut Context,
) -> JsResult<JsValue> {
// 1. Return ? GeneratorResume(this value, value, empty).
Self::generator_resume(this, args.get_or_undefined(0).clone(), context)
}
/// `Generator.prototype.return ( value )`
///
/// The `return()` method returns the given value and finishes the generator.
///
/// More information:
/// - [ECMAScript reference][spec]
/// - [MDN documentation][mdn]
///
/// [spec]: https://tc39.es/ecma262/#sec-generator.prototype.return
/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator/return
pub(crate) fn r#return(
this: &JsValue,
args: &[JsValue],
context: &mut Context,
) -> JsResult<JsValue> {
// 1. Let g be the this value.
// 2. Let C be Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
// 3. Return ? GeneratorResumeAbrupt(g, C, empty).
Self::generator_resume_abrupt(this, Ok(args.get_or_undefined(0).clone()), context)
}
/// `Generator.prototype.throw ( exception )`
///
/// The `throw()` method resumes the execution of a generator by throwing an error into it
/// and returns an object with two properties done and value.
///
/// More information:
/// - [ECMAScript reference][spec]
/// - [MDN documentation][mdn]
///
/// [spec]: https://tc39.es/ecma262/#sec-generator.prototype.throw
/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator/throw
pub(crate) fn throw(
this: &JsValue,
args: &[JsValue],
context: &mut Context,
) -> JsResult<JsValue> {
// 1. Let g be the this value.
// 2. Let C be ThrowCompletion(exception).
// 3. Return ? GeneratorResumeAbrupt(g, C, empty).
Self::generator_resume_abrupt(
this,
Err(JsError::from_opaque(args.get_or_undefined(0).clone())),
context,
)
}
/// `27.5.3.3 GeneratorResume ( generator, value, generatorBrand )`
///
/// More information:
/// - [ECMAScript reference][spec]
///
/// [spec]: https://tc39.es/ecma262/#sec-generatorresume
pub(crate) fn generator_resume(
gen: &JsValue,
value: JsValue,
context: &mut Context,
) -> JsResult<JsValue> {
// 1. Let state be ? GeneratorValidate(generator, generatorBrand).
let Some(generator_obj) = gen.as_object() else {
return Err(JsNativeError::typ()
.with_message("Generator method called on non generator")
.into());
};
let mut gen = generator_obj.downcast_mut::<Self>().ok_or_else(|| {
JsNativeError::typ().with_message("generator resumed on non generator object")
})?;
// 4. Let genContext be generator.[[GeneratorContext]].
// 5. Let methodContext be the running execution context.
// 6. Suspend methodContext.
// 7. Set generator.[[GeneratorState]] to executing.
let (mut generator_context, first_execution) =
match std::mem::replace(&mut gen.state, GeneratorState::Executing) {
GeneratorState::Executing => {
return Err(JsNativeError::typ()
.with_message("Generator should not be executing")
.into());
}
// 2. If state is completed, return CreateIterResultObject(undefined, true).
GeneratorState::Completed => {
gen.state = GeneratorState::Completed;
return Ok(create_iter_result_object(
JsValue::undefined(),
true,
context,
));
}
// 3. Assert: state is either suspendedStart or suspendedYield.
GeneratorState::SuspendedStart { context } => (context, true),
GeneratorState::SuspendedYield { context } => (context, false),
};
drop(gen);
let record = generator_context.resume(
(!first_execution).then_some(value),
GeneratorResumeKind::Normal,
context,
);
let mut gen = generator_obj
.downcast_mut::<Self>()
.expect("already checked this object type");
// 8. Push genContext onto the execution context stack; genContext is now the running execution context.
// 9. Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
// 10. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
// 11. Return Completion(result).
match record {
CompletionRecord::Return(value) => {
gen.state = GeneratorState::SuspendedYield {
context: generator_context,
};
Ok(value)
}
CompletionRecord::Normal(value) => {
gen.state = GeneratorState::Completed;
Ok(create_iter_result_object(value, true, context))
}
CompletionRecord::Throw(err) => {
gen.state = GeneratorState::Completed;
Err(err)
}
}
}
/// `27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand )`
///
/// More information:
/// - [ECMAScript reference][spec]
///
/// [spec]: https://tc39.es/ecma262/#sec-generatorresumeabrupt
pub(crate) fn generator_resume_abrupt(
gen: &JsValue,
abrupt_completion: JsResult<JsValue>,
context: &mut Context,
) -> JsResult<JsValue> {
// 1. Let state be ? GeneratorValidate(generator, generatorBrand).
let Some(generator_obj) = gen.as_object() else {
return Err(JsNativeError::typ()
.with_message("Generator method called on non generator")
.into());
};
let mut gen = generator_obj.downcast_mut::<Self>().ok_or_else(|| {
JsNativeError::typ().with_message("generator resumed on non generator object")
})?;
// 4. Assert: state is suspendedYield.
// 5. Let genContext be generator.[[GeneratorContext]].
// 6. Let methodContext be the running execution context.
// 7. Suspend methodContext.
// 8. Set generator.[[GeneratorState]] to executing.
let mut generator_context =
match std::mem::replace(&mut gen.state, GeneratorState::Executing) {
GeneratorState::Executing => {
return Err(JsNativeError::typ()
.with_message("Generator should not be executing")
.into());
}
// 2. If state is suspendedStart, then
// 3. If state is completed, then
GeneratorState::SuspendedStart { .. } | GeneratorState::Completed => {
// a. Set generator.[[GeneratorState]] to completed.
gen.state = GeneratorState::Completed;
// b. Once a generator enters the completed state it never leaves it and its
// associated execution context is never resumed. Any execution state associated
// with generator can be discarded at this point.
// a. If abruptCompletion.[[Type]] is return, then
if let Ok(value) = abrupt_completion {
// i. Return CreateIterResultObject(abruptCompletion.[[Value]], true).
let value = create_iter_result_object(value, true, context);
return Ok(value);
}
// b. Return Completion(abruptCompletion).
return abrupt_completion;
}
GeneratorState::SuspendedYield { context } => context,
};
// 9. Push genContext onto the execution context stack; genContext is now the running execution context.
// 10. Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
// 11. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
// 12. Return Completion(result).
drop(gen);
let (value, resume_kind) = match abrupt_completion {
Ok(value) => (value, GeneratorResumeKind::Return),
Err(err) => (err.to_opaque(context), GeneratorResumeKind::Throw),
};
let record = generator_context.resume(Some(value), resume_kind, context);
let mut gen = generator_obj.downcast_mut::<Self>().ok_or_else(|| {
JsNativeError::typ().with_message("generator resumed on non generator object")
})?;
match record {
CompletionRecord::Return(value) => {
gen.state = GeneratorState::SuspendedYield {
context: generator_context,
};
Ok(value)
}
CompletionRecord::Normal(value) => {
gen.state = GeneratorState::Completed;
Ok(create_iter_result_object(value, true, context))
}
CompletionRecord::Throw(err) => {
gen.state = GeneratorState::Completed;
Err(err)
}
}
}
}