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33679a8445
We decided that we want to move away from throwing exceptions in AOs and regular functions implicitly and then returning some default-constructed value (usually an empty JS::Value) - this requires remembering to check for an exception at the call site, which is error-prone. It's also awkward for return values that cannot be default-constructed, e.g. MarkedValueList. Instead, the thrown value should be part of the function return value. The solution to this is moving closer to the spec and using something they call "completion records": https://tc39.es/ecma262/#sec-completion-record-specification-type This has various advantages: - It becomes crystal clear whether some AO can fail or not, and errors need to be handled and values unwrapped explicitly (for that reason compatibility with the TRY() macro is already built-in, and a similar TRY_OR_DISCARD() macro has been added specifically for use in LibJS, while the majority of functions doesn't return ThrowCompletionOr yet) - We no longer need to mix "valid" and "invalid" values of various types for the success and exception outcomes (e.g. null/non-null AK::String, empty/non-empty JS::Value) - Subsequently it's no longer possible to accidentally use an exception outcome return value as a success outcome return value (e.g. any AO that returns a numeric type would return 0 even after throwing an exception, at least before we started making use of Optional for that) - Eventually the VM will no longer need to store an exception, and temporarily clearing an exception (e.g. to call a function) becomes obsolete - instead, completions will simply propagate up to the caller outside of LibJS, which then can deal with it in any way - Similar to throw we'll be able to implement the functionality of break, continue, and return using completions, which will lead to easier to understand code and fewer workarounds - the current unwinding mechanism is not even remotely similar to the spec's approach The spec's NormalCompletion and ThrowCompletion AOs have been implemented as simple wrappers around the JS::Completion constructor. UpdateEmpty has been implemented as a JS::Completion method. There's also a new VM::throw_completion<T>() helper, which basically works like VM::throw_exception<T>() - it creates a T object (usually a JS::Error), and returns it wrapped in a JS::Completion of Type::Throw. Two temporary usage patterns have emerged: 1. Callee already returns ThrowCompletionOr, but caller doesn't: auto foo = TRY_OR_DISCARD(bar()); 2. Caller already returns ThrowCompletionOr, but callee doesn't: auto foo = bar(); if (auto* exception = vm.exception()) return throw_completion(exception->value()); Eventually all error handling and unwrapping can be done with just TRY() or possibly even operator? in the future :^) Co-authored-by: Andreas Kling <kling@serenityos.org> |
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| .. | ||
| Bytecode | ||
| Heap | ||
| Runtime | ||
| Tests | ||
| AST.cpp | ||
| AST.h | ||
| CMakeLists.txt | ||
| Console.cpp | ||
| Console.h | ||
| Forward.h | ||
| Interpreter.cpp | ||
| Interpreter.h | ||
| Lexer.cpp | ||
| Lexer.h | ||
| MarkupGenerator.cpp | ||
| MarkupGenerator.h | ||
| Module.cpp | ||
| Module.h | ||
| Parser.cpp | ||
| Parser.h | ||
| Script.cpp | ||
| Script.h | ||
| SourceRange.h | ||
| SourceTextModule.cpp | ||
| SourceTextModule.h | ||
| SyntaxHighlighter.cpp | ||
| SyntaxHighlighter.h | ||
| Token.cpp | ||
| Token.h | ||