mirror of
https://github.com/carbon-language/carbon-lang.git
synced 2026-09-24 22:00:12 +01:00
Richard Smith
181a592b8c
When parsing a pattern, if we encounter something that isn't pattern syntax, try parsing as an expression instead. We only need one-token lookahead to distinguish pattern syntax from expression syntax. Track a precedence group through pattern parsing so that we can allow different kinds of expressions in a top-level pattern (such as the operand of `let`) and in a nested pattern (such as a subpattern of a tuple pattern or within grouping parens). For example, we do not allow `case if ...`, and for now I've chosen to also not allow logical or relational operators at the top level of a pattern, so `case 1 + 1` is OK, but `case 1 == 1` and `case true and false` require parentheses. This decision should be ratified or revisited by a design proposal. Very basic check support is also provided, only sufficient to form an `ExprPattern` instruction and nothing beyond that. For now, all pattern matching against an `ExprPattern` fails with a TODO error. To support that, I've switched from calling `BeginSubpattern` in the parent handler of a pattern and `EndSubpatternAs*` in the pattern handler itself to calling both functions in parent handlers, with `EndSubpattern` converting an expression into an expression pattern where needed. Depends on #6976. Assisted-by: Gemini via Google Antigravity
87 lines
3.2 KiB
C++
87 lines
3.2 KiB
C++
// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
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// Exceptions. See /LICENSE for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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#include "toolchain/parse/context.h"
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#include "toolchain/parse/handle.h"
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namespace Carbon::Parse {
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static auto IsBindingPatternOperator(Lex::TokenKind kind) -> bool {
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return kind == Lex::TokenKind::Colon ||
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kind == Lex::TokenKind::ColonExclaim ||
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kind == Lex::TokenKind::ColonQuestion;
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}
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auto HandlePattern(Context& context) -> void {
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auto state = context.PopState();
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switch (context.PositionKind()) {
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case Lex::TokenKind::OpenParen:
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context.PushStateForPattern(StateKind::PatternListAsTuple,
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state.in_var_pattern, state.in_unused_pattern,
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state.ambient_precedence);
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break;
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case Lex::TokenKind::Var:
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context.PushStateForPattern(StateKind::VariablePattern,
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state.in_var_pattern, state.in_unused_pattern,
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state.ambient_precedence);
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break;
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case Lex::TokenKind::Unused:
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context.PushStateForPattern(StateKind::UnusedPattern,
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state.in_var_pattern, state.in_unused_pattern,
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state.ambient_precedence);
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break;
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case Lex::TokenKind::Template:
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case Lex::TokenKind::Ref:
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context.PushStateForPattern(StateKind::BindingPattern,
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state.in_var_pattern, state.in_unused_pattern,
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state.ambient_precedence);
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break;
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case Lex::TokenKind::Identifier:
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case Lex::TokenKind::SelfValueIdentifier:
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case Lex::TokenKind::Underscore: {
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if (IsBindingPatternOperator(
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context.PositionKind(Lookahead::NextToken))) {
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context.PushStateForPattern(
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StateKind::BindingPattern, state.in_var_pattern,
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state.in_unused_pattern, state.ambient_precedence);
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break;
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}
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[[fallthrough]];
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}
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default:
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context.PushState(StateKind::ExprPattern);
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context.PushStateForExpr(state.ambient_precedence);
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break;
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}
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}
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auto HandleExprPattern(Context& context) -> void {
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auto state = context.PopState();
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// If we parsed an expression followed by a binding operator, we most likely
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// have a malformed attempt to introduce a binding pattern that we interpreted
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// as an expression pattern, so diagnose that here rather than diagnosing a
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// missing `;` at an outer level.
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if (IsBindingPatternOperator(context.PositionKind())) {
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if (!state.has_error) {
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CARBON_DIAGNOSTIC(ExpectedBindingName, Error,
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"unexpected expression before {0} in binding pattern",
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Lex::TokenKind);
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// TODO: Underline the parsed expression.
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context.emitter().Emit(*context.position(), ExpectedBindingName,
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context.PositionKind());
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state.has_error = true;
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}
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context.Consume();
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// It'd be nice to skip the type expression here too, but we can't determine
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// the end of it.
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}
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if (state.has_error) {
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context.ReturnErrorOnState();
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}
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}
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} // namespace Carbon::Parse
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