carbon-lang/executable_semantics/syntax/paren_contents.h

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#ifndef EXECUTABLE_SEMANTICS_SYNTAX_PAREN_CONTENTS_H_
#define EXECUTABLE_SEMANTICS_SYNTAX_PAREN_CONTENTS_H_

#include <optional>
#include <string>
#include <vector>

#include "executable_semantics/common/error.h"

namespace Carbon {

// Represents the syntactic contents of an expression or pattern delimited by
// parentheses. In those syntaxes, parentheses can be used either for grouping
// or for forming a tuple, depending on their context and the syntax of their
// contents; this class helps calling code resolve that ambiguity. Since that
// ambiguity is purely syntactic, this class should only be needed during
// parsing.
//
// `Term` is the type of the syntactic grouping being built, and the type of
// the individual syntactic units it's built from; typically it should be
// either `Expression` or `Pattern`.
template <typename Term>
struct ParenContents {
  struct Element {
    std::optional<std::string> name;
    const Term* term;
  };

  // If this object represents a single term, with no name and no trailing
  // comma, this method returns that term. This typically means the parentheses
  // can be interpreted as grouping.
  auto SingleTerm() const -> std::optional<const Term*>;

  // Converts `elements` to std::vector<TupleElement>. TupleElement must
  // have a constructor that takes a std::string and a const Term*.
  //
  // TODO: Find a way to deduce TupleElement from Term.
  template <typename TupleElement>
  auto TupleElements(int line_num) const -> std::vector<TupleElement>;

  std::vector<Element> elements;
  bool has_trailing_comma;
};

// Implementation details only below here.

template <typename Term>
auto ParenContents<Term>::SingleTerm() const -> std::optional<const Term*> {
  if (elements.size() == 1 && !elements.front().name.has_value() &&
      !has_trailing_comma) {
    return elements.front().term;
  } else {
    return std::nullopt;
  }
}

template <typename Term>
template <typename TupleElement>
auto ParenContents<Term>::TupleElements(int line_num) const
    -> std::vector<TupleElement> {
  std::vector<TupleElement> result;
  int i = 0;
  bool seen_named_member = false;
  for (auto element : elements) {
    if (element.name.has_value()) {
      seen_named_member = true;
      result.push_back(TupleElement(*element.name, element.term));
    } else {
      if (seen_named_member) {
        FATAL_PROGRAM_ERROR(line_num)
            << "positional members must come before named members";
      }
      result.push_back(TupleElement(std::to_string(i), element.term));
    }
    ++i;
  }
  return result;
}

}  // namespace Carbon

#endif  // EXECUTABLE_SEMANTICS_SYNTAX_PAREN_CONTENTS_H_