carbon-lang/explorer/ast/static_scope.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 CARBON_EXPLORER_AST_STATIC_SCOPE_H_
#define CARBON_EXPLORER_AST_STATIC_SCOPE_H_

#include <functional>
#include <string>
#include <unordered_map>
#include <variant>
#include <vector>

#include "common/check.h"
#include "common/error.h"
#include "explorer/ast/ast_node.h"
#include "explorer/ast/value_category.h"
#include "explorer/common/nonnull.h"
#include "explorer/common/source_location.h"
#include "llvm/Support/Error.h"

namespace Carbon {

class Value;

// The placeholder name exposed by anonymous ValueNodes.
static constexpr std::string_view AnonymousName = "_";

// ImplementsValueNode is true if NodeType::ImplementsCarbonValueNode
// is valid and names a type, indicating that NodeType implements the
// ValueNode interface, defined below.

template <typename NodeType, typename = void>
static constexpr bool ImplementsValueNode = false;

/*
  ValueNode is an interface implemented by AstNodes that can be associated
  with a value, such as declarations and bindings. The interface consists of
  the following methods:

  // Returns the constant associated with the node.
  // This is called by the interpreter, not the type checker.
  auto constant_value() const -> std::optional<Nonnull<const Value*>>;

  // Returns the symbolic compile-time identity of the node.
  // This is called by the type checker, not the interpreter.
  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>>;

  // Returns the static type of an IdentifierExpression that names *this.
  auto static_type() const -> const Value&;

  // Returns the value category of an IdentifierExpression that names *this.
  auto value_category() const -> ValueCategory;

  // Print the node's identity (e.g. its name).
  void PrintID(llvm::raw_ostream& out) const;

*/
// TODO: consider turning the above documentation into real code, as sketched
// at https://godbolt.org/z/186oEozhc

template <typename T>
static constexpr bool
    ImplementsValueNode<T, typename T::ImplementsCarbonValueNode> = true;

class ValueNodeView {
 public:
  template <typename NodeType,
            typename = std::enable_if_t<ImplementsValueNode<NodeType>>>
  // NOLINTNEXTLINE(google-explicit-constructor)
  ValueNodeView(Nonnull<const NodeType*> node)
      // Type-erase NodeType, retaining a pointer to the base class AstNode
      // and using std::function to encapsulate the ability to call
      // the derived class's methods.
      : base_(node),
        constant_value_(
            [](const AstNode& base) -> std::optional<Nonnull<const Value*>> {
              return llvm::cast<NodeType>(base).constant_value();
            }),
        symbolic_identity_(
            [](const AstNode& base) -> std::optional<Nonnull<const Value*>> {
              return llvm::cast<NodeType>(base).symbolic_identity();
            }),
        print_([](const AstNode& base, llvm::raw_ostream& out) -> void {
          // TODO: change this to print a summary of the node
          return llvm::cast<NodeType>(base).PrintID(out);
        }),
        static_type_([](const AstNode& base) -> const Value& {
          return llvm::cast<NodeType>(base).static_type();
        }),
        value_category_([](const AstNode& base) -> ValueCategory {
          return llvm::cast<NodeType>(base).value_category();
        }) {}

  ValueNodeView(const ValueNodeView&) = default;
  ValueNodeView(ValueNodeView&&) = default;
  auto operator=(const ValueNodeView&) -> ValueNodeView& = default;
  auto operator=(ValueNodeView&&) -> ValueNodeView& = default;

  // Returns `node` as an instance of the base class AstNode.
  auto base() const -> const AstNode& { return *base_; }

  // Returns node->constant_value()
  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
    return constant_value_(*base_);
  }

  // Returns node->symbolic_identity()
  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>> {
    return symbolic_identity_(*base_);
  }

  void Print(llvm::raw_ostream& out) const { print_(*base_, out); }

  // Returns node->static_type()
  auto static_type() const -> const Value& { return static_type_(*base_); }

  // Returns node->value_category()
  auto value_category() const -> ValueCategory {
    return value_category_(*base_);
  }

  friend auto operator==(const ValueNodeView& lhs, const ValueNodeView& rhs)
      -> bool {
    return lhs.base_ == rhs.base_;
  }

  friend auto operator!=(const ValueNodeView& lhs, const ValueNodeView& rhs)
      -> bool {
    return lhs.base_ != rhs.base_;
  }

  friend auto operator<(const ValueNodeView& lhs, const ValueNodeView& rhs)
      -> bool {
    return std::less<>()(lhs.base_, rhs.base_);
  }

 private:
  Nonnull<const AstNode*> base_;
  std::function<std::optional<Nonnull<const Value*>>(const AstNode&)>
      constant_value_;
  std::function<std::optional<Nonnull<const Value*>>(const AstNode&)>
      symbolic_identity_;
  std::function<void(const AstNode&, llvm::raw_ostream&)> print_;
  std::function<const Value&(const AstNode&)> static_type_;
  std::function<ValueCategory(const AstNode&)> value_category_;
};

// Maps the names visible in a given scope to the entities they name.
// A scope may have parent scopes, whose names will also be visible in the
// child scope.
class StaticScope {
 public:
  // The status of a name. Later enumerators with higher values correspond to
  // more completely declared names.
  enum class NameStatus {
    // The name is known to exist in this scope, and any lookups finding it
    // should be rejected because it's not declared yet.
    KnownButNotDeclared,
    // We've started processing a declaration of this name, but it's not yet
    // fully declared, so any lookups finding it should be rejected.
    DeclaredButNotUsable,
    // The name is usable in this context.
    Usable,
  };

  // Defines `name` to be `entity` in this scope, or reports a compilation error
  // if `name` is already defined to be a different entity in this scope.
  // If `usable` is `false`, `name` cannot yet be referenced and `Resolve()`
  // methods will fail for it.
  auto Add(const std::string& name, ValueNodeView entity,
           NameStatus status = NameStatus::Usable) -> ErrorOr<Success>;

  // Marks `name` as being past its point of declaration.
  void MarkDeclared(const std::string& name);
  // Marks `name` as being completely declared and hence usable.
  void MarkUsable(const std::string& name);

  // Make `parent` a parent of this scope.
  // REQUIRES: `parent` is not already a parent of this scope.
  void AddParent(Nonnull<const StaticScope*> parent) {
    parent_scopes_.push_back(parent);
  }

  // Returns the nearest definition of `name` in the ancestor graph of this
  // scope, or reports a compilation error at `source_loc` there isn't exactly
  // one such definition.
  auto Resolve(const std::string& name, SourceLocation source_loc) const
      -> ErrorOr<ValueNodeView>;

  // Returns the value node of the BindingPattern of the returned var definition
  // if it exists in the ancestor graph.
  auto ResolveReturned() const -> std::optional<ValueNodeView>;

  // Adds the value node of the BindingPattern of the returned var definition to
  // this scope. Returns a compilation error when there is an existing returned
  // var in the ancestor graph.
  auto AddReturnedVar(ValueNodeView returned_var_def_view) -> ErrorOr<Success>;

 private:
  // Equivalent to Resolve, but returns `nullopt` instead of raising an error
  // if no definition can be found. Still raises a compilation error if more
  // than one definition is found.
  auto TryResolve(const std::string& name, SourceLocation source_loc) const
      -> ErrorOr<std::optional<ValueNodeView>>;

  struct Entry {
    ValueNodeView entity;
    NameStatus status;
  };
  // Maps locally declared names to their entities.
  std::unordered_map<std::string, Entry> declared_names_;

  // A list of scopes used for name lookup within this scope.
  std::vector<Nonnull<const StaticScope*>> parent_scopes_;

  // Stores the value node of the BindingPattern of the returned var definition.
  std::optional<ValueNodeView> returned_var_def_view_;
};

}  // namespace Carbon

#endif  // CARBON_EXPLORER_AST_STATIC_SCOPE_H_