Store named constant values in the AST (#1011)

This enables us to stop using `Env` in the typechecker. As a byproduct, this commit also restructures the interpreter to handle run-time global initialization as part of ordinary execution, using the Action stack.

executable_semantics/ast/declaration.h

@@ -108,11 +108,22 @@ class GenericBinding : public AstNode {
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Let; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return constant_value_;
+  }
+
+  // Sets the value returned by constant_value(). Can only be called once,
+  // during typechecking.
+  void set_constant_value(Nonnull<const Value*> value) {
+    CHECK(!constant_value_.has_value());
+    constant_value_ = value;
+  }
 
  private:
   std::string name_;
   Nonnull<Expression*> type_;
   std::optional<Nonnull<const Value*>> static_type_;
+  std::optional<Nonnull<const Value*>> constant_value_;
 };
 
 // The syntactic representation of a function declaration's return type.
@@ -232,6 +243,16 @@ class FunctionDeclaration : public Declaration {
   auto body() -> std::optional<Nonnull<Block*>> { return body_; }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Let; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return constant_value_;
+  }
+
+  // Sets the value returned by constant_value(). Can only be called once,
+  // during typechecking.
+  void set_constant_value(Nonnull<const Value*> value) {
+    CHECK(!constant_value_.has_value());
+    constant_value_ = value;
+  }
 
  private:
   std::string name_;
@@ -239,6 +260,7 @@ class FunctionDeclaration : public Declaration {
   Nonnull<TuplePattern*> param_pattern_;
   ReturnTerm return_term_;
   std::optional<Nonnull<Block*>> body_;
+  std::optional<Nonnull<const Value*>> constant_value_;
 };
 
 class ClassDeclaration : public Declaration {
@@ -259,10 +281,21 @@ class ClassDeclaration : public Declaration {
   auto members() const -> llvm::ArrayRef<Nonnull<Member*>> { return members_; }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Let; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return constant_value_;
+  }
+
+  // Sets the value returned by constant_value(). Can only be called once,
+  // during typechecking.
+  void set_constant_value(Nonnull<const Value*> value) {
+    CHECK(!constant_value_.has_value());
+    constant_value_ = value;
+  }
 
  private:
   std::string name_;
   std::vector<Nonnull<Member*>> members_;
+  std::optional<Nonnull<const Value*>> constant_value_;
 };
 
 class AlternativeSignature : public AstNode {
@@ -312,10 +345,21 @@ class ChoiceDeclaration : public Declaration {
   }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Let; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return constant_value_;
+  }
+
+  // Sets the value returned by constant_value(). Can only be called once,
+  // during typechecking.
+  void set_constant_value(Nonnull<const Value*> value) {
+    CHECK(!constant_value_.has_value());
+    constant_value_ = value;
+  }
 
  private:
   std::string name_;
   std::vector<Nonnull<AlternativeSignature*>> alternatives_;
+  std::optional<Nonnull<const Value*>> constant_value_;
 };
 
 // Global variable definition implements the Declaration concept.

executable_semantics/ast/pattern.h

@@ -61,6 +61,7 @@ class Pattern : public AstNode {
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
   // The value of this pattern. Cannot be called before typechecking.
+  // TODO rename to avoid confusion with BindingPattern::constant_value
   auto value() const -> const Value& { return **value_; }
 
   // Sets the value of this pattern. Can only be called once, during
@@ -122,6 +123,10 @@ class BindingPattern : public Pattern {
 
   auto value_category() const -> ValueCategory { return ValueCategory::Var; }
 
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
+
  private:
   std::string name_;
   Nonnull<Pattern*> type_;

executable_semantics/ast/statement.h

@@ -344,6 +344,9 @@ class Continuation : public Statement {
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Var; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
 
  private:
   std::string name_;

executable_semantics/ast/static_scope.h

@@ -37,6 +37,10 @@ static constexpr std::string_view AnonymousName = "_";
   // Returns the name of an IdentifierExpression that names *this. If *this
   // is anonymous, returns AnonymousName.
   auto name() const -> std::string_view;
+
+  // If *this names a compile-time constant whose value is known, returns that
+  // value. Otherwise returns std::nullopt.
+  auto constant_value() const -> std::optional<Nonnull<const Value*>>;
 */
 // NodeType must be derived from AstNode.
 //
@@ -70,7 +74,11 @@ class NamedEntityView {
         }),
         value_category_([](const AstNode& base) -> ValueCategory {
           return llvm::cast<NodeType>(base).value_category();
-        }) {
+        }),
+        constant_value_(
+            [](const AstNode& base) -> std::optional<Nonnull<const Value*>> {
+              return llvm::cast<NodeType>(base).constant_value();
+            }) {
     CHECK(node->name() != AnonymousName)
         << "Entity with no name used as NamedEntity: " << *node;
   }
@@ -94,6 +102,11 @@ class NamedEntityView {
     return value_category_(*base_);
   }
 
+  // Returns node->constant_value()
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return constant_value_(*base_);
+  }
+
   friend auto operator==(const NamedEntityView& lhs, const NamedEntityView& rhs)
       -> bool {
     return lhs.base_ == rhs.base_;
@@ -114,6 +127,8 @@ class NamedEntityView {
   std::function<std::string_view(const AstNode&)> name_;
   std::function<const Value&(const AstNode&)> static_type_;
   std::function<ValueCategory(const AstNode&)> value_category_;
+  std::function<std::optional<Nonnull<const Value*>>(const AstNode&)>
+      constant_value_;
 };
 
 // Maps the names visible in a given scope to the entities they name.

executable_semantics/interpreter/BUILD

@@ -130,6 +130,7 @@ cc_library(
         ":stack",
         "//common:check",
         "//common:ostream",
+        "//executable_semantics/ast",
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
         "//executable_semantics/ast:pattern",

executable_semantics/interpreter/action.cpp

@@ -55,6 +55,9 @@ void Action::Print(llvm::raw_ostream& out) const {
     case Action::Kind::StatementAction:
       cast<StatementAction>(*this).statement().PrintDepth(1, out);
       break;
+    case Action::Kind::DeclarationAction:
+      cast<DeclarationAction>(*this).declaration().Print(out);
+      break;
     case Action::Kind::ScopeAction:
       out << "ScopeAction";
   }

executable_semantics/interpreter/action.h

@@ -65,6 +65,7 @@ class Action {
     ExpressionAction,
     PatternAction,
     StatementAction,
+    DeclarationAction,
     ScopeAction,
   };
 
@@ -203,6 +204,24 @@ class StatementAction : public Action {
   Nonnull<const Statement*> statement_;
 };
 
+// Action which implements the run-time effects of executing a Declaration.
+// Does not produce a result.
+class DeclarationAction : public Action {
+ public:
+  explicit DeclarationAction(Nonnull<const Declaration*> declaration)
+      : Action(Kind::DeclarationAction), declaration_(declaration) {}
+
+  static auto classof(const Action* action) -> bool {
+    return action->kind() == Kind::DeclarationAction;
+  }
+
+  // The Declaration this Action executes.
+  auto declaration() const -> const Declaration& { return *declaration_; }
+
+ private:
+  Nonnull<const Declaration*> declaration_;
+};
+
 // Action which does nothing except introduce a new scope into the action
 // stack. This is useful when a distinct scope doesn't otherwise have an
 // Action it can naturally be associated with. ScopeActions are not associated

executable_semantics/interpreter/action_stack.cpp

@@ -17,10 +17,10 @@ void ActionStack::Print(llvm::raw_ostream& out) const {
   }
 }
 
-void ActionStack::Start(std::unique_ptr<Action> action, Scope scope) {
+void ActionStack::Start(std::unique_ptr<Action> action) {
   result_ = std::nullopt;
+  CHECK(todo_.IsEmpty());
   todo_ = {};
-  todo_.Push(std::make_unique<ScopeAction>(std::move(scope)));
   todo_.Push(std::move(action));
 }
 
@@ -30,7 +30,7 @@ auto ActionStack::CurrentScope() const -> Scope& {
       return *action->scope();
     }
   }
-  FATAL() << "No current scope";
+  return globals_;
 }
 
 void ActionStack::FinishAction() {
@@ -39,12 +39,12 @@ void ActionStack::FinishAction() {
     case Action::Kind::ExpressionAction:
     case Action::Kind::LValAction:
     case Action::Kind::PatternAction:
-      FATAL() << "This kind of action must produce a result.";
+      FATAL() << "This kind of action must produce a result: " << *act;
     case Action::Kind::ScopeAction:
       FATAL() << "ScopeAction at top of stack";
     case Action::Kind::StatementAction:
+    case Action::Kind::DeclarationAction:
       PopScopes();
-      CHECK(!IsEmpty());
   }
 }
 
@@ -52,7 +52,8 @@ void ActionStack::FinishAction(Nonnull<const Value*> result) {
   std::unique_ptr<Action> act = todo_.Pop();
   switch (act->kind()) {
     case Action::Kind::StatementAction:
-      FATAL() << "Statements cannot produce results.";
+    case Action::Kind::DeclarationAction:
+      FATAL() << "This kind of Action cannot produce results: " << *act;
     case Action::Kind::ScopeAction:
       FATAL() << "ScopeAction at top of stack";
     case Action::Kind::ExpressionAction:

executable_semantics/interpreter/action_stack.h

@@ -19,14 +19,17 @@ namespace Carbon {
 class ActionStack {
  public:
   // Constructs an empty ActionStack
-  ActionStack() = default;
+  explicit ActionStack(Scope globals) : globals_(std::move(globals)) {}
 
   void Print(llvm::raw_ostream& out) const;
   LLVM_DUMP_METHOD void Dump() const { Print(llvm::errs()); }
 
-  // Starts execution with `action` at the top of the stack, in the given scope.
-  // `action` must be an `ExpressionAction` or `PatternAction`.
-  void Start(std::unique_ptr<Action> action, Scope scope);
+  // Returns an Env containing the currently-defined global variables.
+  auto GlobalEnv() const -> Env { return globals_.values(); }
+
+  // Starts execution with `action` at the top of the stack. Cannot be called
+  // when IsEmpty() is false.
+  void Start(std::unique_ptr<Action> action);
 
   // True if the stack is empty.
   auto IsEmpty() const -> bool { return todo_.IsEmpty(); }
@@ -97,6 +100,7 @@ class ActionStack {
   // TODO: consider defining a non-nullable unique_ptr-like type to use here.
   Stack<std::unique_ptr<Action>> todo_;
   std::optional<Nonnull<const Value*>> result_;
+  mutable Scope globals_;
 };
 
 }  // namespace Carbon

executable_semantics/interpreter/exec_program.cpp

@@ -41,8 +41,7 @@ void ExecProgram(Nonnull<Arena*> arena, AST ast, bool trace) {
     }
     llvm::outs() << "********** starting execution **********\n";
   }
-  int result =
-      Interpreter(arena, trace).InterpProgram(ast.declarations, *ast.main_call);
+  int result = Interpreter(arena, trace).InterpProgram(ast);
   llvm::outs() << "result: " << result << "\n";
 }
 

executable_semantics/interpreter/interpreter.cpp

@@ -96,80 +96,6 @@ auto Interpreter::EvalPrim(Operator op,
   }
 }
 
-void Interpreter::InitEnv(const Declaration& d, Env* env) {
-  switch (d.kind()) {
-    case DeclarationKind::FunctionDeclaration: {
-      const auto& func_def = cast<FunctionDeclaration>(d);
-      Env new_env = *env;
-      // Bring the deduced parameters into scope.
-      for (Nonnull<const GenericBinding*> deduced :
-           func_def.deduced_parameters()) {
-        AllocationId a =
-            heap_.AllocateValue(arena_->New<VariableType>(deduced));
-        new_env.Set(deduced->name(), a);
-      }
-      Nonnull<const FunctionValue*> f = arena_->New<FunctionValue>(&func_def);
-      AllocationId a = heap_.AllocateValue(f);
-      env->Set(func_def.name(), a);
-      break;
-    }
-
-    case DeclarationKind::ClassDeclaration: {
-      const auto& class_decl = cast<ClassDeclaration>(d);
-      std::vector<NamedValue> fields;
-      std::vector<NamedValue> methods;
-      for (Nonnull<const Member*> m : class_decl.members()) {
-        switch (m->kind()) {
-          case MemberKind::FieldMember: {
-            const BindingPattern& binding = cast<FieldMember>(*m).binding();
-            const Expression& type_expression =
-                cast<ExpressionPattern>(binding.type()).expression();
-            auto type = InterpExp(Env(arena_), &type_expression);
-            fields.push_back({.name = binding.name(), .value = type});
-            break;
-          }
-        }
-      }
-      auto st = arena_->New<NominalClassType>(
-          class_decl.name(), std::move(fields), std::move(methods));
-      AllocationId a = heap_.AllocateValue(st);
-      env->Set(class_decl.name(), a);
-      break;
-    }
-
-    case DeclarationKind::ChoiceDeclaration: {
-      const auto& choice = cast<ChoiceDeclaration>(d);
-      std::vector<NamedValue> alts;
-      for (Nonnull<const AlternativeSignature*> alternative :
-           choice.alternatives()) {
-        auto t = InterpExp(Env(arena_), &alternative->signature());
-        alts.push_back({.name = alternative->name(), .value = t});
-      }
-      auto ct = arena_->New<ChoiceType>(choice.name(), std::move(alts));
-      AllocationId a = heap_.AllocateValue(ct);
-      env->Set(choice.name(), a);
-      break;
-    }
-
-    case DeclarationKind::VariableDeclaration: {
-      const auto& var = cast<VariableDeclaration>(d);
-      // Adds an entry in `globals` mapping the variable's name to the
-      // result of evaluating the initializer.
-      Nonnull<const Value*> v =
-          Convert(InterpExp(*env, &var.initializer()), &var.static_type());
-      AllocationId a = heap_.AllocateValue(v);
-      env->Set(var.binding().name(), a);
-      break;
-    }
-  }
-}
-
-void Interpreter::InitGlobals(llvm::ArrayRef<Nonnull<Declaration*>> fs) {
-  for (const auto d : fs) {
-    InitEnv(*d, &globals_);
-  }
-}
-
 auto Interpreter::CreateStruct(const std::vector<FieldInitializer>& fields,
                                const std::vector<Nonnull<const Value*>>& values)
     -> Nonnull<const Value*> {
@@ -521,6 +447,11 @@ void Interpreter::StepExp() {
           << "Identifier '" << exp << "' at " << exp.source_loc()
           << " was not resolved";
       // { {x :: C, E, F} :: S, H} -> { {H(E(x)) :: C, E, F} :: S, H}
+      if (std::optional<Nonnull<const Value*>> value =
+              ident.named_entity().constant_value();
+          value.has_value()) {
+        return todo_.FinishAction(*value);
+      }
       Address pointer = GetFromEnv(exp.source_loc(), ident.name());
       return todo_.FinishAction(heap_.Read(pointer, exp.source_loc()));
     }
@@ -579,7 +510,7 @@ void Interpreter::StepExp() {
                              exp.source_loc());
             CHECK(matches.has_value())
                 << "internal error in call_function, pattern match failed";
-            Scope new_scope(globals_, &heap_);
+            Scope new_scope(todo_.GlobalEnv(), &heap_);
             for (const auto& [name, value] : *matches) {
               new_scope.AddLocal(name, value);
             }
@@ -948,6 +879,32 @@ void Interpreter::StepStmt() {
   }
 }
 
+void Interpreter::StepDeclaration() {
+  Action& act = todo_.CurrentAction();
+  const Declaration& decl = cast<DeclarationAction>(act).declaration();
+  if (trace_) {
+    llvm::outs() << "--- step declaration (" << decl.source_loc() << ") --->\n";
+  }
+  switch (decl.kind()) {
+    case DeclarationKind::VariableDeclaration: {
+      const auto& var_decl = cast<VariableDeclaration>(decl);
+      if (act.pos() == 0) {
+        return todo_.Spawn(
+            std::make_unique<ExpressionAction>(&var_decl.initializer()));
+      } else {
+        todo_.CurrentScope().AddLocal(var_decl.binding().name(),
+                                      heap_.AllocateValue(act.results()[0]));
+        return todo_.FinishAction();
+      }
+    }
+    case DeclarationKind::FunctionDeclaration:
+    case DeclarationKind::ClassDeclaration:
+    case DeclarationKind::ChoiceDeclaration:
+      // These declarations have no run-time effects.
+      return todo_.FinishAction();
+  }
+}
+
 // State transition.
 void Interpreter::Step() {
   Action& act = todo_.CurrentAction();
@@ -964,62 +921,65 @@ void Interpreter::Step() {
     case Action::Kind::StatementAction:
       StepStmt();
       break;
+    case Action::Kind::DeclarationAction:
+      StepDeclaration();
+      break;
     case Action::Kind::ScopeAction:
       FATAL() << "ScopeAction escaped ActionStack";
   }  // switch
 }
 
-auto Interpreter::ExecuteAction(std::unique_ptr<Action> action, Env values,
-                                bool trace_steps) -> Nonnull<const Value*> {
-  todo_.Start(std::move(action), Scope(values, &heap_));
-
+void Interpreter::RunAllSteps(bool trace_steps) {
   while (!todo_.IsEmpty()) {
     Step();
     if (trace_steps) {
       PrintState(llvm::outs());
     }
   }
-
-  // Clean up any remaining suspended continuations.
-  for (Nonnull<ContinuationValue::StackFragment*> fragment : stack_fragments_) {
-    fragment->Clear();
-  }
-
-  return todo_.result();
 }
 
-auto Interpreter::InterpProgram(llvm::ArrayRef<Nonnull<Declaration*>> fs,
-                                Nonnull<const Expression*> call_main) -> int {
-  // Check that the interpreter is in a clean state.
-  CHECK(globals_.IsEmpty());
-  CHECK(todo_.IsEmpty());
-
+auto Interpreter::InterpProgram(const AST& ast) -> int {
   if (trace_) {
     llvm::outs() << "********** initializing globals **********\n";
   }
-  InitGlobals(fs);
+
+  for (Nonnull<Declaration*> declaration : ast.declarations) {
+    todo_.Start(std::make_unique<DeclarationAction>(declaration));
+    RunAllSteps(trace_);
+  }
 
   if (trace_) {
     llvm::outs() << "********** calling main function **********\n";
     PrintState(llvm::outs());
   }
 
-  return cast<IntValue>(
-             *ExecuteAction(std::make_unique<ExpressionAction>(call_main),
-                            globals_, trace_))
-      .value();
+  todo_.Start(std::make_unique<ExpressionAction>(*ast.main_call));
+  RunAllSteps(trace_);
+
+  // Clean up any remaining suspended continuations.
+  for (Nonnull<ContinuationValue::StackFragment*> fragment : stack_fragments_) {
+    fragment->Clear();
+  }
+
+  return cast<IntValue>(*todo_.result()).value();
 }
 
-auto Interpreter::InterpExp(Env values, Nonnull<const Expression*> e)
+auto Interpreter::RunCompileTimeAction(std::unique_ptr<Action> action)
     -> Nonnull<const Value*> {
-  return ExecuteAction(std::make_unique<ExpressionAction>(e), values,
-                       /*trace_steps=*/false);
+  todo_.Start(std::move(action));
+  RunAllSteps(/*trace_steps=*/false);
+  CHECK(stack_fragments_.empty());
+  return todo_.result();
 }
 
-auto Interpreter::InterpPattern(Env values, Nonnull<const Pattern*> p)
+auto Interpreter::InterpExp(Nonnull<const Expression*> e)
     -> Nonnull<const Value*> {
-  return ExecuteAction(std::make_unique<PatternAction>(p), values,
-                       /*trace_steps=*/false);
+  return RunCompileTimeAction(std::make_unique<ExpressionAction>(e));
+}
+
+auto Interpreter::InterpPattern(Nonnull<const Pattern*> p)
+    -> Nonnull<const Value*> {
+  return RunCompileTimeAction(std::make_unique<PatternAction>(p));
 }
 
 }  // namespace Carbon

executable_semantics/interpreter/interpreter.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "common/ostream.h"
+#include "executable_semantics/ast/ast.h"
 #include "executable_semantics/ast/declaration.h"
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/pattern.h"
@@ -24,19 +25,19 @@ namespace Carbon {
 class Interpreter {
  public:
   explicit Interpreter(Nonnull<Arena*> arena, bool trace)
-      : arena_(arena), globals_(arena), heap_(arena), trace_(trace) {}
+      : arena_(arena),
+        heap_(arena),
+        todo_(Scope(Env(arena_), &heap_)),
+        trace_(trace) {}
 
   // Interpret the whole program.
-  auto InterpProgram(llvm::ArrayRef<Nonnull<Declaration*>> fs,
-                     Nonnull<const Expression*> call_main) -> int;
+  auto InterpProgram(const AST& ast) -> int;
 
   // Interpret an expression at compile-time.
-  auto InterpExp(Env values, Nonnull<const Expression*> e)
-      -> Nonnull<const Value*>;
+  auto InterpExp(Nonnull<const Expression*> e) -> Nonnull<const Value*>;
 
   // Interpret a pattern at compile-time.
-  auto InterpPattern(Env values, Nonnull<const Pattern*> p)
-      -> Nonnull<const Value*>;
+  auto InterpPattern(Nonnull<const Pattern*> p) -> Nonnull<const Value*>;
 
   // Attempts to match `v` against the pattern `p`. If matching succeeds,
   // returns the bindings of pattern variables to their matched values.
@@ -48,7 +49,6 @@ class Interpreter {
     return heap_.AllocateValue(v);
   }
 
-  void InitEnv(const Declaration& d, Env* env);
   void PrintEnv(Env values, llvm::raw_ostream& out);
 
  private:
@@ -62,12 +62,17 @@ class Interpreter {
   void StepPattern();
   // State transition for statements.
   void StepStmt();
+  // State transition for declarations.
+  void StepDeclaration();
 
-  void InitGlobals(llvm::ArrayRef<Nonnull<Declaration*>> fs);
   auto CurrentEnv() -> Env;
   auto GetFromEnv(SourceLocation source_loc, const std::string& name)
       -> Address;
 
+  // Calls Step() repeatedly until there are no steps left to execute. Produces
+  // trace output if trace_steps is true.
+  void RunAllSteps(bool trace_steps);
+
   auto CreateStruct(const std::vector<FieldInitializer>& fields,
                     const std::vector<Nonnull<const Value*>>& values)
       -> Nonnull<const Value*>;
@@ -82,22 +87,17 @@ class Interpreter {
 
   void PrintState(llvm::raw_ostream& out);
 
-  // Runs `action` in a scope consisting of `values`, and returns the result.
-  // `action` must produce a result. In other words, it must not be a
-  // StatementAction or ScopeAction.
-  //
-  // TODO: consider whether to use this->trace_ rather than a separate
-  // trace_steps parameter.
-  auto ExecuteAction(std::unique_ptr<Action> action, Env values,
-                     bool trace_steps) -> Nonnull<const Value*>;
+  // Runs `action` in an environment where the given constants are defined, and
+  // returns the result. `action` must produce a result. In other words, it must
+  // not be a StatementAction, ScopeAction, or DeclarationAction. Can only be
+  // called at compile time (before InterpProgram), and while `todo_` is empty.
+  auto RunCompileTimeAction(std::unique_ptr<Action> action)
+      -> Nonnull<const Value*>;
 
   Nonnull<Arena*> arena_;
 
-  // Globally-defined entities, such as functions, structs, or choices.
-  Env globals_;
-
-  ActionStack todo_;
   Heap heap_;
+  ActionStack todo_;
 
   // The underlying states of continuation values. All StackFragments created
   // during execution are tracked here, in order to safely deallocate the

executable_semantics/interpreter/type_checker.cpp

@@ -390,23 +390,22 @@ auto TypeChecker::Substitute(
   }
 }
 
-void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
+void TypeChecker::TypeCheckExp(Nonnull<Expression*> e) {
   if (trace_) {
-    llvm::outs() << "checking expression " << *e << "\nvalues: ";
-    interpreter_.PrintEnv(values, llvm::outs());
+    llvm::outs() << "checking expression " << *e << "\nconstants: ";
+    PrintConstants(llvm::outs());
     llvm::outs() << "\n";
   }
   switch (e->kind()) {
     case ExpressionKind::IndexExpression: {
       auto& index = cast<IndexExpression>(*e);
-      TypeCheckExp(&index.aggregate(), values);
+      TypeCheckExp(&index.aggregate());
       const Value& aggregate_type = index.aggregate().static_type();
       switch (aggregate_type.kind()) {
         case Value::Kind::TupleValue: {
           const auto& tuple_type = cast<TupleValue>(aggregate_type);
           int i =
-              cast<IntValue>(*interpreter_.InterpExp(values, &index.offset()))
-                  .value();
+              cast<IntValue>(*interpreter_.InterpExp(&index.offset())).value();
           if (i < 0 || i >= static_cast<int>(tuple_type.elements().size())) {
             FATAL_COMPILATION_ERROR(e->source_loc())
                 << "index " << i << " is out of range for type " << tuple_type;
@@ -422,7 +421,7 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     case ExpressionKind::TupleLiteral: {
       std::vector<Nonnull<const Value*>> arg_types;
       for (auto& arg : cast<TupleLiteral>(*e).fields()) {
-        TypeCheckExp(arg, values);
+        TypeCheckExp(arg);
         arg_types.push_back(&arg->static_type());
       }
       SetStaticType(e, arena_->New<TupleValue>(std::move(arg_types)));
@@ -432,7 +431,7 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     case ExpressionKind::StructLiteral: {
       std::vector<NamedValue> arg_types;
       for (auto& arg : cast<StructLiteral>(*e).fields()) {
-        TypeCheckExp(&arg.expression(), values);
+        TypeCheckExp(&arg.expression());
         arg_types.push_back({arg.name(), &arg.expression().static_type()});
       }
       SetStaticType(e, arena_->New<StructType>(std::move(arg_types)));
@@ -442,9 +441,9 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     case ExpressionKind::StructTypeLiteral: {
       auto& struct_type = cast<StructTypeLiteral>(*e);
       for (auto& arg : struct_type.fields()) {
-        TypeCheckExp(&arg.expression(), values);
+        TypeCheckExp(&arg.expression());
         ExpectIsConcreteType(arg.expression().source_loc(),
-                             interpreter_.InterpExp(values, &arg.expression()));
+                             interpreter_.InterpExp(&arg.expression()));
       }
       if (struct_type.fields().empty()) {
         // `{}` is the type of `{}`, just as `()` is the type of `()`.
@@ -460,7 +459,7 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     }
     case ExpressionKind::FieldAccessExpression: {
       auto& access = cast<FieldAccessExpression>(*e);
-      TypeCheckExp(&access.aggregate(), values);
+      TypeCheckExp(&access.aggregate());
       const Value& aggregate_type = access.aggregate().static_type();
       switch (aggregate_type.kind()) {
         case Value::Kind::StructType: {
@@ -549,7 +548,7 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
       auto& op = cast<PrimitiveOperatorExpression>(*e);
       std::vector<Nonnull<const Value*>> ts;
       for (Nonnull<Expression*> argument : op.arguments()) {
-        TypeCheckExp(argument, values);
+        TypeCheckExp(argument);
         ts.push_back(&argument->static_type());
       }
       switch (op.op()) {
@@ -624,11 +623,11 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     }
     case ExpressionKind::CallExpression: {
       auto& call = cast<CallExpression>(*e);
-      TypeCheckExp(&call.function(), values);
+      TypeCheckExp(&call.function());
       switch (call.function().static_type().kind()) {
         case Value::Kind::FunctionType: {
           const auto& fun_t = cast<FunctionType>(call.function().static_type());
-          TypeCheckExp(&call.argument(), values);
+          TypeCheckExp(&call.argument());
           Nonnull<const Value*> parameters = &fun_t.parameters();
           Nonnull<const Value*> return_type = &fun_t.return_type();
           if (!fun_t.deduced().empty()) {
@@ -668,9 +667,9 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
     case ExpressionKind::FunctionTypeLiteral: {
       auto& fn = cast<FunctionTypeLiteral>(*e);
       ExpectIsConcreteType(fn.parameter().source_loc(),
-                           interpreter_.InterpExp(values, &fn.parameter()));
+                           interpreter_.InterpExp(&fn.parameter()));
       ExpectIsConcreteType(fn.return_type().source_loc(),
-                           interpreter_.InterpExp(values, &fn.return_type()));
+                           interpreter_.InterpExp(&fn.return_type()));
       SetStaticType(&fn, arena_->New<TypeType>());
       fn.set_value_category(ValueCategory::Let);
       return;
@@ -681,7 +680,7 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
       return;
     case ExpressionKind::IntrinsicExpression: {
       auto& intrinsic_exp = cast<IntrinsicExpression>(*e);
-      TypeCheckExp(&intrinsic_exp.args(), values);
+      TypeCheckExp(&intrinsic_exp.args());
       switch (cast<IntrinsicExpression>(*e).intrinsic()) {
         case IntrinsicExpression::Intrinsic::Print:
           if (intrinsic_exp.args().fields().size() != 1) {
@@ -710,15 +709,14 @@ void TypeChecker::TypeCheckExp(Nonnull<Expression*> e, Env values) {
 }
 
 void TypeChecker::TypeCheckPattern(
-    Nonnull<Pattern*> p, Env values,
-    std::optional<Nonnull<const Value*>> expected) {
+    Nonnull<Pattern*> p, std::optional<Nonnull<const Value*>> expected) {
   if (trace_) {
     llvm::outs() << "checking pattern " << *p;
     if (expected) {
       llvm::outs() << ", expecting " << **expected;
     }
-    llvm::outs() << "\nvalues: ";
-    interpreter_.PrintEnv(values, llvm::outs());
+    llvm::outs() << "\nconstants: ";
+    PrintConstants(llvm::outs());
     llvm::outs() << "\n";
   }
   switch (p->kind()) {
@@ -728,9 +726,8 @@ void TypeChecker::TypeCheckPattern(
     }
     case PatternKind::BindingPattern: {
       auto& binding = cast<BindingPattern>(*p);
-      TypeCheckPattern(&binding.type(), values, std::nullopt);
-      Nonnull<const Value*> type =
-          interpreter_.InterpPattern(values, &binding.type());
+      TypeCheckPattern(&binding.type(), std::nullopt);
+      Nonnull<const Value*> type = interpreter_.InterpPattern(&binding.type());
       if (expected) {
         if (IsConcreteType(type)) {
           ExpectType(p->source_loc(), "name binding", type, *expected);
@@ -742,14 +739,12 @@ void TypeChecker::TypeCheckPattern(
                 << "Type pattern '" << *type << "' does not match actual type '"
                 << **expected << "'";
           }
-          CHECK(values->begin() == values->end())
-              << "Name bindings within type patterns are unsupported";
           type = *expected;
         }
       }
       ExpectIsConcreteType(binding.source_loc(), type);
       SetStaticType(&binding, type);
-      SetValue(&binding, interpreter_.InterpPattern(values, &binding));
+      SetValue(&binding, interpreter_.InterpPattern(&binding));
       return;
     }
     case PatternKind::TuplePattern: {
@@ -769,16 +764,16 @@ void TypeChecker::TypeCheckPattern(
         if (expected) {
           expected_field_type = cast<TupleValue>(**expected).elements()[i];
         }
-        TypeCheckPattern(field, values, expected_field_type);
+        TypeCheckPattern(field, expected_field_type);
         field_types.push_back(&field->static_type());
       }
       SetStaticType(&tuple, arena_->New<TupleValue>(std::move(field_types)));
-      SetValue(&tuple, interpreter_.InterpPattern(values, &tuple));
+      SetValue(&tuple, interpreter_.InterpPattern(&tuple));
       return;
     }
     case PatternKind::AlternativePattern: {
       auto& alternative = cast<AlternativePattern>(*p);
-      TypeCheckExp(&alternative.choice_type(), values);
+      TypeCheckExp(&alternative.choice_type());
       if (alternative.choice_type().static_type().kind() !=
           Value::Kind::TypeOfChoiceType) {
         FATAL_COMPILATION_ERROR(alternative.source_loc())
@@ -799,41 +794,40 @@ void TypeChecker::TypeCheckPattern(
             << "'" << alternative.alternative_name()
             << "' is not an alternative of " << choice_type;
       }
-      TypeCheckPattern(&alternative.arguments(), values, *parameter_types);
+      TypeCheckPattern(&alternative.arguments(), *parameter_types);
       SetStaticType(&alternative, &choice_type);
-      SetValue(&alternative, interpreter_.InterpPattern(values, &alternative));
+      SetValue(&alternative, interpreter_.InterpPattern(&alternative));
       return;
     }
     case PatternKind::ExpressionPattern: {
       auto& expression = cast<ExpressionPattern>(*p).expression();
-      TypeCheckExp(&expression, values);
+      TypeCheckExp(&expression);
       SetStaticType(p, &expression.static_type());
-      SetValue(p, interpreter_.InterpPattern(values, p));
+      SetValue(p, interpreter_.InterpPattern(p));
       return;
     }
   }
 }
 
-void TypeChecker::TypeCheckStmt(Nonnull<Statement*> s, Env values) {
+void TypeChecker::TypeCheckStmt(Nonnull<Statement*> s) {
   switch (s->kind()) {
     case StatementKind::Match: {
       auto& match = cast<Match>(*s);
-      TypeCheckExp(&match.expression(), values);
+      TypeCheckExp(&match.expression());
       std::vector<Match::Clause> new_clauses;
       for (auto& clause : match.clauses()) {
-        TypeCheckPattern(&clause.pattern(), values,
-                         &match.expression().static_type());
-        TypeCheckStmt(&clause.statement(), values);
+        TypeCheckPattern(&clause.pattern(), &match.expression().static_type());
+        TypeCheckStmt(&clause.statement());
       }
       return;
     }
     case StatementKind::While: {
       auto& while_stmt = cast<While>(*s);
-      TypeCheckExp(&while_stmt.condition(), values);
+      TypeCheckExp(&while_stmt.condition());
       ExpectType(s->source_loc(), "condition of `while`",
                  arena_->New<BoolType>(),
                  &while_stmt.condition().static_type());
-      TypeCheckStmt(&while_stmt.body(), values);
+      TypeCheckStmt(&while_stmt.body());
       return;
     }
     case StatementKind::Break:
@@ -842,21 +836,21 @@ void TypeChecker::TypeCheckStmt(Nonnull<Statement*> s, Env values) {
     case StatementKind::Block: {
       auto& block = cast<Block>(*s);
       for (auto* block_statement : block.statements()) {
-        TypeCheckStmt(block_statement, values);
+        TypeCheckStmt(block_statement);
       }
       return;
     }
     case StatementKind::VariableDefinition: {
       auto& var = cast<VariableDefinition>(*s);
-      TypeCheckExp(&var.init(), values);
+      TypeCheckExp(&var.init());
       const Value& rhs_ty = var.init().static_type();
-      TypeCheckPattern(&var.pattern(), values, &rhs_ty);
+      TypeCheckPattern(&var.pattern(), &rhs_ty);
       return;
     }
     case StatementKind::Assign: {
       auto& assign = cast<Assign>(*s);
-      TypeCheckExp(&assign.rhs(), values);
-      TypeCheckExp(&assign.lhs(), values);
+      TypeCheckExp(&assign.rhs());
+      TypeCheckExp(&assign.lhs());
       ExpectType(s->source_loc(), "assign", &assign.lhs().static_type(),
                  &assign.rhs().static_type());
       if (assign.lhs().value_category() != ValueCategory::Var) {
@@ -866,23 +860,23 @@ void TypeChecker::TypeCheckStmt(Nonnull<Statement*> s, Env values) {
       return;
     }
     case StatementKind::ExpressionStatement: {
-      TypeCheckExp(&cast<ExpressionStatement>(*s).expression(), values);
+      TypeCheckExp(&cast<ExpressionStatement>(*s).expression());
       return;
     }
     case StatementKind::If: {
       auto& if_stmt = cast<If>(*s);
-      TypeCheckExp(&if_stmt.condition(), values);
+      TypeCheckExp(&if_stmt.condition());
       ExpectType(s->source_loc(), "condition of `if`", arena_->New<BoolType>(),
                  &if_stmt.condition().static_type());
-      TypeCheckStmt(&if_stmt.then_block(), values);
+      TypeCheckStmt(&if_stmt.then_block());
       if (if_stmt.else_block()) {
-        TypeCheckStmt(*if_stmt.else_block(), values);
+        TypeCheckStmt(*if_stmt.else_block());
       }
       return;
     }
     case StatementKind::Return: {
       auto& ret = cast<Return>(*s);
-      TypeCheckExp(&ret.expression(), values);
+      TypeCheckExp(&ret.expression());
       ReturnTerm& return_term = ret.function().return_term();
       if (return_term.is_auto()) {
         SetStaticType(&return_term, &ret.expression().static_type());
@@ -894,13 +888,13 @@ void TypeChecker::TypeCheckStmt(Nonnull<Statement*> s, Env values) {
     }
     case StatementKind::Continuation: {
       auto& cont = cast<Continuation>(*s);
-      TypeCheckStmt(&cont.body(), values);
+      TypeCheckStmt(&cont.body());
       SetStaticType(&cont, arena_->New<ContinuationType>());
       return;
     }
     case StatementKind::Run: {
       auto& run = cast<Run>(*s);
-      TypeCheckExp(&run.argument(), values);
+      TypeCheckExp(&run.argument());
       ExpectType(s->source_loc(), "argument of `run`",
                  arena_->New<ContinuationType>(),
                  &run.argument().static_type());
@@ -988,17 +982,16 @@ void TypeChecker::ExpectReturnOnAllPaths(
 // TODO: Add checking to function definitions to ensure that
 //   all deduced type parameters will be deduced.
 void TypeChecker::TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f,
-                                               Env values, bool check_body) {
+                                               bool check_body) {
   // Bring the deduced parameters into scope
   for (Nonnull<GenericBinding*> deduced : f->deduced_parameters()) {
-    TypeCheckExp(&deduced->type(), values);
+    TypeCheckExp(&deduced->type());
     // auto t = interpreter_.InterpExp(values, deduced.type);
     SetStaticType(deduced, arena_->New<VariableType>(deduced));
-    AllocationId a = interpreter_.AllocateValue(&deduced->static_type());
-    values.Set(deduced->name(), a);
+    SetConstantValue(deduced, &deduced->static_type());
   }
   // Type check the parameter pattern
-  TypeCheckPattern(&f->param_pattern(), values, std::nullopt);
+  TypeCheckPattern(&f->param_pattern(), std::nullopt);
 
   // Evaluate the return type, if we can do so without examining the body.
   if (std::optional<Nonnull<Expression*>> return_expression =
@@ -1006,9 +999,9 @@ void TypeChecker::TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f,
       return_expression.has_value()) {
     // We ignore the return value because return type expressions can't bring
     // new types into scope.
-    TypeCheckExp(*return_expression, values);
+    TypeCheckExp(*return_expression);
     SetStaticType(&f->return_term(),
-                  interpreter_.InterpExp(values, *return_expression));
+                  interpreter_.InterpExp(*return_expression));
   } else if (f->return_term().is_omitted()) {
     SetStaticType(&f->return_term(), TupleValue::Empty());
   } else {
@@ -1021,7 +1014,7 @@ void TypeChecker::TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f,
   }
 
   if (f->body().has_value() && check_body) {
-    TypeCheckStmt(*f->body(), values);
+    TypeCheckStmt(*f->body());
     if (!f->return_term().is_omitted()) {
       ExpectReturnOnAllPaths(f->body(), f->source_loc());
     }
@@ -1044,7 +1037,7 @@ void TypeChecker::TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f,
 }
 
 void TypeChecker::TypeCheckClassDeclaration(
-    Nonnull<ClassDeclaration*> class_decl, Env ct_top) {
+    Nonnull<ClassDeclaration*> class_decl) {
   std::vector<NamedValue> fields;
   std::vector<NamedValue> methods;
   for (Nonnull<Member*> m : class_decl->members()) {
@@ -1055,7 +1048,7 @@ void TypeChecker::TypeCheckClassDeclaration(
           FATAL_COMPILATION_ERROR(binding.source_loc())
               << "Struct members must have names";
         }
-        TypeCheckPattern(&binding, ct_top, std::nullopt);
+        TypeCheckPattern(&binding, std::nullopt);
         fields.push_back(
             {.name = binding.name(), .value = &binding.static_type()});
         break;
@@ -1068,12 +1061,12 @@ void TypeChecker::TypeCheckClassDeclaration(
           class_decl->name(), std::move(fields), std::move(methods))));
 }
 
-void TypeChecker::TypeCheckChoiceDeclaration(Nonnull<ChoiceDeclaration*> choice,
-                                             Env ct_top) {
+void TypeChecker::TypeCheckChoiceDeclaration(
+    Nonnull<ChoiceDeclaration*> choice) {
   std::vector<NamedValue> alternatives;
   for (Nonnull<AlternativeSignature*> alternative : choice->alternatives()) {
-    TypeCheckExp(&alternative->signature(), ct_top);
-    auto signature = interpreter_.InterpExp(ct_top, &alternative->signature());
+    TypeCheckExp(&alternative->signature());
+    auto signature = interpreter_.InterpExp(&alternative->signature());
     alternatives.push_back({.name = alternative->name(), .value = signature});
   }
   auto ct = arena_->New<ChoiceType>(choice->name(), std::move(alternatives));
@@ -1081,35 +1074,33 @@ void TypeChecker::TypeCheckChoiceDeclaration(Nonnull<ChoiceDeclaration*> choice,
 }
 
 void TypeChecker::TypeCheck(AST& ast) {
-  Env values(arena_);
   for (Nonnull<Declaration*> declaration : ast.declarations) {
-    TopLevel(declaration, &values);
+    TopLevel(declaration);
   }
   for (Nonnull<Declaration*> decl : ast.declarations) {
-    TypeCheckDeclaration(decl, values);
+    TypeCheckDeclaration(decl);
   }
-  TypeCheckExp(*ast.main_call, values);
+  TypeCheckExp(*ast.main_call);
 }
 
-void TypeChecker::TypeCheckDeclaration(Nonnull<Declaration*> d,
-                                       const Env& values) {
+void TypeChecker::TypeCheckDeclaration(Nonnull<Declaration*> d) {
   switch (d->kind()) {
     case DeclarationKind::FunctionDeclaration:
-      TypeCheckFunctionDeclaration(&cast<FunctionDeclaration>(*d), values,
+      TypeCheckFunctionDeclaration(&cast<FunctionDeclaration>(*d),
                                    /*check_body=*/true);
       return;
     case DeclarationKind::ClassDeclaration:
-      TypeCheckClassDeclaration(&cast<ClassDeclaration>(*d), values);
+      TypeCheckClassDeclaration(&cast<ClassDeclaration>(*d));
       return;
     case DeclarationKind::ChoiceDeclaration:
-      TypeCheckChoiceDeclaration(&cast<ChoiceDeclaration>(*d), values);
+      TypeCheckChoiceDeclaration(&cast<ChoiceDeclaration>(*d));
       return;
     case DeclarationKind::VariableDeclaration: {
       auto& var = cast<VariableDeclaration>(*d);
       // Signals a type error if the initializing expression does not have
       // the declared type of the variable, otherwise returns this
       // declaration with annotated types.
-      TypeCheckExp(&var.initializer(), values);
+      TypeCheckExp(&var.initializer());
       const auto* binding_type =
           dyn_cast<ExpressionPattern>(&var.binding().type());
       if (binding_type == nullptr) {
@@ -1118,7 +1109,7 @@ void TypeChecker::TypeCheckDeclaration(Nonnull<Declaration*> d,
             << "Type of a top-level variable must be an expression.";
       }
       Nonnull<const Value*> declared_type =
-          interpreter_.InterpExp(values, &binding_type->expression());
+          interpreter_.InterpExp(&binding_type->expression());
       SetStaticType(&var, declared_type);
       ExpectType(var.source_loc(), "initializer of variable", declared_type,
                  &var.initializer().static_type());
@@ -1127,33 +1118,28 @@ void TypeChecker::TypeCheckDeclaration(Nonnull<Declaration*> d,
   }
 }
 
-void TypeChecker::TopLevel(Nonnull<Declaration*> d, Nonnull<Env*> values) {
+void TypeChecker::TopLevel(Nonnull<Declaration*> d) {
   switch (d->kind()) {
     case DeclarationKind::FunctionDeclaration: {
       auto& func_def = cast<FunctionDeclaration>(*d);
-      TypeCheckFunctionDeclaration(&func_def, *values,
-                                   /*check_body=*/false);
-      interpreter_.InitEnv(*d, values);
+      TypeCheckFunctionDeclaration(&func_def, /*check_body=*/false);
+      SetConstantValue(&func_def, arena_->New<FunctionValue>(&func_def));
       break;
     }
 
     case DeclarationKind::ClassDeclaration: {
       auto& class_decl = cast<ClassDeclaration>(*d);
-      TypeCheckClassDeclaration(&class_decl, *values);
+      TypeCheckClassDeclaration(&class_decl);
       const auto& type = cast<TypeOfClassType>(class_decl.static_type());
-      const NominalClassType& value = type.class_type();
-      AllocationId a = interpreter_.AllocateValue(&value);
-      values->Set(class_decl.name(), a);  // Is this obsolete?
+      SetConstantValue(&class_decl, &type.class_type());
       break;
     }
 
     case DeclarationKind::ChoiceDeclaration: {
       auto& choice = cast<ChoiceDeclaration>(*d);
-      TypeCheckChoiceDeclaration(&choice, *values);
+      TypeCheckChoiceDeclaration(&choice);
       const auto& type = cast<TypeOfChoiceType>(choice.static_type());
-      const ChoiceType& value = type.choice_type();
-      AllocationId a = interpreter_.AllocateValue(&value);
-      values->Set(choice.name(), a);  // Is this obsolete?
+      SetConstantValue(&choice, &type.choice_type());
       break;
     }
 
@@ -1163,13 +1149,33 @@ void TypeChecker::TopLevel(Nonnull<Declaration*> d, Nonnull<Env*> values) {
       // compile-time symbol table.
       Expression& type =
           cast<ExpressionPattern>(var.binding().type()).expression();
-      TypeCheckPattern(&var.binding(), *values, std::nullopt);
-      Nonnull<const Value*> declared_type =
-          interpreter_.InterpExp(*values, &type);
+      TypeCheckPattern(&var.binding(), std::nullopt);
+      Nonnull<const Value*> declared_type = interpreter_.InterpExp(&type);
       SetStaticType(&var, declared_type);
       break;
     }
   }
 }
 
+template <typename T>
+void TypeChecker::SetConstantValue(Nonnull<T*> named_entity,
+                                   Nonnull<const Value*> value) {
+  std::optional<Nonnull<const Value*>> old_value =
+      named_entity->constant_value();
+  if (old_value.has_value()) {
+    CHECK(ValueEqual(*old_value, value));
+  } else {
+    named_entity->set_constant_value(value);
+    CHECK(constants_.insert(named_entity).second);
+  }
+}
+
+void TypeChecker::PrintConstants(llvm::raw_ostream& out) {
+  llvm::ListSeparator sep;
+  for (const auto& named_entity : constants_) {
+    out << sep << named_entity.name() << ": "
+        << **named_entity.constant_value();
+  }
+}
+
 }  // namespace Carbon

executable_semantics/interpreter/type_checker.h

@@ -42,39 +42,37 @@ class TypeChecker {
   //
   // `values` maps variable names to their compile-time values. It is not
   //    directly used in this function but is passed to InterExp.
-  void TypeCheckExp(Nonnull<Expression*> e, Env values);
+  void TypeCheckExp(Nonnull<Expression*> e);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at `p`.
   //
   // `expected` is the type that this pattern is expected to have, if the
   // surrounding context gives us that information. Otherwise, it is
   // nullopt.
-  void TypeCheckPattern(Nonnull<Pattern*> p, Env values,
+  void TypeCheckPattern(Nonnull<Pattern*> p,
                         std::optional<Nonnull<const Value*>> expected);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at `d`.
-  void TypeCheckDeclaration(Nonnull<Declaration*> d, const Env& values);
+  void TypeCheckDeclaration(Nonnull<Declaration*> d);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at `s`.
   //
   // REQUIRES: f.return_term().has_static_type() || f.return_term().is_auto(),
   // where `f` is nearest enclosing FunctionDeclaration of `s`.
-  void TypeCheckStmt(Nonnull<Statement*> s, Env values);
+  void TypeCheckStmt(Nonnull<Statement*> s);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at `f`,
   // and may not traverse f->body() if `check_body` is false.
-  void TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f, Env values,
+  void TypeCheckFunctionDeclaration(Nonnull<FunctionDeclaration*> f,
                                     bool check_body);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at class_decl.
-  void TypeCheckClassDeclaration(Nonnull<ClassDeclaration*> class_decl,
-                                 Env ct_top);
+  void TypeCheckClassDeclaration(Nonnull<ClassDeclaration*> class_decl);
 
   // Equivalent to TypeCheckExp, but operates on the AST rooted at choice_decl.
-  void TypeCheckChoiceDeclaration(Nonnull<ChoiceDeclaration*> choice,
-                                  Env ct_top);
+  void TypeCheckChoiceDeclaration(Nonnull<ChoiceDeclaration*> choice);
 
-  void TopLevel(Nonnull<Declaration*> d, Nonnull<Env*> values);
+  void TopLevel(Nonnull<Declaration*> d);
 
   // Verifies that opt_stmt holds a statement, and it is structurally impossible
   // for control flow to leave that statement except via a `return`.
@@ -90,8 +88,17 @@ class TypeChecker {
                                  Nonnull<const Value*>>& dict,
                   Nonnull<const Value*> type) -> Nonnull<const Value*>;
 
+  // Sets named_entity.constant_value() to `value`. Can be called multiple
+  // times on the same named_entity, so long as it is always called with
+  // the same value.
+  template <typename T>
+  void SetConstantValue(Nonnull<T*> named_entity, Nonnull<const Value*> value);
+
+  void PrintConstants(llvm::raw_ostream& out);
+
   Nonnull<Arena*> arena_;
   Interpreter interpreter_;
+  std::set<NamedEntityView> constants_;
 
   bool trace_;
 };

executable_semantics/testdata/basic_syntax/trace.carbon

@@ -13,7 +13,7 @@
 // CHECK: fn Print (format_str: String) {
 // CHECK: ********** type checking **********
 // CHECK: checking pattern (format_str: String)
-// CHECK: values: Print: fun<Print>
+// CHECK: constants: Main: fun<Main>, Print: fun<Print>
 // CHECK: ********** type checking complete **********
 // CHECK: fn Print (format_str: String) {
 // CHECK: ********** starting execution **********
@@ -21,7 +21,7 @@
 // CHECK: ********** calling main function **********
 // CHECK: {
 // CHECK: stack:
-// CHECK: heap: fun<Print>, fun<Main>
+// CHECK: heap:
 // CHECK: }
 // CHECK: --- step exp Main() (<Main()>:0) --->
 // CHECK: result: 0