Add TupleValue::Empty for the common empty tuple case (#708)

executable_semantics/interpreter/interpreter.cpp

@@ -759,8 +759,7 @@ void StepPattern() {
       if (act->pos == 0) {
         if (tuple.Fields().empty()) {
           frame->todo.Pop(1);
-          frame->todo.Push(Action::MakeValAction(
-              global_arena->New<TupleValue>(std::vector<TupleElement>())));
+          frame->todo.Push(Action::MakeValAction(&TupleValue::Empty()));
         } else {
           const Pattern* p1 = tuple.Fields()[0].pattern;
           frame->todo.Push(Action::MakePatternAction(p1));

executable_semantics/interpreter/typecheck.cpp

@@ -855,8 +855,7 @@ auto TypeCheckFunDef(const FunctionDefinition* f, TypeEnv types, Env values)
     // TODO: Check that main doesn't have any parameters.
   }
   auto res = TypeCheckStmt(f->body, param_res.types, values, return_type);
-  bool void_return = TypeEqual(
-      return_type, global_arena->New<TupleValue>(std::vector<TupleElement>()));
+  bool void_return = TypeEqual(return_type, &TupleValue::Empty());
   auto body = CheckOrEnsureReturn(res.stmt, void_return, f->line_num);
   return global_arena->New<FunctionDefinition>(
       f->line_num, f->name, f->deduced_parameters, f->param_pattern,

executable_semantics/interpreter/value.h

@@ -233,6 +233,12 @@ class AlternativeValue : public Value {
 // A function value.
 class TupleValue : public Value {
  public:
+  // An empty tuple, also known as the unit type.
+  static const TupleValue& Empty() {
+    static const TupleValue empty = TupleValue(std::vector<TupleElement>());
+    return empty;
+  }
+
   explicit TupleValue(std::vector<TupleElement> elements)
       : Value(Kind::TupleValue), elements(std::move(elements)) {}