Add support for _ placeholder. (#661)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/expression.cpp

@@ -105,7 +105,8 @@ auto Expression::MakeIdentifierExpression(int line_num, std::string var)
   return v;
 }
 
-auto Expression::MakeBindingExpression(int line_num, std::string var,
+auto Expression::MakeBindingExpression(int line_num,
+                                       std::optional<std::string> var,
                                        const Expression* type)
     -> const Expression* {
   auto* v = new Expression();
@@ -280,11 +281,17 @@ void PrintExp(const Expression* e) {
     case ExpressionKind::IdentifierExpression:
       std::cout << e->GetIdentifierExpression().name;
       break;
-    case ExpressionKind::BindingExpression:
-      PrintExp(e->GetBindingExpression().type);
+    case ExpressionKind::BindingExpression: {
+      const BindingExpression& binding = e->GetBindingExpression();
+      if (binding.name.has_value()) {
+        std::cout << *binding.name;
+      } else {
+        std::cout << "_";
+      }
       std::cout << ": ";
-      std::cout << e->GetBindingExpression().name;
+      PrintExp(e->GetBindingExpression().type);
       break;
+    }
     case ExpressionKind::CallExpression:
       PrintExp(e->GetCallExpression().function);
       if (e->GetCallExpression().argument->tag() ==

executable_semantics/ast/expression.h

@@ -5,6 +5,7 @@
 #ifndef EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_
 #define EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_
 
+#include <optional>
 #include <string>
 #include <variant>
 #include <vector>
@@ -74,7 +75,8 @@ struct IndexExpression {
 
 struct BindingExpression {
   static constexpr ExpressionKind Kind = ExpressionKind::BindingExpression;
-  std::string name;
+  // nullopt represents the `_` placeholder.
+  std::optional<std::string> name;
   const Expression* type;
 };
 
@@ -136,7 +138,8 @@ struct TypeTypeLiteral {
 struct Expression {
   static auto MakeIdentifierExpression(int line_num, std::string var)
       -> const Expression*;
-  static auto MakeBindingExpression(int line_num, std::string var,
+  static auto MakeBindingExpression(int line_num,
+                                    std::optional<std::string> var,
                                     const Expression* type)
       -> const Expression*;
   static auto MakeIntLiteral(int line_num, int i) -> const Expression*;