Stop using std::pair in the implementation of tuples (#479)

executable_semantics/ast/expression.cpp

@@ -143,8 +143,7 @@ auto MakeGetField(int line_num, const Expression* exp, std::string field)
   return e;
 }
 
-auto MakeTuple(int line_num,
-               std::vector<std::pair<std::string, const Expression*>>* args)
+auto MakeTuple(int line_num, std::vector<FieldInitializer>* args)
     -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
@@ -152,14 +151,14 @@ auto MakeTuple(int line_num,
   int i = 0;
   bool seen_named_member = false;
   for (auto& arg : *args) {
-    if (arg.first == "") {
+    if (arg.name == "") {
       if (seen_named_member) {
         std::cerr << line_num
                   << ": positional members must come before named members"
                   << std::endl;
         exit(-1);
       }
-      arg.first = std::to_string(i);
+      arg.name = std::to_string(i);
       ++i;
     } else {
       seen_named_member = true;
@@ -176,7 +175,7 @@ auto MakeUnit(int line_num) -> const Expression* {
   auto* unit = new Expression();
   unit->line_num = line_num;
   unit->tag = ExpressionKind::Tuple;
-  auto* args = new std::vector<std::pair<std::string, const Expression*>>();
+  auto* args = new std::vector<FieldInitializer>();
   unit->u.tuple.fields = args;
   return unit;
 }
@@ -217,15 +216,14 @@ static void PrintOp(Operator op) {
   }
 }
 
-static void PrintFields(
-    std::vector<std::pair<std::string, const Expression*>>* fields) {
+static void PrintFields(std::vector<FieldInitializer>* fields) {
   int i = 0;
   for (auto iter = fields->begin(); iter != fields->end(); ++iter, ++i) {
     if (i != 0) {
       std::cout << ", ";
     }
-    std::cout << iter->first << " = ";
-    PrintExp(iter->second);
+    std::cout << iter->name << " = ";
+    PrintExp(iter->expression);
   }
 }
 

executable_semantics/ast/expression.h

@@ -10,6 +10,17 @@
 
 namespace Carbon {
 
+struct Expression;
+
+// A FieldInitializer represents the initialization of a single tuple field.
+struct FieldInitializer {
+  // The field name. For a positional field, this may be empty.
+  std::string name;
+
+  // The expression that initializes the field.
+  const Expression* expression;
+};
+
 enum class ExpressionKind {
   AutoT,
   BoolT,
@@ -65,7 +76,7 @@ struct Expression {
     bool boolean;
 
     struct {
-      std::vector<std::pair<std::string, const Expression*>>* fields;
+      std::vector<FieldInitializer>* fields;
     } tuple;
 
     struct {
@@ -101,8 +112,7 @@ auto MakeCall(int line_num, const Expression* fun, const Expression* arg)
     -> const Expression*;
 auto MakeGetField(int line_num, const Expression* exp, std::string field)
     -> const Expression*;
-auto MakeTuple(int line_num,
-               std::vector<std::pair<std::string, const Expression*>>* args)
+auto MakeTuple(int line_num, std::vector<FieldInitializer>* args)
     -> const Expression*;
 // Create an AST node for an empty tuple.
 auto MakeUnit(int line_num) -> const Expression*;