Use llvm::ListSeparator for simpler separators (#671)

I was looking for something like this, I think it's a straightforward simplification for code. Internally, it handles skipping the separator on the first print.

See the bottom of: https://llvm.org/doxygen/StringExtras_8h_source.html

executable_semantics/ast/expression.cpp

@@ -5,6 +5,7 @@
 #include "executable_semantics/ast/expression.h"
 
 #include "executable_semantics/common/error.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -221,12 +222,9 @@ static void PrintOp(llvm::raw_ostream& out, Operator op) {
 
 static void PrintFields(llvm::raw_ostream& out,
                         const std::vector<FieldInitializer>& fields) {
-  int i = 0;
-  for (auto iter = fields.begin(); iter != fields.end(); ++iter, ++i) {
-    if (i != 0) {
-      out << ", ";
-    }
-    out << iter->name << " = " << *iter->expression;
+  llvm::ListSeparator sep;
+  for (const auto& field : fields) {
+    out << sep << field.name << " = " << field.expression;
   }
 }
 

executable_semantics/interpreter/action.cpp

@@ -13,6 +13,7 @@
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/interpreter/stack.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -74,24 +75,21 @@ void Action::Print(llvm::raw_ostream& out) const {
   out << "<" << pos << ">";
   if (results.size() > 0) {
     out << "(";
+    llvm::ListSeparator sep;
     for (auto& result : results) {
+      out << sep;
       if (result) {
         out << *result;
       }
-      out << ",";
     }
     out << ")";
   }
 }
 
 void Action::PrintList(const Stack<Action*>& ls, llvm::raw_ostream& out) {
-  auto it = ls.begin();
-  while (it != ls.end()) {
-    out << **it;
-    ++it;
-    if (it != ls.end()) {
-      out << " :: ";
-    }
+  llvm::ListSeparator sep(" :: ");
+  for (const auto& action : ls) {
+    out << sep << *action;
   }
 }
 

executable_semantics/interpreter/heap.cpp

@@ -4,6 +4,8 @@
 
 #include "executable_semantics/interpreter/heap.h"
 
+#include "llvm/ADT/StringExtras.h"
+
 namespace Carbon {
 
 auto Heap::AllocateValue(const Value* v) -> Address {
@@ -48,9 +50,10 @@ void Heap::Deallocate(const Address& address) {
 }
 
 void Heap::Print(llvm::raw_ostream& out) const {
+  llvm::ListSeparator sep;
   for (size_t i = 0; i < values_.size(); ++i) {
+    out << sep;
     PrintAddress(Address(i), out);
-    out << ", ";
   }
 }
 

executable_semantics/interpreter/interpreter.cpp

@@ -18,6 +18,7 @@
 #include "executable_semantics/interpreter/action.h"
 #include "executable_semantics/interpreter/frame.h"
 #include "executable_semantics/interpreter/stack.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -31,10 +32,10 @@ void Step();
 //
 
 void PrintEnv(Env values, llvm::raw_ostream& out) {
+  llvm::ListSeparator sep;
   for (const auto& [name, address] : values) {
-    out << name << ": ";
+    out << sep << name << ": ";
     state->heap.PrintAddress(address, out);
-    out << ", ";
   }
 }
 
@@ -43,13 +44,9 @@ void PrintEnv(Env values, llvm::raw_ostream& out) {
 //
 
 void PrintStack(const Stack<Frame*>& ls, llvm::raw_ostream& out) {
-  auto it = ls.begin();
-  while (it != ls.end()) {
-    out << **it;
-    ++it;
-    if (it != ls.end()) {
-      out << " :: ";
-    }
+  llvm::ListSeparator sep(" :: ");
+  for (const auto& frame : ls) {
+    out << sep << *frame;
   }
 }
 

executable_semantics/interpreter/value.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 
 #include "common/check.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -373,15 +374,9 @@ void Value::Print(llvm::raw_ostream& out) const {
     }
     case ValKind::TupleValue: {
       out << "(";
-      bool add_commas = false;
+      llvm::ListSeparator sep;
       for (const TupleElement& element : GetTupleValue().elements) {
-        if (add_commas) {
-          out << ", ";
-        } else {
-          add_commas = true;
-        }
-
-        out << element.name << " = " << *element.value;
+        out << sep << element.name << " = " << *element.value;
       }
       out << ")";
       break;