diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 4480e80bf..04da075ed 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -5,9 +5,8 @@
 
 namespace nix {
 
-optional<string> ErrorInfo::programName = std::nullopt;
+std::optional<string> ErrorInfo::programName = std::nullopt;
 
-// return basic_format?
 string showErrLine(ErrLine &errLine)
 {
     if (errLine.columnRange.has_value()) 
@@ -27,19 +26,19 @@ void printCodeLines(string &prefix, NixCode &nixCode)
     {
         // previous line of code.
         if (nixCode.errLine->prevLineOfCode.has_value()) { 
-            cout << format("%1% %|2$5d|| %3%")
+            std::cout << format("%1% %|2$5d|| %3%")
                 %    prefix
                 % (nixCode.errLine->lineNumber - 1)
                 % *nixCode.errLine->prevLineOfCode
-                << endl;
+                << std::endl;
         }
 
         // line of code containing the error.%2$+5d%
-        cout << format("%1% %|2$5d|| %3%")
+        std::cout << format("%1% %|2$5d|| %3%")
             %    prefix
             % (nixCode.errLine->lineNumber)
             % nixCode.errLine->errLineOfCode
-            << endl;
+            << std::endl;
 
       // error arrows for the column range.
         if (nixCode.errLine->columnRange.has_value()) 
@@ -58,18 +57,18 @@ void printCodeLines(string &prefix, NixCode &nixCode)
                 arrows.append("^");
             }
 
-          cout << format("%1%            |%2%" ANSI_RED "%3%" ANSI_NORMAL) % prefix % spaces % arrows << endl;
+          std::cout << format("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL) % prefix % spaces % arrows << std::endl;
         }
 
 
 
         // next line of code.
         if (nixCode.errLine->nextLineOfCode.has_value()) { 
-            cout << format("%1% %|2$5d|| %3%")
+            std::cout << format("%1% %|2$5d|| %3%")
             %    prefix
             % (nixCode.errLine->lineNumber + 1)
             % *nixCode.errLine->nextLineOfCode
-            << endl;
+            << std::endl;
         }
 
     }
@@ -113,14 +112,14 @@ void printErrorInfo(ErrorInfo &einfo)
         dashes.append("-");
 
     // divider.
-    cout << format("%1%%2%" ANSI_BLUE " %3% %4% %5% %6%" ANSI_NORMAL)
+    std::cout << format("%1%%2%" ANSI_BLUE " %3% %4% %5% %6%" ANSI_NORMAL)
         % prefix
         % levelString
         % "---"
         % einfo.name
         % dashes
         % einfo.programName.value_or("")
-        << endl;
+        << std::endl;
 
     // filename.
     if (einfo.nixCode.has_value()) 
@@ -131,33 +130,33 @@ void printErrorInfo(ErrorInfo &einfo)
                 ? string(" ") + showErrLine(*einfo.nixCode->errLine)
                 : "";
 
-            cout << format("%1%in file: " ANSI_BLUE "%2%%3%" ANSI_NORMAL) 
-                % prefix % *einfo.nixCode->nixFile % eline << endl;
-            cout << prefix << endl;
+            std::cout << format("%1%in file: " ANSI_BLUE "%2%%3%" ANSI_NORMAL) 
+                % prefix % *einfo.nixCode->nixFile % eline << std::endl;
+            std::cout << prefix << std::endl;
         }
         else
         {
-            cout << format("%1%from command line argument") % prefix << endl;
-            cout << prefix << endl;
+            std::cout << format("%1%from command line argument") % prefix << std::endl;
+            std::cout << prefix << std::endl;
         }
     }
 
     // description
-    cout << prefix << einfo.description << endl;
-    cout << prefix << endl;
+    std::cout << prefix << einfo.description << std::endl;
+    std::cout << prefix << std::endl;
 
     // lines of code.
     if (einfo.nixCode.has_value())
     {
         printCodeLines(prefix, *einfo.nixCode);
-        cout << prefix << endl;
+        std::cout << prefix << std::endl;
     }
 
     // hint
     if (einfo.hint.has_value()) 
     {
-        cout << prefix << *einfo.hint << endl;
-        cout << prefix << endl;
+        std::cout << prefix << *einfo.hint << std::endl;
+        std::cout << prefix << std::endl;
     }
 }
 
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 00c32ff21..9dcb42e14 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -27,21 +27,21 @@ class ErrorInfo;
 class ErrLine { 
     public:
         int lineNumber;
-        optional<ColumnRange> columnRange;
-        optional<string> prevLineOfCode;
+        std::optional<ColumnRange> columnRange;
+        std::optional<string> prevLineOfCode;
         string errLineOfCode;
-        optional<string> nextLineOfCode;
+        std::optional<string> nextLineOfCode;
 };
 
 class NixCode { 
     public:
-        optional<string> nixFile;
-        optional<ErrLine> errLine;
+        std::optional<string> nixFile;
+        std::optional<ErrLine> errLine;
 
         ErrLine& ensureErrLine() 
         { 
             if (!this->errLine.has_value())
-                 this->errLine = optional(ErrLine());
+                 this->errLine = std::optional(ErrLine());
             return *this->errLine; 
         }
 };
@@ -80,11 +80,11 @@ class ErrorInfo {
         ErrLevel level;
         string name;
         string description;
-        optional<NixCode> nixCode;
-        optional<string> hint;
+        std::optional<NixCode> nixCode;
+        std::optional<string> hint;
         ErrorInfo& GetEI() { return *this; }
 
-        static optional<string> programName;
+        static std::optional<string> programName;
 
         // give these access to the private constructor, 
         // when they are direct descendants (children but not grandchildren).
@@ -100,7 +100,7 @@ class ErrorInfo {
         NixCode& ensureNixCode() 
         { 
             if (!this->nixCode.has_value())
-                 this->nixCode = optional(NixCode());
+                 this->nixCode = std::optional(NixCode());
             return *this->nixCode; 
         }
     protected:
@@ -187,7 +187,7 @@ template <class T>
 class AddLOC : private T
 {
     public:
-        T& linesOfCode(optional<string> prevloc, string loc, optional<string> nextloc) { 
+        T& linesOfCode(std::optional<string> prevloc, string loc, std::optional<string> nextloc) { 
             GetEI().ensureNixCode().ensureErrLine().prevLineOfCode = prevloc;
             GetEI().ensureNixCode().ensureErrLine().errLineOfCode = loc;
             GetEI().ensureNixCode().ensureErrLine().nextLineOfCode = nextloc;
@@ -197,6 +197,11 @@ class AddLOC : private T
         ErrorInfo& GetEI() { return T::GetEI(); }
 };
 
+
+// ----------------------------------------------------------------
+// format for hints.  same as boost format, except templated values 
+// are always in yellow.
+
 template <class T>
 class yellowify
 {
@@ -211,11 +216,12 @@ std::ostream& operator<<(std::ostream &out, const yellowify<T> &y)
     return out << ANSI_YELLOW << y.value << ANSI_NORMAL;
 }
 
-// hint format shows templated values in yellow.
 class hintfmt 
 {
     public:
-        hintfmt(string format) :fmt(format) {}
+        hintfmt(string format) :fmt(format) {
+            fmt.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
+          }
         template<class T>
             hintfmt& operator%(const T &value) { fmt % yellowify(value); return *this; }
 
@@ -226,12 +232,13 @@ class hintfmt
 
 };
 
+// the template layer for adding a hint.
 template <class T> 
 class AddHint : private T
 {
     public:
         T& hint(hintfmt &hfmt) {
-            GetEI().hint = optional(hfmt.fmt.str());
+            GetEI().hint = std::optional(hfmt.fmt.str());
             return *this;
         }
         T& nohint() {
diff --git a/tests/errors/main.cc b/tests/errors/main.cc
index f11390c73..870af75d3 100644
--- a/tests/errors/main.cc
+++ b/tests/errors/main.cc
@@ -3,14 +3,12 @@
 #include <optional>
 #include <iostream>
 
-
-
 int main() 
 {
     using namespace nix; 
 
     // In each program where errors occur, this has to be set.
-    ErrorInfo::programName = optional("error-test");
+    ErrorInfo::programName = std::optional("error-test");
 
     // There are currently four error types:
     //  
@@ -83,9 +81,9 @@ int main()
             .nixFile("myfile.nix")
             .lineNumber(40)
             .columnRange(13,7)
-            .linesOfCode(optional("previous line of code")
+            .linesOfCode(std::optional("previous line of code")
                         ,"this is the problem line of code"
-                        ,optional("next line of code"))
+                        ,std::optional("next line of code"))
             .hint(hintfmt("this hint has %1% templated %2%!!") % "yellow" % "values")
             );