diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 81c1f1805..f258d6a83 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -94,7 +94,11 @@ void print_error(ErrorInfo &einfo)
         level_string = "warning:";  // TODO make yellow.
         break;
       }
-  }
+    default: 
+      {
+        level_string = "wat:";  
+        break;
+      }}
 
   int ndl = level_string.length() + 3 + einfo.name.length() + einfo.program.length();
   int dashwidth = errwidth - 3 ? 3 : 80 - ndl; 
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 69776eb8c..9449540af 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -3,6 +3,7 @@
 #include "types.hh"
 #include <string>
 #include <optional>
+#include <iostream>
 
 using std::string;
 using std::optional;
@@ -16,35 +17,171 @@ enum ErrLevel
 
 class ColumnRange { 
   public:
-  unsigned int start;
-  unsigned int len;
+    unsigned int start;
+    unsigned int len;
 };
 
+class ErrorInfo;
+
+// -------------------------------------------------
+// forward declarations before ErrLine.
+template <class T> 
+class AddLineNumber;
+
+template <class T> 
+class AddColumnRange;
+
+template <class T> 
+class AddLOC;
+
 class ErrLine { 
   public:
-  int lineNumber;
-  optional<ColumnRange> columnRange;
-  optional<string> prevLineOfCode;
-  string errLineOfCode;
-  optional<string> nextLineOfCode;
+    int lineNumber;
+    optional<ColumnRange> columnRange;
+    optional<string> prevLineOfCode;
+    string errLineOfCode;
+    optional<string> nextLineOfCode;
+
+    friend AddLineNumber<ErrLine>;
+    friend AddColumnRange<ErrLine>;
+    friend AddLOC<ErrLine>;
+    ErrLine& GetEL() { return *this; }
+  private:
+    ErrLine() {}
 };
 
+template <class T> 
+class AddLineNumber : public T
+{
+  public:
+    T& lineNumber(int lineNumber) { 
+      GetEL().lineNumber = lineNumber;
+      return *this;
+    }
+  protected:
+    ErrLine& GetEL() { return T::GetEL(); }
+};
+
+template <class T> 
+class AddColumnRange : public T
+{
+  public:
+    T& columnRange(unsigned int start, unsigned int len) { 
+      GetEL().columnRange = { start, len };
+      return *this;
+    }
+  protected:
+    ErrLine& GetEL() { return T::GetEL(); }
+};
+
+template <class T> 
+class AddLOC : public T
+{
+  public:
+    T& linesOfCode(optional<string> prevloc, string loc, optional<string> nextloc) { 
+      GetEL().prevLineOfCode = prevloc;
+      GetEL().errLineOfCode = loc;
+      GetEL().nextLineOfCode = nextloc;
+      return *this;
+    }
+  protected:
+    ErrLine& GetEL() { return T::GetEL(); }
+};
+
+typedef AddLineNumber<AddColumnRange<AddLOC<ErrLine>>> MkErrLine;
+MkErrLine mkErrLine;
+
+
+// -------------------------------------------------
+// NixCode.
+
+template <class T>
+class AddNixFile;
+
+template <class T>
+class AddErrLine;
+
 class NixCode { 
   public:
-  optional<string> nixFile;
-  optional<ErrLine> errLine;
+    optional<string> nixFile;
+    optional<ErrLine> errLine;
+
+    friend AddNixFile<NixCode>;
+    friend AddErrLine<NixCode>;
+    friend ErrorInfo;
+    NixCode& GetNC() { return *this; }
+  private:
+    NixCode() {}
 };
 
+template <class T> 
+class AddNixFile : public T
+{
+  public:
+    T& nixFile(string filename) { 
+      GetNC().nixFile = filename;
+      return *this;
+    }
+  protected:
+    NixCode& GetNC() { return T::GetNC(); }
+};
+
+template <class T> 
+class AddErrLine : public T
+{
+  public:
+    T& errLine(ErrLine errline) { 
+      GetNC().errLine = errline;
+      return *this;
+    }
+  protected:
+    NixCode& GetNC() { return T::GetNC(); }
+};
+
+typedef AddNixFile<AddErrLine<NixCode>> MkNixCode;
+
+// -------------------------------------------------
+// ErrorInfo.
+
+template <class T>
+class AddName;
+
+template <class T>
+class AddDescription;
+
+template <class T>
+class AddNixCode;
 
 class ErrorInfo { 
   public:
-  ErrLevel level;
-  string name;
-  string description;
-  string program;
-  optional<NixCode> nixCode;
-  string hint;
-  ErrorInfo& GetEI() { return *this; }
+    ErrLevel level;
+    string name;
+    string description;
+    string program;
+    optional<NixCode> nixCode;
+    string hint;
+    ErrorInfo& GetEI() { return *this; }
+
+    // give these access to the private constructor, 
+    // when they are direct descendants.
+    friend AddName<ErrorInfo>;
+    friend AddDescription<ErrorInfo>;
+    friend AddNixCode<ErrorInfo>;
+    
+  protected:
+    ErrorInfo(ErrLevel level) { this->level = level; }
+};
+
+class EIError : public ErrorInfo
+{
+  protected:
+    EIError() : ErrorInfo(elError) {}
+};
+
+class EIWarning : public ErrorInfo
+{
+  protected:
+    EIWarning() : ErrorInfo(elWarning) {}
 };
 
 template <class T>
@@ -71,7 +208,23 @@ class AddDescription : private T
     ErrorInfo& GetEI() { return T::GetEI(); }
 };
 
-typedef AddName<AddDescription<ErrorInfo>> StandardError;
+template <class T>
+class AddNixCode : private T 
+{
+  public:
+    T& nixcode(const NixCode &nixcode){
+      GetEI().nixCode = nixcode;
+      return *this;
+    }
+  protected:
+    ErrorInfo& GetEI() { return T::GetEI(); }
+};
+
+typedef AddName<AddDescription<EIError>> StandardError;
+typedef AddName<AddDescription<EIWarning>> StandardWarning;
+
+typedef AddName<AddDescription<AddNixCode<EIError>>> MkNixError;
+typedef AddName<AddDescription<AddNixCode<EIWarning>>> MkNixWarning;
 
 string showErrLine(ErrLine &errLine);
 
diff --git a/tests/errors/main.cc b/tests/errors/main.cc
index 0e6d781c9..20dba046b 100644
--- a/tests/errors/main.cc
+++ b/tests/errors/main.cc
@@ -4,11 +4,13 @@
 #include <iostream>
 
 using std::optional;
+using std::nullopt;
 
 int main() {
 
 using namespace nix; 
 
+  /*
   ColumnRange columnRange;
   columnRange.start = 24;
   columnRange.len = 14;
@@ -30,11 +32,43 @@ using namespace nix;
   generic.nixCode = nixcode;
 
   print_error(generic);
-
+  */
 
   StandardError standardError;
 
-  print_error(standardError.name("blah").description("blah"));
+  print_error(standardError
+                .name("name")
+                .description("description"));
+
+  StandardWarning standardWarning;
+
+  print_error(standardWarning
+                .name("warning name")
+                .description("warning description"));
+
+  print_error(MkNixError()
+                .name("name")
+                .description("description")
+                .nixcode(
+                  MkNixCode()
+                    .nixFile("myfile.nix")
+                    .errLine(MkErrLine().lineNumber(40)
+                           .columnRange(50,10)
+                           .linesOfCode(nullopt
+                                      ,"this is the problem line of code"
+                                      ,nullopt))));
+
+  print_error(MkNixWarning()
+                .name("name")
+                .description("description")
+                .nixcode(
+                  MkNixCode()
+                    .nixFile("myfile.nix")
+                    .errLine(MkErrLine().lineNumber(40)
+                           .columnRange(50,10)
+                           .linesOfCode(nullopt
+                                      ,"this is the problem line of code"
+                                      ,nullopt))));
 
   return 0;
 }