diff --git a/src/libutil/error.cc b/src/libutil/error.cc
new file mode 100644
index 000000000..4af4691d4
--- /dev/null
+++ b/src/libutil/error.cc
@@ -0,0 +1,137 @@
+#include "error.hh"
+
+#include <iostream>
+
+namespace nix {
+
+using std::cout;
+using std::endl;
+
+// return basic_format?
+string showErrLine(ErrLine &errLine) 
+{
+  if (errLine.columnRange.has_value()) 
+  {
+    return (format("(%1%:%2%)") % errLine.lineNumber % errLine.columnRange->start).str();
+  }
+  else
+  {
+    return (format("(%1%)") % errLine.lineNumber).str();
+  };
+}
+
+void print_code_lines(string &prefix, NixCode &nix_code) 
+{
+  if (nix_code.errLine.has_value()) 
+  {
+    // previous line of code.
+    if (nix_code.errLine->prevLineOfCode.has_value()) { 
+      cout << format("%1% %2$+5d%| %3%") %  prefix % (nix_code.errLine->lineNumber - 1) % *nix_code.errLine->prevLineOfCode;
+    }
+
+    // line of code containing the error.
+    cout << format("%1% %2$+5d%| %3%") %  prefix % (nix_code.errLine->lineNumber) % nix_code.errLine->errLineOfCode;
+
+    // error arrows for the column range.
+    if (nix_code.errLine->columnRange.has_value()) 
+    {
+      int start = nix_code.errLine->columnRange->start;
+      std::string spaces;
+      for (int i = 0; i < start; ++i)
+      {
+        spaces.append(" ");
+      }
+
+      int len = nix_code.errLine->columnRange->len;
+      std::string arrows;
+      for (int i = 0; i < len; ++i)
+      {
+        arrows.append("^");
+      }
+    
+      cout << format("%1%     |%2%%3%") % prefix % spaces % arrows;
+    }
+
+
+    // next line of code.
+    if (nix_code.errLine->nextLineOfCode.has_value()) { 
+      cout << format("%1% %2$+5d%| %3%") %  prefix % (nix_code.errLine->lineNumber + 1) % *nix_code.errLine->nextLineOfCode;
+    }
+
+  }
+  else 
+  {
+  }
+    
+}
+
+void print_error(ErrorInfo &einfo) 
+{
+  int errwidth = 80;
+  string prefix = "  ";
+
+  string level_string;
+  switch (einfo.level) 
+  {
+    case ErrLevel::elError: 
+      {
+        level_string = "error:";  // TODO make red.
+        break;
+      }
+    case ErrLevel::elWarning: 
+      {
+        level_string = "warning:";  // TODO make yellow.
+        break;
+      }
+  }
+
+  int ndl = level_string.length() + 3 + einfo.errName.length() + einfo.toolName.length();
+  int dashwidth = errwidth - 3 ? 3 : 80 - ndl; 
+
+  string dashes;
+  for (int i = 0; i < dashwidth; ++i)
+    dashes.append("-");
+
+  // divider.
+  cout << format("%1%%2% %3% %4% %5% %6%")
+    % prefix
+    % level_string
+    % "---"
+    % einfo.errName
+    % dashes
+    % einfo.toolName;
+
+  // filename.
+  if (einfo.nixCode.has_value()) 
+  {
+    if (einfo.nixCode->nixFile.has_value()) 
+    {
+      string eline = einfo.nixCode->errLine.has_value()  
+        ? string(" ") + showErrLine(*einfo.nixCode->errLine)
+        : "";
+
+      cout << format("%1%in file: %2%%3%") % prefix % *einfo.nixCode->nixFile % eline;
+      cout << prefix << endl;
+    }
+    else
+    {
+      cout << format("%1%from command line argument") % prefix << endl;
+      cout << prefix << endl;
+    }
+  }
+
+  // description
+  cout << prefix << einfo.description << endl;
+  cout << prefix << endl;
+
+  // lines of code.
+  if (einfo.nixCode.has_value())
+    print_code_lines(prefix, *einfo.nixCode);
+
+  // hint
+  cout << prefix << einfo.hint << endl;
+  cout << prefix << endl;
+
+}
+
+}
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
new file mode 100644
index 000000000..0bc1f4e24
--- /dev/null
+++ b/src/libutil/error.hh
@@ -0,0 +1,57 @@
+#pragma once
+
+#include "types.hh"
+#include <string>
+#include <optional>
+// #include <boost/format.hpp>
+
+using std::string;
+using std::optional;
+// using boost::format;
+
+namespace nix {
+
+enum ErrLevel 
+  { elWarning
+  , elError
+  };
+
+class ColumnRange { 
+  public:
+  unsigned int start;
+  unsigned int len;
+};
+
+class ErrLine { 
+  public:
+  int lineNumber;
+  optional<ColumnRange> columnRange;
+  optional<string> prevLineOfCode;
+  string errLineOfCode;
+  optional<string> nextLineOfCode;
+};
+
+class NixCode { 
+  public:
+  optional<string> nixFile;
+  optional<ErrLine> errLine;
+};
+
+
+class ErrorInfo { 
+  public:
+  ErrLevel level;
+  string errName;
+  string description;
+  string toolName;
+  optional<NixCode> nixCode;
+  string hint;
+};
+
+string showErrLine(ErrLine &errLine);
+
+void print_code_lines(string &prefix, NixCode &nix_code); 
+
+void print_error(ErrorInfo &einfo);
+}
+