diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 72a3bf9b9..8c33fd224 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -249,6 +249,14 @@ void mkString(Value & v, const string & s, const PathSet & context)
 }
 
 
+void mkString(Value & v, const Symbol & s)
+{
+    v.type = tString;
+    v.string.s = ((string) s).c_str();
+    v.string.context = 0;
+}
+
+
 void mkPath(Value & v, const char * s)
 {
     clearValue(v);
@@ -429,7 +437,7 @@ void ExprInt::eval(EvalState & state, Env & env, Value & v)
 
 void ExprString::eval(EvalState & state, Env & env, Value & v)
 {
-    mkString(v, s.c_str());
+    mkString(v, s);
 }
 
 
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index f29f9b684..5b27e2582 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -46,7 +46,7 @@ static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 }
 
 
-static Expr * unescapeStr(const char * s)
+static Expr * unescapeStr(SymbolTable & symbols, const char * s)
 {
     string t;
     char c;
@@ -66,7 +66,7 @@ static Expr * unescapeStr(const char * s)
         }
         else t += c;
     }
-    return new ExprString(t);
+    return new ExprString(symbols.create(t));
 }
 
  
@@ -119,7 +119,7 @@ inherit     { return INHERIT; }
                  "$\"" will be consumed as part of a string, rather
                  than a "$" followed by the string terminator.
                  Disallow "$\"" for now. */
-              yylval->e = unescapeStr(yytext);
+              yylval->e = unescapeStr(data->symbols, yytext);
               return STR;
             }
 <STRING>\$\{  { BEGIN(INITIAL); return DOLLAR_CURLY; }
@@ -140,7 +140,7 @@ inherit     { return INHERIT; }
                    return IND_STR;
                  }
 <IND_STRING>\'\'\\. {
-                   yylval->e = unescapeStr(yytext + 2);
+                   yylval->e = unescapeStr(data->symbols, yytext + 2);
                    return IND_STR;
                  }
 <IND_STRING>\$\{ { BEGIN(INITIAL); return DOLLAR_CURLY; }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 5c0071dca..1aa3df368 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -65,8 +65,8 @@ struct ExprInt : Expr
 
 struct ExprString : Expr
 {
-    string s;
-    ExprString(const string & s) : s(s) { };
+    Symbol s;
+    ExprString(const Symbol & s) : s(s) { };
     COMMON_METHODS
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index b0c54339b..3a72a4ade 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -7,19 +7,44 @@
 %parse-param { yyscan_t scanner }
 %parse-param { ParseData * data }
 %lex-param { yyscan_t scanner }
+%lex-param { ParseData * data }
 
-
-%{
-/* Newer versions of Bison copy the declarations below to
-   parser-tab.hh, which sucks bigtime since lexer.l doesn't want that
-   stuff.  So allow it to be excluded. */
-#ifndef BISON_HEADER_HACK
-#define BISON_HEADER_HACK
-
+%code requires {
+    
+#ifndef BISON_HEADER
+#define BISON_HEADER
+    
 #include "util.hh"
     
 #include "nixexpr.hh"
 
+namespace nix {
+
+    struct ParseData 
+    {
+        SymbolTable & symbols;
+        Expr * result;
+        Path basePath;
+        Path path;
+        string error;
+        Symbol sLetBody;
+        ParseData(SymbolTable & symbols)
+            : symbols(symbols)
+            , sLetBody(symbols.create("<let-body>"))
+            { };
+    };
+    
+}
+
+#define YY_DECL int yylex \
+    (YYSTYPE * yylval_param, YYLTYPE * yylloc_param, yyscan_t yyscanner, nix::ParseData * data)
+
+#endif
+
+}
+
+%{
+
 #include "parser-tab.hh"
 #include "lexer-tab.hh"
 #define YYSTYPE YYSTYPE // workaround a bug in Bison 2.4
@@ -28,27 +53,13 @@
 #include <stdlib.h>
 #include <string.h>
 
+YY_DECL;
 
 using namespace nix;
 
 
 namespace nix {
-
     
-struct ParseData 
-{
-    SymbolTable & symbols;
-    Expr * result;
-    Path basePath;
-    Path path;
-    string error;
-    Symbol sLetBody;
-    ParseData(SymbolTable & symbols)
-        : symbols(symbols)
-        , sLetBody(symbols.create("<let-body>"))
-    { };
-};
-
 
 static string showAttrPath(const vector<Symbol> & attrPath)
 {
@@ -113,9 +124,9 @@ static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
 }
 
 
-static Expr * stripIndentation(vector<Expr *> & es)
+static Expr * stripIndentation(SymbolTable & symbols, vector<Expr *> & es)
 {
-    if (es.empty()) return new ExprString("");
+    if (es.empty()) return new ExprString(symbols.create(""));
     
     /* Figure out the minimum indentation.  Note that by design
        whitespace-only final lines are not taken into account.  (So
@@ -195,7 +206,7 @@ static Expr * stripIndentation(vector<Expr *> & es)
                 s2 = string(s2, 0, p + 1);
         }
 
-        es2->push_back(new ExprString(s2));
+        es2->push_back(new ExprString(symbols.create(s2)));
     }
 
     return new ExprConcatStrings(es2);
@@ -224,9 +235,6 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
 }
 
 
-#endif
-
-
 %}
 
 %union {
@@ -337,15 +345,15 @@ expr_simple
   | INT { $$ = new ExprInt($1); }
   | '"' string_parts '"' {
       /* For efficiency, and to simplify parse trees a bit. */
-      if ($2->empty()) $$ = new ExprString("");
+      if ($2->empty()) $$ = new ExprString(data->symbols.create(""));
       else if ($2->size() == 1) $$ = $2->front();
       else $$ = new ExprConcatStrings($2);
   }
   | IND_STRING_OPEN ind_string_parts IND_STRING_CLOSE {
-      $$ = stripIndentation(*$2);
+      $$ = stripIndentation(data->symbols, *$2);
   }
   | PATH { $$ = new ExprPath(absPath($1, data->basePath)); }
-  | URI { $$ = new ExprString($1); }
+  | URI { $$ = new ExprString(data->symbols.create($1)); }
   | '(' expr ')' { $$ = $2; }
   /* Let expressions `let {..., body = ...}' are just desugared
      into `(rec {..., body = ...}).body'. */