diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 56b23cc622..06ca8c928e 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -308,3 +308,27 @@ string showType(Expr e)
     return "an unknown type";
 }
 
+
+string showValue(Expr e)
+{
+    ATerm s;
+    int i;
+    if (matchStr(e, s)) {
+        string t = aterm2String(s), u;
+        for (string::iterator i = t.begin(); i != t.end(); ++i)
+            if (*i == '\"' || *i == '\\') u += "\\" + *i;
+            else if (*i == '\n') u += "\\n";
+            else if (*i == '\r') u += "\\r";
+            else if (*i == '\t') u += "\\t";
+            else u += *i;
+        return "\"" + u + "\"";
+    }
+    if (matchPath(e, s)) return aterm2String(s);
+    if (matchUri(e, s)) return aterm2String(s);
+    if (matchNull(e)) return "null";
+    if (matchInt(e, i)) return (format("%1%") % i).str();
+    if (e == eTrue) return "true";
+    if (e == eFalse) return "false";
+    /* !!! incomplete */
+    return "<unknown>";
+}
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 6e67e5ebd1..3fd22ecfed 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -92,5 +92,7 @@ Expr makeBool(bool b);
 
 string showType(Expr e);
 
+string showValue(Expr e);
+
 
 #endif /* !__NIXEXPR_H */
diff --git a/src/nix-instantiate/main.cc b/src/nix-instantiate/main.cc
index 963e81bac4..857aeaa247 100644
--- a/src/nix-instantiate/main.cc
+++ b/src/nix-instantiate/main.cc
@@ -44,9 +44,17 @@ static void printResult(EvalState & state, Expr e,
         ATerm body, pos;
         if (matchFunction(e, formals, body, pos)) {
             for (ATermIterator i(formals); i; ++i) {
-                Expr name; ATerm d1, d2;
-                if (!matchFormal(*i, name, d1, d2)) abort();
-                cout << format("%1%\n") % aterm2String(name);
+                Expr name; ValidValues valids; ATerm dummy;
+                if (!matchFormal(*i, name, valids, dummy)) abort();
+                cout << format("%1%: ") % aterm2String(name);
+                ATermList valids2;
+                if (matchValidValues(valids, valids2)) {
+                    for (ATermIterator j(valids2); j; ++j) {
+                        Expr e = evalExpr(state, *j);
+                        cout << format("%1% ") % showValue(e);
+                    }
+                }
+                cout << format("\n");
             }
         } else
             printMsg(lvlError, "warning: expression does not evaluate to a function");