3 files changed, 28 insertions, 7 deletions
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index c7fc52db15e5..736c3e522fcd 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -124,15 +124,15 @@ static void getDerivations(EvalState & state, Expr e,
     DrvInfos & drvs, Exprs & doneExprs, const string & attrPath,
     const string & pathTaken)
 {
-    /* Automatically call functions that have defaults for all
-       arguments. */
+    /* Automatically call functions for which each argument has a
+       default value. */
     ATermList formals;
     ATerm body, pos;
     if (matchFunction(e, formals, body, pos)) {
         for (ATermIterator i(formals); i; ++i) {
             Expr name, def; ATerm values, def2;
             if (!matchFormal(*i, name, values, def2)) abort();
-            if (!matchDefaultValue(def2, def)) 
+            if (!matchDefaultValue(def2, def))
                 throw TypeError(format("cannot auto-call a function that has an argument without a default value (`%1%')")
                     % aterm2String(name));
         }
diff --git a/src/nix-instantiate/help.txt b/src/nix-instantiate/help.txt
index 45607b71af00..18b1d8bd0d99 100644
--- a/src/nix-instantiate/help.txt
+++ b/src/nix-instantiate/help.txt
@@ -15,4 +15,6 @@ Options:
   --eval-only: evaluate and print resulting term; do not instantiate
   --parse-only: parse and print abstract syntax tree
 
+  --attr / -A PATH: select an attribute from the top-level expression
+
   --add-root: add garbage collector roots for the result
diff --git a/src/nix-instantiate/main.cc b/src/nix-instantiate/main.cc
index d73d0e613f72..31d61f3afebd 100644
--- a/src/nix-instantiate/main.cc
+++ b/src/nix-instantiate/main.cc
@@ -33,11 +33,25 @@ static int rootNr = 0;
 static bool indirectRoot = false;
 
 
-static void printResult(EvalState & state, Expr e, bool evalOnly,
-    const string & attrPath)
+static void printResult(EvalState & state, Expr e,
+    bool evalOnly, bool printArgs, const string & attrPath)
 {
     if (evalOnly)
         cout << format("%1%\n") % e;
+    
+    else if (printArgs) {
+        ATermList formals;
+        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);
+            }
+        } else
+            printMsg(lvlError, "warning: expression does not evaluate to a function");
+    }
+    
     else {
         DrvInfos drvs;
         getDerivations(state, e, drvs, attrPath);
@@ -62,6 +76,7 @@ void run(Strings args)
     bool readStdin = false;
     bool evalOnly = false;
     bool parseOnly = false;
+    bool printArgs = false;
     string attrPath;
 
     for (Strings::iterator i = args.begin();
@@ -79,6 +94,10 @@ void run(Strings args)
             readOnlyMode = true;
             parseOnly = evalOnly = true;
         }
+        else if (arg == "--print-args") {
+            readOnlyMode = true;
+            printArgs = true;
+        }
         else if (arg == "--add-root") {
             if (i == args.end())
                 throw UsageError("`--add-root requires an argument");
@@ -101,7 +120,7 @@ void run(Strings args)
 
     if (readStdin) {
         Expr e = evalStdin(state, parseOnly);
-        printResult(state, e, evalOnly, attrPath);
+        printResult(state, e, evalOnly, printArgs, attrPath);
     }
 
     for (Strings::iterator i = files.begin();
@@ -111,7 +130,7 @@ void run(Strings args)
         Expr e = parseOnly
             ? parseExprFromFile(state, path)
             : evalFile(state, path);
-        printResult(state, e, evalOnly, attrPath);
+        printResult(state, e, evalOnly, printArgs, attrPath);
     }
 
     printEvalStats(state);