* New built-in function `builtins.attrNames' that returns the

  names of the attributes in an attribute set.

4 files changed, 51 insertions, 0 deletions

diff --git a/doc/manual/writing-nix-expressions.xml b/doc/manual/writing-nix-expressions.xml
index 1cc5bb95b8c3..9770e91822b5 100644
--- a/doc/manual/writing-nix-expressions.xml
+++ b/doc/manual/writing-nix-expressions.xml
@@ -1362,6 +1362,24 @@ is also available as <function>builtins.derivation</function>.</para>
   </varlistentry>
 
   
+  <varlistentry><term><function>builtins.attrNames</function>
+  <replaceable>attrs</replaceable></term>
+
+    <listitem><para>Return the names of the attributes in the
+    attribute set <replaceable>attrs</replaceable> in a sorted list.
+    For instance, <literal>builtins.attrNames {y = 1; x =
+    "foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
+    There is no built-in function <function>attrValues</function>, but
+    you can easily define it yourself:
+
+<programlisting>
+attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>
+
+    </para></listitem>
+
+  </varlistentry>
+
+  
   <varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
 
     <listitem><para>Return the <emphasis>base name</emphasis> of the
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 1d0d9c6b2245..fcf0354509a2 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -620,6 +620,26 @@ static Expr primGetEnv(EvalState & state, const ATermVector & args)
 }
 
 
+/* Return the names of the attributes in an attribute set as a sorted
+   list of strings. */
+static Expr primAttrNames(EvalState & state, const ATermVector & args)
+{
+    ATermMap attrs(128); /* !!! */
+    queryAllAttrs(evalExpr(state, args[0]), attrs);
+
+    StringSet names;
+    for (ATermMap::const_iterator i = attrs.begin(); i != attrs.end(); ++i)
+        names.insert(aterm2String(i->key));
+
+    ATermList list = ATempty;
+    for (StringSet::const_reverse_iterator i = names.rbegin();
+         i != names.rend(); ++i)
+        list = ATinsert(list, makeStr(*i, PathSet()));
+
+    return makeList(list);
+}
+
+
 /* Apply a function to every element of a list. */
 static Expr primMap(EvalState & state, const ATermVector & args)
 {
@@ -732,6 +752,7 @@ void EvalState::addPrimOps()
     addPrimOp("__head", 1, primHead);
     addPrimOp("__tail", 1, primTail);
     addPrimOp("__getEnv", 1, primGetEnv);
+    addPrimOp("__attrNames", 1, primAttrNames);
 
     addPrimOp("map", 2, primMap);
     addPrimOp("__getAttr", 2, primGetAttr);
diff --git a/tests/lang/eval-okay-attrnames.exp b/tests/lang/eval-okay-attrnames.exp
new file mode 100644
index 000000000000..98af99a0cab0
--- /dev/null
+++ b/tests/lang/eval-okay-attrnames.exp
@@ -0,0 +1 @@
+Str("newxfoonewxy",[])
diff --git a/tests/lang/eval-okay-attrnames.nix b/tests/lang/eval-okay-attrnames.nix
new file mode 100644
index 000000000000..978138f0c0d9
--- /dev/null
+++ b/tests/lang/eval-okay-attrnames.nix
@@ -0,0 +1,11 @@
+with import ./lib.nix;
+
+let
+
+  attrs = {y = "y"; x = "x"; foo = "foo";} // rec {x = "newx"; bar = x;};
+
+  names = builtins.attrNames attrs;
+
+  values = map (name: builtins.getAttr name attrs) names;
+
+in concat values