diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a1a227c8a..871dfa64c 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -724,7 +724,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
     }
     
     if (fun.type != tLambda)
-        throwTypeError("attempt to call something which is neither a function nor a primop (built-in operation) but %1%",
+        throwTypeError("attempt to call something which is not a function but %1%",
             showType(fun));
 
     unsigned int size =
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index b833413ea..db7aa9e89 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -906,6 +906,30 @@ static void prim_map(EvalState & state, Value * * args, Value & v)
 }
 
 
+/* Filter a list using a predicate; that is, return a list containing
+   every element from the list for which the predicate function
+   returns true. */
+static void prim_filter(EvalState & state, Value * * args, Value & v)
+{
+    state.forceFunction(*args[0]);
+    state.forceList(*args[1]);
+
+    // FIXME: putting this on the stack is risky.
+    Value * vs[args[1]->list.length];
+    unsigned int k = 0;
+
+    for (unsigned int n = 0; n < args[1]->list.length; ++n) {
+        Value res;
+        state.callFunction(*args[0], *args[1]->list.elems[n], res);
+        if (state.forceBool(res))
+            vs[k++] = args[1]->list.elems[n];
+    }
+
+    state.mkList(v, k);
+    for (unsigned int n = 0; n < k; ++n) v.list.elems[n] = vs[n];
+}
+
+
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, Value * * args, Value & v)
 {
@@ -1120,6 +1144,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__head", 1, prim_head);
     addPrimOp("__tail", 1, prim_tail);
     addPrimOp("map", 2, prim_map);
+    addPrimOp("__filter", 2, prim_filter);
     addPrimOp("__length", 1, prim_length);
     
     // Integer arithmetic