diff --git a/dhallToNix.nix b/dhallToNix.nix
index 384017a..fe98fa2 100644
--- a/dhallToNix.nix
+++ b/dhallToNix.nix
@@ -15,4 +15,4 @@
     nativeBuildInputs = [pkgs.dhall-nix];
   };
 in
-  import "${drv}" (import ./runtime.nix)
+  import "${drv}" (import ./runtime.nix pkgs.lib)
diff --git a/primops.dhall b/primops.dhall
index 19d4848..247bfeb 100644
--- a/primops.dhall
+++ b/primops.dhall
@@ -5,6 +5,7 @@ let Set = ./Set/Type.dhall
 in  { equal : Any → Any → Bool
     , fix : ∀(t : Type) → (t → t) → t
     , `fix'` : (Set → Set) → Set
+    , ignoreOtherArgs : ∀(A: Type) → ∀(B: Type) → (A → B) → A → B
     , mergeAttrs : Set → Set → Set
     , toAny : ∀(t : Type) → ∀(v : t) → Any
     , toTypeUnchecked : ∀(t : Type) → ∀(v : Any) → t
diff --git a/runtime.nix b/runtime.nix
index e7478e8..580efb9 100644
--- a/runtime.nix
+++ b/runtime.nix
@@ -1,8 +1,13 @@
-{
+lib: {
   primops = {
     equal = a: b: a == b;
     fix = _: f: let x = f x; in x;
     fix' = f: let x = f x // { __unfix__ = f; }; in x;
+    ignoreOtherArgs = _: _: f: let
+      args = lib.trivial.functionArgs f;
+      filterFunction = k: _: builtins.hasAttrs k set;
+      filteredFunction = s: f (lib.attrsets.filterAttrs filterFunction s);
+    in lib.trivial.setFunctionArgs filteredFunction args;
     mergeAttrs = a: b: a // b;
     toAny = _: v: v;
     toTypeUnchecked = _: v: v;