diff --git a/projects/s-js-utils/src/lib/callable-object.spec.ts b/projects/s-js-utils/src/lib/callable-object.spec.ts
index 8e9b2a8..2b4abbe 100644
--- a/projects/s-js-utils/src/lib/callable-object.spec.ts
+++ b/projects/s-js-utils/src/lib/callable-object.spec.ts
@@ -1,11 +1,6 @@
 import { CallableObject } from "./callable-object";
 
-interface Multiplier {
-  // tslint:disable:callable-types
-  (value: number): number;
-}
-
-class Multiplier extends CallableObject {
+class Multiplier extends CallableObject<(value: number) => number> {
   constructor(public factor: number) {
     super((value: number) => value * this.factor);
   }
diff --git a/projects/s-js-utils/src/lib/callable-object.ts b/projects/s-js-utils/src/lib/callable-object.ts
index 5e4fac7..d00f242 100644
--- a/projects/s-js-utils/src/lib/callable-object.ts
+++ b/projects/s-js-utils/src/lib/callable-object.ts
@@ -1,12 +1,14 @@
+/** @hidden */
+export interface CallableObject<F extends (...args: any[]) => any> {
+  // tslint:disable-next-line:callable-types
+  (...args: Parameters<F>): ReturnType<F>;
+}
+
 /**
- * Extend this for classes whose objects are directly callable. Sadly, you'll need to define an extra interface and repeat the typing information, for now. (Maybe fixable with TypeScript 3 tuples?)
+ * Extend this for classes whose objects are directly callable.
  *
  * ```ts
- * interface Multiplier {
- *   (value: number): number;
- * }
- *
- * class Multiplier extends CallableObject {
+ * class Multiplier extends CallableObject<(value: number) => number> {
  *   constructor(public factor: number) {
  *     super((value: number) => value * this.factor);
  *   }
@@ -18,8 +20,8 @@
  * doubler(2); // 6
  * ```
  */
-export abstract class CallableObject {
-  constructor(f: Function) {
-    return Object.setPrototypeOf(f, new.target.prototype);
+export abstract class CallableObject<F extends (...args: any[]) => any> {
+  constructor(callAction: F) {
+    return Object.setPrototypeOf(callAction, new.target.prototype);
   }
 }
diff --git a/src/app/multiplier.ts b/src/app/multiplier.ts
index 690e9c3..e42ade5 100644
--- a/src/app/multiplier.ts
+++ b/src/app/multiplier.ts
@@ -1,11 +1,6 @@
-import { CallableObject } from "../../projects/s-js-utils/src/lib/callable-object";
+import { CallableObject } from "s-js-utils";
 
-export interface Multiplier {
-  // tslint:disable:callable-types
-  (value: number): number;
-}
-
-export class Multiplier extends CallableObject {
+export class Multiplier extends CallableObject<(value: number) => number> {
   constructor(public factor: number) {
     super((value: number) => value * this.factor);
   }