Short for No-operation Kit. Set of functions making work with empty closures a breeze.

# NoopKit

Short for No-operation Kit. Set of functions making work with empty closures a breeze.

By empty closures we mean:

  • Closures not doing anything (not calling any other code).
  • Closures returning constant value.
  • Closures returning the same parameters which were passed to them.
  • More broadly closures with no side-effects.


Day by day we are using more functional programming in our apps. The turning point for mainstream developer on Apple platforms was the introduction of SwiftUI.

In UIKit it was enough to write empty method or not connect IBAction in the interface builder. But as you can see this has a downside, it is hard to search for an empty method or unused control.

Similarly, it is hard to search for unimplemented closures passed to higher-order functions. They can be a mess of underlines, brackets and parentheses. I wanted to improve the experience during active development by passing named functions, which enable us to make more readable code for human and are more discoverable.

There are typical use-cases where these free functions shine:

  • SwiftUI previews.
  • Tests.
  • Unimplemented view bindings.
  • Unimplemented completion handlers.

Secondarily, I wanted to learn Sourcery and see how variadic generics can be used in Swift until proposed solution from Generics manifesto is added to the language.

Usage examples

This library provides four free pure functions with variadic generics.


Can be used either as a temporary placeholder when calling higher-order functions where no action is required. Sometimes the parameter is not optional and you need to pass a value.

// With anonymous closure
URLSession.shared.dataTask(with: url) { _, _, _ in }.resume()
// Named function
URLSession.shared.dataTask(with: url, completionHandler: noop).resume()

Another typical place for noop function is as a default parameter in higher order functions.

// With anonymous closure
func animate(with completion: (Bool) -> Void = { _ in })
// Named function
func animate(with completion: (Bool) -> Void = noop)


Similar to noop, undefined is a placeholder function, but instead of doing nothing it calls fatal error. This function makes it clear to the developer, that it needs to be implemented. It is easy to find, which is not the case for empty closures.

Button("Tap and crash", action: undefined)
Stepper("Step to crash", onIncrement: undefined, onDecrement: undefined)


Undefined and noop do not work with return values. Const function takes one parameter and returns a closure, which throws away all the parameters and constantly returns the provided value.

// Using closure
validate(string: "Some string", using: { _ in true })
// Using const function
validate(string: "Some string", using: const(true))


Identity (id) function returns all the arguments it receives as a tuple. This is useful in a function with one parameter. Typical use-case is the compactMap method.

// Using closures
[1, nil, 2, nil].compactMap { $0 }
// Using id function
[1, nil, 2, nil].compactMap(id)

As you can see, the id can replace any { $0 } closure returning one anonymous parameter.


Add this package dependency to your Xcode project or add following line to your Package.swift file:

.package(url: "", from: "0.1.0")

Feel free to add some of these free functions using copy & paste if you want to experiment and don't need the full package.


Most of these functions are just a syntactic sugar. Be mindful when adding them to your project. Swift is a complicated language as is and introducing more syntactic sugar to your project can increase the complexity and understandability of your code. I personally don't use those functions on team projects, but experimentally on my personal project. If introducing these functions to your codebase make sure every contributor understands them and accepts their use.

Authors & contributing

All contributions are welcome.

Project was created by Matěj Kašpar Jirásek.

Project is licensed under MIT license.


Some people explored similar concepts in the past, notably:

Many of the ideas come from Haskell and other functional programming languages.

I iterated on their work mainly by adding „variadic generics“ using code generation.


  • Swift Tools 5.0.0
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Last updated: Sat Mar 18 2023 09:16:46 GMT-0500 (GMT-05:00)