? Let your code flow.

This library provides a bunch of extension methods for a better fluent syntax in Swift. This style is very useful for some operations that benefit from being able to be chained (composed) together.


  • .then to configure reference and value types. Useful for configuration at the point of initialization.

  • .mutate in place value types.

  • .let to transform an object into another.

  • .do to perform multiple actions with the same object.

  • Free function variants, for when you prefer this syntax or don’t want to conform to the protocol:

    • with (similar to .then)
    • withLet (similar to .let)
  • run as an alternative to immediately executed closures.


Use .then to perform an object configuration inline. It applies statements in the closure to the object. It’s very useful to set the properties of an object when defining it.

let label = UILabel().then {
  $0.text = "Hello"
  $0.textColor = .red
  $0.font = .preferredFont(forTextStyle: .largeTitle)

let size = CGSize().then {
	$0.width = 20

There are two overloads of this method provided. One that works on AnyObject (a.k.a. classes) and another that operates on Any (intended for value types). The compiler picks the correct one appropriately.

  • In the closure you get a reference to self or an inout copy in case of value types.
  • It returns the same reference to the object, or the mutated copy for value types.



Mutates a value in place. It s like .then but applies to self instead of a new copy. The value needs to be defined as a var.

view.frame.mutate {
  $0.origin.y = 200
  $0.size.width = 300
  • In the closure you get an inout reference to self .
  • It returns nothing.

This should be used only for value types. For reference types is recommended to use .then.


You can think of .let as a map operation but for all the types (not only for Functors). It lets you transform the object into an object of another type.

let dateString: String = Date().let {
    let formatter = DateFormatter()
    return formatter.string(from: $0)

It works especially well for type conversions based on initializers:

let number: Int? = "42".let(Int.init)

Don’t overuse this when you can use just plain dot syntax. You can use it to access a member of the object Date().let { $0.timeIntervalSince1970 } but that’s just the same as Date().timeIntervalSince1970.

  • You get a reference to self in the closure.
  • It returns the object returned in the closure.



Use this method to perform multiple actions (side effects) with the same object. It helps to reduce the verbosity of typing the same name multiple times.

UserDefaults.standard.do {
    $0.set(42, forKey: "number")
    $0.set("hello", forKey: "string")
    $0.set(true, forKey: "bool")

This behaves like other methods if you discard their return, but is preferred to use do to convey the intention better. It also lets you avoid writing the return on some occasions.

  • You get a reference to self in the closure.
  • It returns nothing.



By default, it prints self to the console. This method is useful for debugging intermediate values of a chain of method calls.

let result = Object()
   .then { ... }
   .let { ... }
  • You get a reference to self in the closure.
  • It returns the same object without touching it.

Free function with

Executes a closure with the object. This free function it’s a substitute for .then when you can’t use the method or if you prefer the free function style.

let label = with(UILabel()) {
    $0.text = "Hello"
    $0.textColor = .red
    $0.font = .preferredFont(forTextStyle: .largeTitle)
  • You get a reference to an inout copy of self in the closure.
  • It returns the returned object in the closure.


Free function withLet

Variant of with that let’s you return a different type. It’s a free function alternative of let.

Free function run

Executes a closure of statements, useful to be used when you need an expression. This is like making a closure and invoking immediately but sometimes is clearer to have a specific name for it.

let result = run { ... } // same as { ... }()


Supported Types

Since Swift doesn’t let us extend non-nominal types like Any we need to conform each type to the Flowable protocol.

The library provides out of the box conformances for a bunch of Standard Library, Foundation and UIKit types. See Conformances.swift for the entire list.

You can conform any type yourself by just extending it:

extension YourType: Flowable {}

Note that you can use the free function variants without the types conforming to the protocol.


  • devxoul/Then
  • Functional style approaches (like Overture)
  • Kotlin Scope Functions. Note that Swift can’t “rebind self” inside a closure, so most of Kotlin’s scope functions are redundant.
  • Other languages that have a similar with or using functions.


Alejandro Martinez | https://alejandromp.com | @alexito4


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