Swift Custom Operator Spotlight: Optional Unwrapping

How often have you written code like this:

var object: MyObject?
...
if let unwrapped = object {
  doSomething(with: unwrapped)
}

Or, worse, in order to minimize indentation or because you’re relying on a library you don’t control, how often have you written a function like this:

func optionalDoSomething(with myObject: MyObject?) {
  if let unwrapped = myObject {
    doSomething(with: unwrapped)
  }
}

Gross! Well, out of frustration from this happening frequently, I’ve written an operator to alleviate this and allow for more ergonomic functional programming.

infix operator ?>: MultiplicationPrecedence
public func ?><T>(t: T?, f: (T) -> Void) {
    if let t = t {
        f(t)
    }
}

This may look complicated, and feel free to refer back to my article on what a custom operator is. But this will allow us to use optionals and functions that only accept unwrapped values together easily:

var object: MyObject?
object ?> doSomething

which will compile without warnings (whereas trying to call doSomething(with: object) would result in an error popping up). What’s better is that we can now use this to compose two functions into one by creating a new operator that builds on the one above:

infix operator >?>: Composition
public func >?><A, B>(f: @escaping (A) -> B?, g: @escaping (B) -> Void) -> (A) -> Void {
  return { a in
    f(a) ?> g
  }
}

which we can use like this:

let objectFromString: (String) -> MyObject? = { ... }
let newFunction = objectFromString >?> doSomething

That is, suppose we have a function objectFromString which returns an optional, but doSomething requires a non-optional; we could pass an unwrapped version from one to the other by using this >?> operator. To do this the old way, we’d need to do this:

let objectFromString: (String) -> MyObject? = { ... }
let newFunction: (String) -> Void = { string in
  if let myObject = objectFromString(string) {
    doSomething(with: myObject)
  }
}

Which I find much less readable, not to mention more than double the number of lines.

With operators of these sorts, it becomes very easy to work with optionals in a functional programming codebase.