Always Write Functions to Cope with all Possible Parameter Values

Matt Galagher is back writing at Cocoa with Love. His goal is maintainability, which is the greatest of all, I think. It’s easy to copy code samples together to create an app, bur it’s hard to create a product you can keep alive and make better over years.

In that vein, his first article, “Partial functions in Swift, Part 1: Avoidance”, includes a lot of details why partial functions will hurt you. This is a great topic. Read his post for the basic set theory involved.

In a nutshell, a partial function is a function that advertises to take in more values than it actually does:

  • accepting Int as a parameter but only dealing with positive numbers properly, like array subscripts
  • everything NSArray and NSDictionary did in Objective-C where you couldn’t know what was inside
  • similarly, accepting id or AnyObject in the API bur requiring some protocol conformance behind the scenes

This is dangerous because you have yet another detail to remember when using a function to not make your app crash. So a partial function seems to accept a wider amount of values than it actually does, implementation-wise.

The corollary advise is to avoid functions that raise fatal errors when the input doesn’t meet expectations. Instead prefer to limit the scope of input instead, for example accepting UInt only. If your algorithm requires even more strict rules, create a value type to capture that. A function should limit the parameter types in such a way that it can deal with all possible values.

Matt’s example is a non-zero integer:

struct NonZeroInt {
    let value: Int
    init?(fromInt: Int) {
        guard fromInt != 0 else { return nil }
        value = fromInt

func divideFiveBy(x: NonZeroInt) -> Int {
    return 5 / x.value

Now divideFiveBy doesn’t need to guard against 0 to fail silently, or crash the program with a precondition. The client will have the power to supply correct values 100% of the time, making the resulting app more robust.

This may seem like overkill at first because you create more types – but it’s making things crystal clear to everyone involved. That’s a huge payoff.