inessential by Brent Simmons

Why I Prefer Protocol-Oriented-Programming in Objective-C to Swift

I first learned protocol-oriented-programming with Objective-C, and I was very pleased to see the Swift team emphasize this style.

But, at least at this writing at the end of 2016, I still run into problems when I use this style of programming in Swift.

Here’s the problem I’m trying to solve:

Variant Value

I’m working on a schema-less hierarchical database. Tables can contain tables, and they can contain values such as strings, numbers, booleans, dates, arrays, and so on.

To represent these values, I’d rather use a Value protocol rather than a Value class. If it’s a protocol, then I can have completely separate implementations: BoolValue and DateValue and ArrayValue and so on would conform to Value, but otherwise would have different implementations.

But here’s the thing: ArrayValue needs to have an actual Swift array of type [Value].

So let’s say you want to add something to that Swift array, where arrayValue is of [Value] type. Let’s say you’ve created trueValue, which is a BoolValue, and BoolValue conforms to the Value protocol.

arrayValue += [trueValue]

You can’t. Because even though BoolValue conforms to the Value protocol, this isn’t handled automatically. The compiler knows all the types involved, but it still won’t let you do this unless you explicitly cast. The following works:

arrayValue += [trueValue as Value]

In Objective-C, no casting is required:

[arrayValue addObject:trueValue];

This may seem like nitpicking on my part, but that added bit of housekeeping makes protocol-oriented-programming in Swift feel a little bit unnatural.

A natural form of protocol-oriented-programming might start with this premise: any time the protocol type is called-for, you can use an object that conforms to that protocol.

More complexity, to push the point

Let’s say you have two arrays:

let array1 = [trueValue as Value]
let array2 = [trueValue]

You and I both know that those arrays contain just one object, and it’s an identical object. Those two arrays are absolutely equal.

But you can’t compare them:

array1 == array2 will not compile, because array1 is of type [Value] and array2 is of type [BoolValue]. Even though BoolValue conforms to Value, this won’t work.

In Objective-C, you’d just use isEqual:, and it would work as expected.

But it gets worse:

let array1 = [trueValue as Value]
let array2 = [trueValue as Value]

Okay: now you know those two arrays couldn’t possibly be more equal.

But array1 == array2 still won’t compile, and you’ll get the following error:

binary operator '==' cannot be applied to two '[Value]' operands

In Objective-C, you’d write:

NSArray \*array1 = @[trueValue];
NSArray \*array2 = @[trueValue];

And [array1 isEqual:array2] will return YES.

(You could type those arrays using lightweight generics, and the outcome is the same: it works.)

So: a second premise for natural protocol-oriented-programming might be: a protocol-conforming object should have the same features as other objects — for instance, you should be able to make it Equatable (the equivalent of, in Objective-C, responding to isEqual:).