Why does Swift require parameter names if it also requires argument order?

TL;DR

To help understand the answer to your question, you first have to understand that unlike other languages which support named arguments, in Swift, you are not actually seeing the argument names at all. You are seeing what are known as outward-facing labels. While they often do match the argument names, that is merely a coincidence/convenience. They are still very much separate things, with an entirely different set of rules, and understanding that difference helps explain the answer to your question.

That's why in Swift, when you're referring to the call site, don't think about argument names at all. Think of what you're seeing as words which add clarity at the call site by helping it read more like a sentence, and just as you can't randomly reorder words in a sentence, you shouldn't be able to randomly reorder those labels either.

The Rules

A simple set or rules describing the above can be summed up as follows...

• Argument labels (external) are meant make sense only at the call site so it's clear what you are passing. As such they may function more in a 'grammar' fashion than an 'identifier' fashion (e.g. the function add(4, to: 3) uses the label 'to:' for the second argument, and no label at all for the first argument as the function name itself completes the grammar.) Also, labels do not have to be unique as they are also bound by ordinal position.

• Argument names (internal) are meant to make sense only in the context of the function/method's internal body. The above function may be written as add(_ valueA: Int, to valueB: Int) which uses valueA and valueB only within the function's implementation.

• If such a name makes sense in both contexts, you only need to specify the argument name, and it will implicitly use that same name for the label (e.g. someFunc(a: Int) is the equivalent of someFunc(a a: Int))

I highly recommend checking out Swift.org's API Design Guidelines as it goes into much more detail about why these decisions were made.

The Details (e.g. the reason)

In most languages, there is no concept of outward-facing labels. You simply specify a function's argument values by ordinal position, usually separated by a comma.

Now in some languages, like C#, in addition to specifying argument values ordinally, you can also/instead specify them by argument name. If you are specifying all arguments by name, it seemingly logically makes sense to be able to specify them in whatever order you want, but there are no requirements to use names so you can mix and match with ordinals.

But if you can do that, what happens when you specify some ordinally and some by name? Now you're forced to put the named ones at the end because if you didn't, how would it know the position of the unnamed arguments? Or more confusing, what happens if you specify an argument value in the third position but you also specify that same argument by name later on? It can be confusing.

In contrast, Swift strives to not only be much more consistent in its calling conventions, but it also strives to be a self-documenting language with a focus on clarity at the point of use (i.e. the call-site). As such, Swift added the concept of external/outward labels which default to being required, but it allows you to make them optional if it makes sense. And if you don't explicitly specify a label for your argument, the label is implicitly set to the same name as the argument, but again, they are still very different things.

Now as to the why this is a good thing, let's look at some examples.

Examples

Let's start with a language like C# that does let you reorder named arguments.

var result = multiplyValues(valA: 20, valB: 5)

Since you can reorder it, you can write this as well...

var result = multiplyValues(valB: 5, valA: 20)

Now in Swift, what you use at the call site aren't argument names, they are labels to help in the overall context of the signature itself.

Consider this slightly more complex function definition...

func multiply(value: Int, by factor: Double, thenAdd addlAmount: Int){}

When called, it reads like this...

let result = multiply(value: 20, by: 1.5, thenAdd: 5)

Isn't that much clearer than the C# example? But it's important to note that 'by' is not the argument name, but rather a label that makes sense given its position in the call. The actual, underlying argument is called 'factor' and that's what's used inside the body of the function's implementation.

Following the Swift design guidelines, one may even make the first argument's label optional, which now reads like this...

let result = multiply(20, by: 1.5, thenAdd: 5)

Again, it's incredibly clear what's going on, almost reading like a sentence.

Now imagine if you didn't use those labels and just used the argument names. Now the names matter in the external context. For instance, look at this

let result = multiply(value: 20, addlAmount: 5, factor: 1.5)

Clarity starts to get obscured. Are you multiplying 20 by 1.5 then adding 5, or are you multiplying 25 by 1.5? Is the answer 35 or 37.5?

And what if you then used the external names/labels? Now it's even worse!

let result = multiply(by: 1.5, thenAdd: 5, 20)

What the heck is going on?

When you reorder them in this way, it's not exactly clear what you're multiplying by 1.5, because if this is in an entity (struct, class, etc.) it could be easily mistaken that you're multiplying the implicit self by 1.5 (i.e. self.multiply(by:)), then there's some rando parameter 20 at the end without context. Again, it's not clear.

Of course, naturally you may respond 'But that's with an optional label! The order shouldn't matter if and only if all labels have to be specified.' but now you're cluttering up the rules and breaking consistency for a single, specific use-case.

Perhaps a more pertinent question is what do you actually get with that one-off use-case? With all these up-sides of the existing rules, what's a down-side? What are you not able to do by not allowing a random order?

It's the above aversion to inconsistency that led Swift to introduce a breaking change in earlier versions of Swift where the first argument's label wasn't required even if you specified one. It implicitly added a _ if you will. But why should that first argument get special treatment when compared to the others? The answer... it shouldn't! All that did was confuse people, so they changed it in a future version because again, consistency is more important than cleverness, even over introducing a breaking change.

Argument Names have to be unique. Label names do not.

Since label names are meant to offer clarity at the call site, helping it read more like a sentence, it is possible to use the same label more than once since the order is also important.

Argument names however must be unique because they need to coexist within the implementation.

Here's a example demonstrating this. Note how the label 'and' is used twice, but the argument names 'a', 'b' and 'c' must be unique...

func add(_ a: Int, and b: Int, and c: Int) -> Int {
    return a + b + c
}

let result = add(1, and:2, and:3)

print(result)

To Sum Up...

External-facing labels should only make sense at the call site and should strive to complete the grammar of a sentence. Argument names however should only make sense in the context of a function's implementation.

Follow those simple guidelines and it will become very clear what should go where and in what order, and why it's a good thing that you can't simply reorder your arguments at the call site.

There are a couple of reasons Swift does this.

One is the influence of Objective-C on the designers of the language. Named parameters are a feature of the Objective-C language (and others as well.)

The other reason is that use of named parameters allows the code to be self documenting.

It would be interesting if the named parameters could be specified in any order, as you can do in other languages.

You can read more about the specifics of Swift functions here: Functions

Names are used for clarifying the purpose of each.

In swift, parameters can be anonymous and you can use the their order number in order to use them. You usually see them in closures and in Swift, functions are closures.

So order matters and the names are for more clarification.

Imagine the sorted function on an [Int]:

[1, 2].sorted(by: <#(Int, Int) throws -> Bool#>)

How could you know which one of these two arguments should be on the right or left side of the < operator? Their order!

For the sake of self-documented code, you may name them as first and second to prevent losing track of their order inside the function:

[1, 2].sorted { first, second -> Bool in
    return first < second
}

But as you may know, you could get rid of the names and just use the order number:

[1, 2].sorted {
    return $0 < $1
}

As you may know, this < is an infix function in Swift like this:

func <(lhs: Int, rhs: Int)->Bool { return lhs < rhs }

So you can pass its name to the entire function as an argument of the sorted:

[1, 2].sorted(by: <)

It's arguments have names, but how many times you use them? Zero! so how compiler knows which one to use when you pass?

function signature!

The types of the arguments, their order, and the type(s) of the function's return value(s)

The way to understand this is to realize that the label's purpose is not to tell the compiler which parameter this is. The order and type, combined with default values, does that.

The label's purpose is purely to dictate to the caller what the caller must say.

So for example:

func f(p1 param1:String = "one", p2 param2:Int = 2) {}

f(p1:"hey", p2:42)
f(p1:"hey")
f(p2:42)
f()
// but not:
// f(p2:42, p1:"hey")

The label p1: does not mean, "Hey, compiler, this is the param1 parameter."

It means, "Hey, caller, if you include this parameter (which must come first if you include both), you must label it p1:."

One thing not mentioned yet is for functions or initializers that have multiple parameters with defaults and you only want to override one or two of the defaults. For example:

func foo(aString: String = "", anInt: Int = 42, aFloat: Float = 10) {}

foo(aFloat: 50) // Skip the params where you want to use the defaults.