replace-ternary.md

Replace the ?: operator with an in-language function

• Proposal: TBD
• Author: Charlotte Tortorella
• Editor: Soroush Khanlou
• Review Manager: TBD
• Status: TBD

Introduction

The ternary operator in Swift was added early in development, as a holdover from C. This document is an attempt to provide a clear look at the ternary operator without the baggage of the languages that came before, and comes to the conclusion that we should deprecate and remove the ternary operator in favor of an extension to Bool.

As a quick refresher, here's what the ternary operator looks like:

let a = 10
let b = 20
// If a is less than b, sets e to "foo", else sets e to "bar"
let e = a < b ? "foo" : "bar"

Advantages of The Ternary Operator

The primary advantage of this operator is its terseness and expressive capability. It's shorthand for (e.g.):

let a = 10
let b = 20
let e: String
if a < b {
  e = "foo"
} else {
  e = "bar"
}

The second advantage of Swift supporting the ternary operator is continuity with C, and other common languages in the extended C family (C++, Objective-C, Java, C#, Javascript, etc). People coming to Swift from these other languages may reasonably expect this operator to exist. That said, there are also popular languages which have kept the majority of C operators but dropped the ternary operator (e.g. Go and Rust).

Disadvantages of The Ternary Operator

1. The existence of the ternary operator as a holdover from C is to increase the familiarity of the Swift language for C family developers, at the expense of newcomers. Established developers do much better with learning concepts than newcomers to programming and probably don't need their hands held with this carry over of an operator.

2. The ternary operator adds complexity to the compiler, because it requires special handling. It is the only operator that requires two components to work (both the ? and the :), it uses a character that is excluded from being used in other operators (:), and it isn't defined in the standard library.

3. The ternary operator's usage of ? can be confusing to new users. Every other instance of ? is associated with Optional values.

4. The ternary operator uses :, which is already a heavily overloaded symbol in Swift. : is used in hash tables, type annotations for variables, class inheritance, and protocol conformance.

5. The ternary operator's short length lends it to being abused in the nested ternary operator anti-pattern. This is similar to the ++ and -- operators, which were removed in Swift 3. While they worked fine and were readable enough when used alone, using them multiple times in a single expression like function(a++, ++a) made them highly unreadable and confusing.

6. This operator is only applicable to a single type, Bool.

7. If the ternary operator weren't in common usage, it would not be proposed for Swift. Higher clarity can be achieved with common language features by creating an extension to Bool.

8. The ternary operator was created for and is much more suited to a language like C, where there were no generics and as such no alternative to an unintuitive operator.

9. Several other modern languages, like Rust and Go discussed earlier, have eschewed the usage of the ternary operator entirely. Other languages that have special constructs similar to ?:, such as if then else in Haskell have discussed removing it. if then else is identical to the ?: operator, excepting that it's prefixed by if, while ?: has no prefix.

Example: if True then 10 else 20

10. On a more personal and anecdotal note, the ternary operator gave me more trouble than any other operator when I was first learning how to program. I’ve also spoken to several other people who expressed similar sentiments about this operator’s inscrutability.

Proposed Approach

We should drop the ternary operator in favor of a new extension to Bool. There are a few possibilities for the naming of this function. We've provided four for consideration in this proposal, but are open to other options as well. This proposal is much more about the concept than the naming of the replacement function.

extension Bool {
    /// If `self == true`, returns `t`, otherwise, returns `f`.
    func transformed<T>(true t: @autoclosure () -> T, false f: @autoclosure () -> T) -> T {
        if self {
            return t()
        } else {
            return f()  
        }
    }

    func when<T>(true t: @autoclosure () -> T, false f: @autoclosure () -> T) -> T {
      ...
    }

    func if<T>(true t: @autoclosure () -> T, false f: @autoclosure () -> T) -> T {
      ...
    }

    func if<T>(then t: @autoclosure () -> T, else f: @autoclosure () -> T) -> T {
      ...
    }
}

Only one of these should be chosen. We're not proposing adding multiple functions that achieve the same thing.

Example usage:

let a = 10
let b = 20
_ = (a < b).transformed(true: "foo", false: "bar")
_ = (a < b).when(true: "foo", false: "bar")
_ = (a < b).if(true: "foo", false: "bar")
_ = (a < b).if(then: "foo", else: "bar")

Impact on existing code

This proposal is breaking and would require migration.

Alternatives considered

Simplest alternative: we could leave the ternary operator as is and not introduce any new concepts.

It'd also be possible to add an if then else Haskell-esque expression. This would have the disadvantages of still needing special handling by the compiler. Since this proposal's intention is partially to remove compiler complexity, this would be counterproductive and would probably confuse new users in a similar way to how ?: does.