I've put together a pitch text to extend the use of trailing comma. Unfortunately, I don't have the skills to implement it myself but if anyone is willing to do at least there's an updated text partially written for the proposal.
Allow trailing comma in tuples, arguments/parameters and if/guard/while conditions
- Proposal: SE-NNNN
- Author: Mateus Rodrigues
- Review Manager: TBD
- Status: Awaiting Review
- Implementation: swift, swift-syntax (gated behind
-enable-experimental-feature TrailingComma). - Review: (pitch)
Introduction
This proposal aims to allow the use of trailing commas, currently restricted to array and dictionary literals, in tuples, arguments/parameters and if/guard/while conditions.
Motivation
Development Quality of Life Improvement
A trailing comma is a optional comma after the last item in a series of elements:
let rank = [
"Player 1",
"Player 3",
"Player 2",
]
Using trailing commas makes it easy to add, remove, reorder and comment in/out elements, without the need to add or delete the comma while doing any of these manipulations.
Consider the following SwiftUI modifier:
func frame(
width: CGFloat? = nil,
height: CGFloat? = nil,
alignment: Alignment = .center
) -> some View
frame(width:), frame(height:), frame(width:alignment:), frame(height:alignment:), frame(width:height:), frame(width:height:alignment:) are all valid calls but you can't easily swipe between frame(width:) and frame(width:alignment:) by commenting in/out alignment without add/remove trailing comma.
.frame(
width: 500,
// alignment: .leading
) ❌ Unexpected ',' separator
The introduction of parameter packs allows more APIs that are list-like at call site and would benefit from trailing comma.
extension [S] {
func sorted<each T: Comparable>(_ keyPath: repeat KeyPath<S, each T>) { }
}
arrayOfS.sorted(
\.a,
\.b,
// \.c
) ❌ Unexpected ',' separator
Since #21381 has been merged back in 2019 enum associated values supports default values and are a good fit for trailing comma as well.
Tuples use are very close to parameter list and, although may not be so frequently used, it seems natural that they adopt trailing comma too.
Multiple conditions in if, guard and while are also list-like and add, remove, reorder and comment in/out are not uncommon practice during development.
if
condition1,
condition2,
// condition3
{ ❌ Cannot convert value of type '() -> ()' to expected condition type 'Bool'
} ❌ Expected '{' after 'if' condition
Code Generation
Plugins and Macros have made it possible to generate code using swift and trailing comma would allow generate list of parameters and conditions without worrying about a special condition for the last element.
Code Diff
A tangential motivation is that trailing comma makes version-control diffs cleaner.
Without trailing comma:
foo(
- a: Int
+ a: Int,
+ b: Int
)
With trailing comma:
foo(
a: Int,
+ b: Int,
)
[!NOTE]
A similar proposal was rejected back in 2016 for Swift 3. It's been 8 years since that, the swift language has evolved a lot, some changes highlighted above as motivation, and the code style that "puts the terminating right parenthesis on a line following the arguments to that call" has been widely adopted by community, swift standard library codebase, swift-format, docc documentation and Xcode. Therefore, not encourage or endorse this code style doesn't hold true anymore nor is a reason for rejection.
Proposed solution
This proposal adds support for trailing comma to:
Arguments/Parameters
Including declaration and call of initializers, functions and enum case associated values.
func foo(
a: Int = 0,
b: Int = 0,
) {
}
foo(
a: 1,
b: 2,
)
Tuples
(1, 2, 3,)
Conditions
Including if, guard and while.
if
condition1,
condition2,
condition3,
{
}
Source compatibility
This change is purely additive and has no impact on existing code.
Future directions
Allow trailing comma anywhere there's a comma-separated list
Although this proposal focuses on the most requested use cases for trailing comma, there's other places with comma-separated list and the restriction could be consistently lifted for all of these.
Subclass and Protocol Conformance
class C2: C1, P1, P2, { }
Generics
struct S<T1, T2,> where T1: P2, T2: P2, { }
Switch Case
switch number {
case 1, 2, 3,:
...
default:
...
}
Alternatives considered
Eliding commas from multiline expression lists
A different approach to address the exact same motivation is to allow the comma between two expressions to be elided when they are separated by a newline.
print(
"red"
"green"
"blue"
)
This was even proposed and returned to revision back in 2019.
Even though both approach are not mutually exclusive, this proposal is about consistently extend an existing behavior in the language while eliding comma is a more serious change to the language.