Trailing comma implementation #2515

mateusrodriguesxyz · 2024-02-29T13:44:24Z

This is a prototype implementation for Allow trailing comma in tuples, arguments and if/guard/while conditions gated behind -enable-experimental-feature TrailingComma.

ahoppen

Some initial thoughts. My biggest comment is that we should implement the disambiguation between an additional condition element and the body using lookahead instead of by modifying the tree.

Sources/SwiftParser/Declarations.swift

Sources/SwiftParser/Expressions.swift

Sources/SwiftParser/Statements.swift

Tests/SwiftParserTest/TrailingCommaTests.swift

mateusrodriguesxyz · 2024-03-07T21:25:47Z

@ahoppen I've updated the implementation for conditionals to use a much simpler lookahead approach

ahoppen

Thank you. The approach is the right one, I think. Getting the lookahead logic right is the key building block here, I think and we are on a very good track. I needed to bang my head really hard to come up with counterexamples, those are very contrived and maybe we can ignore them.

Sources/SwiftParser/Declarations.swift

Sources/SwiftParser/Attributes.swift

Sources/SwiftParser/Expressions.swift

Sources/SwiftParser/Statements.swift

Tests/SwiftParserTest/TrailingCommaTests.swift

mateusrodriguesxyz · 2024-03-14T18:00:27Z

@ahoppen I've fixed minor problems with parseArgumentListElements and updated parseConditionList and atStartOfStatementBody implementations to address our conversation. I've also updated the tests and included some substructure assertions to ensure that the expected block is parsed as the statement body.

ahoppen

I think we’re on the final stretch for this PR. Thanks for your continued work on this.

Sources/SwiftParser/Expressions.swift

Sources/SwiftParser/Statements.swift

Tests/SwiftParserTest/TrailingCommaTests.swift

ahoppen

Nice. Looking good to me 👍🏽

I would still like to hear a second opinion from @rintaro on the implementation.

Tests/SwiftParserTest/TrailingCommaTests.swift

rintaro · 2024-03-19T11:28:58Z

Could you let me know the plan for other comma separated lists like func parameter list, tuple type, generic paremeter/argument list etc?
I believe your proposal includes parameter list support at least, right?

ahoppen · 2024-03-19T11:36:19Z

I just suggested to keep the PR as-is for now and implement trailing closures for other constructs in a follow-up PR (#2515 (comment)). I’ve already spent considerable time reviewing this PR and it’s easier review-wise if we can consider the implications of trailing commas for other constructs in a separate PR.

mateusrodriguesxyz · 2024-03-19T11:39:48Z

I believe your proposal includes parameter list support at least, right?

That's right. The current proposal and implementation includes tuples, functions parameter/argument list and if, guard and while conditions. I'm keeping minimal and included the ones that I consider the most demanded use cases.

mateusrodriguesxyz · 2024-03-19T11:43:04Z

BTW @ahoppen thank you so much for all the help and patience while reviewing this PR and sorry for all the troubles. That's my first time trying to contribute to the language and whatever the outcome I'm happy with all that I've learned.

Sources/SwiftParser/Statements.swift

rintaro · 2024-03-19T12:20:34Z

Sources/SwiftParser/Statements.swift

+      // If the current token is at the start of a line, it is most likely a statement body. The only exceptions are:
+      if withLookahead({ $0.atStartOfStatementBody() }) {
+        // If a statement body starts after this on, we actually have a closure followed by the statement body e.g.
+        // if true, { true }
+        // {
+        //   print("body")
+        // }
+        return false


Sorry if I missed the previous discussion, but do we really care this case? Closure alone cannot be a condition. it must be used/called some way (e.g. call it, or make it a part of a sequence expression). This example case is ambiguous, it can be missing () after { true } or missing () after { print("body") }. I don't think it's worth to lookahead again unless there's any strong reason.

I agree that's a odd case but today that's a valid condition and the parser reports a Function produces expected type 'Bool'; did you mean to call it with '()'? error so without this lookahead there would be a regression.

That's not a valid condition today, it's just parsed as a condition expression and diagnosed in type checker. Since it's invalid anyway, I wouldn't consider it's a regression even if we don't parse it as a closure.

I don't feel comfortable changing existing behavior and as a user I consider the type checker diagnostic more helpful because is probably more close to the user intention.

I can add at(.leftBrace) before the withLookahead to avoid actually looking ahead if not necessary.

The current behavior makes sense before the trailing comma support. There was no ambiguity. But supporting trailing comma introduces some ambiguity, for example, I believe the user intention is that { return true } is the body in the following case:

func test() -> Bool { if condition1, condition2, { return true } { print("something") } }

Sorry, I took I time from this PR and missed your last answer. That's a really good counterexample. Honestly, I'm good with either behavior, whichever has a consensus so this can move forward. Could you weight in @ahoppen?

I agree with @rintaro.

I will change then and also resolve the conflicts with main branch.

rintaro · 2024-03-19T12:38:25Z

The current proposal and implementation includes tuples, functions parameter/argument list and if, guard and while conditions.

I don't see implementation/tests for functions parameter list. e.g. from your proposal

func foo(
    a: Int = 0, 
    b: Int = 0, 
) {
}

Am I missing anything?

rintaro · 2024-03-19T12:58:38Z

To be clear I'm not arguing that this PR should include parameter list support. I just want to make sure I'm not missing anything, and know the plan.

mateusrodriguesxyz · 2024-03-19T13:06:06Z

I don't see implementation/tests for functions parameter list. e.g. from your proposal

I think I didn't add this test case because arguments and parameters are parsed by the same parsePostfixExpressionSuffix so testing arguments covers parameters, but I can add it to make it clear.

rintaro · 2024-03-19T13:23:54Z

arguments and parameters are parsed by the same parsePostfixExpressionSuffix

Parameters are parsed with parseParameterClause() not parsePostfixExpressionSuffix() :)

Sources/SwiftParser/Statements.swift

mateusrodriguesxyz · 2024-03-19T13:38:22Z

Parameters are parsed with parseParameterClause() not parsePostfixExpressionSuffix() :)

Now that's strange because I never touched this functions but if I add your example as a test case parsePostfixExpressionSuffix is also called and it does parse correctly so I assumed that was it 🤔 It was my intention from the beginning to also support parameters.

rintaro · 2024-03-19T13:46:20Z

That's... concerning (for current parser) 😅

FWIW parsePostfixExpressionSuffix is called whenever parsing an expression. In this case parsing 0 (default parameter value) causes parsePostfixExpressionSuffix

Sources/SwiftParser/Expressions.swift

mateusrodriguesxyz · 2024-03-19T13:57:13Z

That's... concerning (for current parser) 😅

I think parseParameterClausealready supports trailing comma because !self.at(.endOfFile, .rightParen) breaks the while even if the last parameter has as trailing comma so I think parsePostfixExpressionSuffix was the one blocking it from working it.

swift-syntax/Sources/SwiftParser/Parameters.swift

Lines 291 to 302 in 18cd7f9

    
           while !self.at(.endOfFile, .rightParen) 
        
             && keepGoing 
        
             && self.hasProgressed(&loopProgress) 
        
           { 
        
             let parameter = parseParameter(&self) 
        
             if parameter.isEmpty { 
        
               keepGoing = false 
        
             } else { 
        
               keepGoing = parameter.trailingComma != nil 
        
               elements.append(parameter) 
        
             } 
        
           }

rintaro · 2024-03-19T14:02:16Z

Yeah, I mean we've just discovered a bug in the current parser. We should disallow it unless trailingComma experimental feature is enabled.
(I don't think it's a scope of this PR)

mateusrodriguesxyz · 2024-03-21T15:06:47Z

I've update apple/swift#71975 implementation to match this PR.

mateusrodriguesxyz · 2024-04-05T17:39:07Z

I've update atStartOfStatementBody implementation and resolved the conflicts. I think that's it? @ahoppen @rintaro Thank you both for the review!

ahoppen · 2024-04-05T20:52:00Z

One last thing: Could you squash your commits? It makes for a nicer Git history https://github.com/apple/swift-syntax/blob/main/CONTRIBUTING.md#authoring-commits

Other than that, there’s anything to be done from my side. I would like to wait for @rintaro’s approval as well though.

mateusrodriguesxyz · 2024-04-17T11:36:52Z

@rintaro do you have any more to add before I squash the commits?

rintaro · 2024-04-18T16:46:43Z

@swift-ci Please test

mateusrodriguesxyz · 2024-04-18T20:05:59Z

@rintaro I had forget to format the source code after I merged with main, sorry. I think it will pass all the checks now.

rintaro · 2024-04-18T21:30:06Z

@swift-ci Please test

ahoppen · 2024-04-18T21:58:57Z

@swift-ci Please test Windows

…ard/while conditions

mateusrodriguesxyz · 2024-04-19T02:03:38Z

One last thing: Could you squash your commits?

@ahoppen done!

kimdv · 2024-04-21T20:14:01Z

@swift-ci please test

kimdv · 2024-04-21T20:14:53Z

@swift-ci please test windows

mateusrodriguesxyz requested review from ahoppen and bnbarham as code owners February 29, 2024 13:44

mateusrodriguesxyz mentioned this pull request Feb 29, 2024

Trailing comma implementation apple/swift#71975

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ahoppen reviewed Feb 29, 2024

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mateusrodriguesxyz requested a review from rintaro April 11, 2024 11:32

rintaro approved these changes Apr 18, 2024

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Add trailing comma support to tuples, arguments, parameters and if/gu…

376ddbb

…ard/while conditions

mateusrodriguesxyz force-pushed the trailing-comma branch from 37ac9f5 to 376ddbb Compare April 19, 2024 02:02

ahoppen merged commit a85aa07 into apple:main Apr 22, 2024
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mateusrodriguesxyz mentioned this pull request May 7, 2024

Allow trailing comma in comma-separated lists swiftlang/swift-evolution#2344

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Trailing comma implementation #2515

Trailing comma implementation #2515

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