SE-0330: Conditionals in Collections

[tkremenek (Ted Kremenek)]

Returned for Revision

This review is circling around two points:

1. Within the current syntax-based model for condition code, the proposal cannot be reasonably evaluated because the implications are not fully clear in both the written proposal and in the implementation (which is not up-to-date on main). Because the implementation is not up-to-date, the impact of the complexity of the implementation on the compiler is difficult to assess, and the behavior of various corner cases is not clear.

2. Whether or not the syntax-based approach for conditional code should be abandoned entirely for a different approach, such as a lexer-based one. This creates and existential question of whether this proposed change should even be considered.

The review has waded into debating the second point. To that end, @allevato raised the following reasonable concern:

@allevato

@johnno1962

All of this is irrelevant however if a firm decision has already been made that the processing of preprocessor conditionals should be moved into the lexer which if this were the case, can this be communicated in the review decision and can someone be assigned to start looking at it please?

Since this would have considerable usability and compatibility impact on SwiftSyntax users like swift-format and others writing Swift source analysis tooling, I hope that such a proposed change would be given its own pitch/discussion thread and not be communicated as part of this review decision.

The core team concurs that such a significant change in direction should have its own properly considered pitch/discussion given the potentially seismic implications. A decision along these lines will not be made as part of this review.

However, the points raised in favor of a lexer-based approach contend that the complexity of the current approach continues to stack up, and is potentially unpredictable to the user. @Chris_Lattner3 framed this perspective succinctly:

@Chris_Lattner3

One of the reasons that I'm in favor of a simple lexical #if model is that it is simple, explainable, and predictable for users. Our current model is already a pile of non-obvious special cases.

In order for this proposal to be considered, the implications of the proposed change needs to be clear. There have been several questions that have been raised in the review that would need to be answered before accepting this proposal as a refinement to the current conditional code model. Thus, the proposal is returned for revision, with the following request:

1. Update the implementation to work with main so that the implications of the implementation can be understood.

2. The proposal speaks to the cases brought up here, and sorts through the broader concerns brought up. An updated implementation will be helpful to this end.

With an updated proposal and implementation, the community and core team can properly weigh the cost/benefit tradeoff of this refinement to the current approach, while also providing signal for a potential discussion of moving the conditional compilation to a lexer-based approach. To be clear, it is not a forgone conclusion that a lexer-based approach is an inevitable outcome. As @allevato points out, this deserves sufficient discourse to make such a call.

On behalf of the core team, I'd like to thank both @johnno1962 as the proposal author and everyone who participated in this discussion.