SE-0257: Eliding commas from multiline expression lists

[jawbroken]

I suppose I'll have to repost this from the pitch thread because it doesn't seem to have been mentioned or addressed at all in the proposal under review (apologies if I somehow missed it).

@jawbroken

An example of some confusion this might cause:

let a = [
   1
  -1
  +1
]

looks like it could be splitting a single expression across multiple lines, but is actually declaring an array with 3 elements. The very similar

let a: [Int] = [
  1
  - 1	
  + 1
]

currently produces an array with a single element. I don't think this is technically ambiguous, because unary operators can't be separated from their operands, but it is very subtle.

Note: Similar confusion already exists to some extent

let b = 1
+ 1 // b = 2

let b = 1 // b = 1
+1 // Warning: Result of operator '+' is unused

but at least you get a warning.

@jawbroken

@Douglas_Gregor made a great argument about the similarities between semicolon elision and comma elision, and everyone accepts that it's not technically ambiguous, but there are key differences in the possible warning behaviour that are being glossed over. How important you find good warnings in cases like these will affect how acceptable this pitch is to you.

@jawbroken

Before I played around with the example I posted in the thread I didn't actually know what the result would be, because I hadn't internalised all the rules around mixing whitespace and prefix/infix operators (and I doubt I'm the only one). I'm not suggesting that the inability to provide good warnings in confusing situations is fatal to the proposal, just that there is a tradeoff here and people will reasonably disagree.

I have come around to supporting the proposal, but I think this tradeoff should have been mentioned. There might be the opportunity for warnings here when operators have both prefix/postfix and infix forms, if deemed necessary.

let solutions = [
  2*(1-sqrt(a))*sqrt(b+sqrt(b))
  -(1+sqrt(a))(sqrt(2*b)-sqrt(2))
] // two solutions

let solutions = [
  2*(1-sqrt(a))*sqrt(b+sqrt(b))
  - (1+sqrt(a))(sqrt(2*b)-sqrt(2))
] // one solution