[SR-12043] Top-level variables in REPL

[swift-ci]

Previous ID	SR-12043
Radar	rdar://problem/58819915
Original Reporter	svanimpe (JIRA User)
Type	Bug

Environment

Swift 5.1.3

Additional Detail from JIRA

Votes	3
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 3aa2a9d5cebddd4796f89d36aa578bf6

Issue Description:

I've noticed that the REPL assigns default values when declaring (but not initializing) a variable:

  1> var d: Double
d: Double = 0
  2> var i: Int
i: Int = 0
  3> var t: (Int, Int)
t: (Int, Int) = {
  0 = 0
  1 = 0
}
  4> var f: (Int) -> Int
f: (Int) -> Int =
  5> var b: Bool
b: Bool = false

Is this behavior documented somewhere? Is this even intentional, or is it just showing the contents of memory?

I believe this is a recent change, as I remember seeing a "top-level variable must be initialized" error for at least some of these examples with Swift 4.

[CodaFi]

It's not documented, and it allows for the creation of undefined behavior: the value you get initialized with is usually the zero bitpattern, so you can create ill-typed null pointers out of thin air.

[CodaFi]

@swift-ci create

[dcci]

Yes, this is intentional.
The whole story is a little messy here, we should really reconsider if we get to reply the REPL to properly match the semantics of the language.

[dcci]

More generally, the REPL does a lot of stuffs that are not really kosher from a language perspective because it was originally thought as a tool to "experiment". If you ask me, the semantics of the REPL and the one of the language should be a 1:1 mapping, but it's really hard to get to that point without breaking, e.g. existing playgrounds.

[swift-ci]

Comment by Steven Van Impe (JIRA)

If Swift is going to support more platforms in the future, the REPL will become an important teaching tool, as the point of entry for new learners.

With that in mind, I hope the REPL will evolve to match the language 1:1 and become more reliable, as issues like this, and SR-8690, and SR-1743, can easily leave a bad impression on new learners.