[swift-dev] Why doesn't CaptureListExpr hold a ClosureExpr?

[John McCall]

> On Feb 8, 2017, at 12:18 AM, Jacob Bandes-Storch via swift-dev <swift-dev at swift.org> wrote:
> I don't think it would be a very big parser change to invert the relationship. Maybe I'll try it out and put up another PR.
> 
> On the other hand, noticed the header comment says:
> 
> /// ...Because the capture list is evaluated outside of the closure, this
> /// CaptureList wraps the ClosureExpr.  The dynamic semantics are that evaluates
> /// the variable bindings from the capture list, then evaluates the
> /// subexpression (the closure itself) and returns the result.

I think the original representation just made the captures part of the ClosureExpr, and IIRC Chris changed it to this intentionally.  I don't remember why; maybe it was causing problems for some tooling that wanted to walk the expression tree and find uses of the variable?  It's certainly kindof weird for IRGen.

John.

> 
> 🤷‍
> 
> On Tue, Feb 7, 2017 at 9:10 PM, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
> 
>> On Feb 7, 2017, at 9:09 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>> 
>> PR'd:  https://github.com/apple/swift/pull/7326 <https://github.com/apple/swift/pull/7326>
>> 
>> Although I would also ask: why is CaptureListExpr a parent of ClosureExpr and not a child?
> 
> I think it’s kind of arbitrary. You could also argue that they should be one and the same AST node. I think right now it’s just a property of how the parser works, the capture list comes first, before the closure body?
> 
> Slava
> 
>> 
>> Jacob
>> 
>> On Tue, Feb 7, 2017 at 7:56 PM, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> 
>>> On Feb 7, 2017, at 7:30 PM, Jacob Bandes-Storch via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>> 
>>> I just learned about CaptureListExpr when working on some diagnostics. Is there a particular reason that its member "closureBody" is an Expr* and not a ClosureExpr*? There seems to be only one place it's built <https://github.com/apple/swift/blob/1e46f13184d7256c991b2ba9424af9efe3cd860f/lib/Parse/ParseExpr.cpp#L2425>, and the body is always a ClosureExpr.
>>> 
>>> I can see one minor place <https://github.com/apple/swift/blob/1e46f13184d7256c991b2ba9424af9efe3cd860f/lib/AST/ASTWalker.cpp#L663-L665> where it might be less convenient to have a ClosureExpr, but otherwise there doesn't seem to be much of a reason to keep it generalized to Expr*.
>> 
>> Since autoclosures cannot have capture lists, I think the change you’re suggesting makes sense.
>> 
>> Slava
>> 
>>> 
>>> Jacob
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>> 
>> 
> 
> 
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