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

[Slava Pestov]

> On Feb 7, 2017, at 9:18 PM, Jacob Bandes-Storch <jtbandes at gmail.com> 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.

Unless you think there’s something to be gained, I’m not sure it’s worth it…

> 
> 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.

This makes sense, but as I understand it, evaluating the closure itself has no side-effects so this is irrelevant.

Slava

> 
> 🤷‍
> 
> 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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