[swift-evolution] Proposal: Introduce User-defined "Dynamic Member Lookup" Types

[Chris Lattner]

> On Dec 4, 2017, at 5:22 PM, Joe DeCapo via swift-evolution <swift-evolution at swift.org> wrote:
>> The first one, has no static type info, no compile time checking, it's not self documenting, no type inference so people will be forced to use a dynamic reference at the call site to store the result, leading to more type loss, and all this spirals down.
>> I'm already starting to fear dynamic.
>> Edit: The danger has passed (Phew!) ... and dynamic wasn't been abused after all, no need to down vote me after 3 years :)
> 
> From what I can gather, `dynamic` is used when declaring types, but there's no indication at call sites that what is being invoked is dynamic. And it even allows for casting basically anything to the `dynamic` type.
> 
> https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/dynamic
> 
> So here we have a language community that was (is?) very vocal about caution when it comes to type inference with `var`, but seems to have accepted the validity of `dynamic`. This seems to show that at least one community has absorbed this sort of change (arguably a more "dangerous" version than what is being proposed here) with no real issues.

Right.  dynamic in C# is far broader (and more “dangerous”) than what I’m proposing.  That said, because there have been absolutely zero specific examples of the sorts of harm DynamicMemberLookup could cause, it is difficult to speculate about exactly which boogieman people are afraid of.

> So I have a few questions:
> 
> - Would it be enough to require annotation of the dynamic nature of a type at the declaration sites, if that still means the call sites aren't explicitly annotated? 

It is impossible to solve a problem if it cannot be explained in enough detail to provide examples.  Personally, I don’t see the problem at all.

> - Why do some think the Swift community would be more at risk of abuse of this feature than the C# community seems to have been? 


People are making bizarre claims about what the spirit of Swift is, informing me of things which are obviously not true, and ignoring the evidence I present to them.  This is doubly humorous given that I have a fairly good sense for the design balance and tradeoffs of existing features in Swift today, along with detailed rationale for why they were added, when, and all of the discussion that backed them.  I chalk this up to the fear of the unknown or perhaps a mistrust for the peers these people work with.

My goal is to make the design and proposal writeup as good as possible, and the fear mongering about abuse has led me to add several options for further narrowing the potential for abuse, including to the point of requiring every new adoptee to go through the Swift evolution process for review.  During the review period for DynamicMemberLookup, people who carry these concerns are welcome to +1 one or more of those.

I personally am far more interested in getting to the bottom of Doug’s concerns - it isn’t clear to me what exactly his preferred direction actually is, but that discussion is based on engineering tradeoffs and may well lead to a change to the proposal or a complete change in direction.

-Chris